describe miller urey experiment

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Miller-Urey Experiment

The Miller-Urey Experiment was a landmark experiment to investigate the chemical conditions that might have led to the origin of life on Earth. The scientist Stanley Miller, under the supervision of the Nobel laureate scientist Harold Urey conducted it in 1952 at the University of Chicago. They tried to recreate the conditions that could have existed in the first billion years of the Earth’s existence (also known as the Early Earth) to check the said chemical transformations.

Miller-Urey Experiment And The Primordial Soup Theory

The experiment tested the primordial or primeval soup theory developed independently by the Soviet biologist A.I. Oparin and English scientist J.B.S. Haldane in 1924 and 1929 respectively. The theory propounds the idea that the complex chemical components of life on Earth originated from simple molecules occurring naturally in the reducing atmosphere of the Early Earth, sans oxygen. Lightning and rain energized the said atmosphere to create simple organic compounds that formed an organic “soup”. The so-called soup underwent further changes giving rise to more complex organic polymers and finally life.

The Miller-Urey Experiment In Support Of Abiogenesis

From what was explained in the previous paragraph, it can undoubtedly be considered as a classic experiment to demonstrate abiogenesis. For those who are not conversant with the term, abiogenesis is the process responsible for the development of living beings from non-living or abiotic matter. It is thought to have taken place on the Earth about 3.8 to 4 billion years ago.

Miller-Urey Experiment Apparatus and Procedure

The groundbreaking experiment used a sterile glass flask of 5 liters attached with a pair of electrodes, to hold water (H 2 O), methane (CH 4 ), ammonia (NH 3 ) and hydrogen (H 2 ), the major components of primitive Earth. This was connected to another glass flask of 500 ml capacity half filled with water. On heating it, the water vaporized to fill the larger container with water vapor. The electrodes induced continuous electrical sparks in the gas mixture to simulate lightning. When the gas was cooled, the condensed water made its way into a U-shaped trap at the base of the apparatus.

After electrical sparking had continued for a day, the solution in the trap turned pink in color. At the end of a week, the boiling flask was removed, and mercuric chloride added to prevent microbial contamination. After stopping the chemical reaction, the scientist duo examined the cooled water collected to find that 10-15% of the carbon present in the system was in the form of organic compounds. 2% of carbon went into the formation of various amino acids, including 13 of the 22 amino acids essential to make proteins in living cells, glycine being the most abundant.

Though the result was the production of only simple organic molecules and not a complete living biochemical system, still the simple prebiotic experiment could, to a considerable extent, prove the primordial soup hypothesis.

Miller-Urey Experiment Animation

Chemistry of the miller and urey experiment.

The components of the mixture can react among themselves to produce formaldehyde (CH 2 O), hydrogen cyanide (HCN) and other intermediate compounds.

CO 2 → CO + [O] (atomic oxygen)

CH 4 + 2[O] → CH 2 O + H 2 O

CO + NH 3 → HCN + H 2 O

CH 4 + NH 3 → HCN + 3H 2

The ammonia, formaldehyde and HCN so produced react by a process known as Strecker synthesis to form biomolecules including amino acids.

CH 2 O + HCN + NH 3 → NH 2 -CH 2 -CN + H 2 O

NH 2 -CH 2 -CN + 2H 2 O → NH 3 + NH 2 -CH 2 -COOH (glycine)

In addition to the above, formaldehyde and water can react by Butlerov’s reaction to produce a variety of sugars like ribose, etc.

Though later studies have indicated that the reducing atmosphere as replicated by Miller and Urey could not have prevailed on primitive Earth, still, the experiment remains to be a milestone in synthesizing the building blocks of life under abiotic conditions and not from living beings themselves.

https://www.bbc.co.uk/bitesize/guides/z2gjtv4/revision/1

https://www.juliantrubin.com/bigten/miller_urey_experiment.html

Article was last reviewed on Thursday, February 2, 2023

One response to “Miller-Urey Experiment”

This experiment is currently seen as not sufficient to support abiogenesis. See Stephen C. Meyer, James Tour.

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Miller Urey Experiment: Hypothesis, Steps, Conclusions, and Limitations

The Miller Urey Experiment played a crucial role in investigating the origin of life on our planet. This comprehensive guide explores the experiment’s hypothesis, step-by-step process, key findings, and limitations, shedding light on its significance in unraveling the mysteries of life’s beginnings.

Oparin-Haldane Hypothesis

The Oparin-Haldane Hypothesis, proposed by Aleksandr Oparin and J.B.S. Haldane, postulates that life didn’t spontaneously emerge on early Earth due to different environmental conditions. It suggests that life gradually evolved from chemical reactions, starting with the combination of atoms into inorganic molecules and the subsequent formation of simple organic compounds. These compounds then assembled into complex organic structures, ultimately leading to the emergence of the first cell.

Steps of the Miller Urey Experiment

The Miller-Urey experiment, conducted in 1953 by Stanley L. Miller and Harold C. Urey, aimed to simulate early Earth’s conditions and test the Oparin-Haldane Hypothesis. Here are the key steps of the experiment:

Simulating Early Earth’s Atmosphere: The researchers recreated early Earth’s atmosphere in a closed system using a mixture of gases believed to be present during that era. They used a mixture of gases, including methane (CH4), ammonia (NH3), water vapor (H2O), and hydrogen (H2).

Introduction of Energy: Sparks or electric discharges were introduced to simulate the energy sources on early Earth, such as lightning strikes.

Circulation and Condensation: The gas mixture and energy were circulated continuously, mimicking Earth’s water cycle and allowing for the formation of various organic compounds.

Collection and Analysis: Samples were collected from the closed system and analyzed using chromatography and spectrometry to identify and characterize the organic compounds formed during the experiment.

Results and Findings: The experiment produced a variety of organic molecules, including amino acids—the building blocks of proteins —supporting the notion that early Earth’s conditions could have facilitated the synthesis of organic compounds essential for life’s origin.

Conclusions of the Miller Urey Experiment

The Miller-Urey experiment yielded significant conclusions, including:

Organic compounds, including amino acids, can be synthesized from inorganic materials under simulated early Earth conditions.
Basic building blocks of life may have emerged spontaneously from non-living matter.
The experiment demonstrated the potential for diverse organic compound formation, including rare amino acids.
External energy sources played a crucial role in facilitating chemical reactions and organic compound synthesis.
The experiment offered insights into the chemical reactions that might have occurred in early Earth’s atmosphere.
The findings supported the concept of abiogenesis, where life can arise from non-living matter through natural processes.
The Miller-Urey experiment laid the foundation for further research in prebiotic chemistry and the study of life’s origins.

Limitations of the Miller Urey Experiment

It’s important to consider the limitations of the Miller-Urey experiment, which include:

The experiment’s simulation of early Earth’s atmosphere may not perfectly represent the actual conditions.
The specific gases used may not accurately reflect the true composition of early Earth’s atmosphere.
The experiment’s short duration and scale may not fully capture the complexity and length of natural processes involved in life’s origin.
While the experiment produced organic compounds, it didn’t address the assembly of complex biomolecules or replicating systems crucial for life’s origin.

Ongoing Debates and Significance

Critics argue that the experiment oversimplifies the interconnected nature of biochemical systems and may not fully represent the processes behind life’s origin. There is an ongoing debate regarding the specific conditions and pathways leading to life’s emergence, with the Miller-Urey experiment presenting one plausible scenario. While it doesn’t address the origin of genetic information or self-replicating systems, subsequent research has refined and expanded upon its findings, leading to revised interpretations. The Miller-Urey experiment remains a significant milestone in our understanding of prebiotic chemistry and contributes to unraveling the complex puzzle of life’s origin.

In conclusion, the Miller-Urey experiment’s hypothesis, steps, conclusions, and limitations provide valuable insights into the origin of life on Earth. It serves as a foundation for further research, stimulating ongoing debates and refining our understanding of life’s emergence from non-living matter.

Learn more:

Amino Acids: Types, Functions, Sources, and Differences between Essential and Non-Essential Amino Acids

Miller Urey Experiment

Miller and urey experiment.

Stanley L. Muller and Harold C. Urey performed an experiment to describe the origin of life on earth. They were of the idea that the early earth’s atmosphere was able to produce amino acids from inorganic matter. The two biologists made use of methane, water, hydrogen, and ammonia which they considered were found in the early earth’s atmosphere. The chemicals were sealed inside sterile glass tubes and flasks connected together in a loop and circulated inside the apparatus.

One flask is half-filled with water and the other flask contains a pair of electrodes. The water vapour was heated and the vapour released was added to the chemical mixture. The released gases circulated around the apparatus imitating the earth’s atmosphere. The water in the flask represents the water on the earth’s surface and the water vapour is just like the water evaporating from lakes, and seas. The electrodes were used to spark the fire to imitate lightning and storm through water vapour.

The vapours were cooled and the water condensed. This condensed water trickles back into the first water flask in a continuous cycle. Miller and Urey examined the cooled water after a week and observed that 10-15% of the carbon was in the form of organic compounds. 2% of carbon had formed 13 amino acids . Yet, the Miller and Urey experiments were condemned by their fellow scientists.

Criticism of the Miller Urey Experiment

The experiment failed to explain how proteins were responsible for the formation of amino acids. A few scientists have contradicted that the gases used by Miller and Urey are not as abundant as shown in the experiment. They were of the notion that the gases released by the volcanic eruptions such as oxygen, nitrogen, and carbon dioxide make up the atmosphere. Therefore, the results are not reliable.

Oparin and Haldane

In the early 20th century, Oparin and Haldane suggested that if the atmosphere of the primitive earth was reducing and if it had sufficient supply of energy such as ultraviolet radiations and lightning, organic compounds would be synthesized at a wide range.

Oparin believed that the organic compounds would have undergone a series of reactions to form complex molecules. He suggested that the molecules formed coacervates in the aqueous environment.

Haldane proposed that the atmosphere of the primordial sea was devoid of oxygen, and was a composed of ammonia, carbon dioxide, and ultraviolet light. This gave rise to a host of organic compounds. The sea contained large amounts of organic monomers and polymers, and the sea was called a ‘hot dilute soup’. He conceived that the polymers and monomers acquired lipid membranes. The molecules further developed and gave rise to the first living organism. ‘Prebiotic soup’ was the term coined by Haldane.

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Conducting Miller-Urey Experiments

Eric t. parker.

1 School of Chemistry and Biochemistry, Georgia Institute of Technology

James H. Cleaves

2 Earth-Life Science Institute, Tokyo Institute of Technology

3 Institute for Advanced Study

Aaron S. Burton

4 Astromaterials Research and Exploration Science Directorate, NASA Johnson Space Center

Daniel P. Glavin

5 Goddard Center for Astrobiology, NASA Goddard Space Flight Center

Jason P. Dworkin

Manshui zhou, jeffrey l. bada.

6 Geosciences Research Division, Scripps Institution of Oceanography, University of California at San Diego

Facundo M. Fernández

In 1953, Stanley Miller reported the production of biomolecules from simple gaseous starting materials, using an apparatus constructed to simulate the primordial Earth's atmosphere-ocean system. Miller introduced 200 ml of water, 100 mmHg of H 2 , 200 mmHg of CH 4 , and 200 mmHg of NH 3 into the apparatus, then subjected this mixture, under reflux, to an electric discharge for a week, while the water was simultaneously heated. The purpose of this manuscript is to provide the reader with a general experimental protocol that can be used to conduct a Miller-Urey type spark discharge experiment, using a simplified 3 L reaction flask. Since the experiment involves exposing inflammable gases to a high voltage electric discharge, it is worth highlighting important steps that reduce the risk of explosion. The general procedures described in this work can be extrapolated to design and conduct a wide variety of electric discharge experiments simulating primitive planetary environments.

Introduction

The nature of the origins of life on Earth remains one of the most inscrutable scientific questions. In the 1920s Russian biologist Alexander Oparin and British evolutionary biologist and geneticist John Haldane proposed the concept of a "primordial soup" 1,2 , describing the primitive terrestrial oceans containing organic compounds that may have facilitated chemical evolution. However, it wasn't until the 1950s when chemists began to conduct deliberate laboratory studies aimed at understanding how organic molecules could have been synthesized from simple starting materials on the early Earth. One of the first reports to this end was the synthesis of formic acid from the irradiation of aqueous CO 2 solutions in 1951 3 .

In 1952, Stanley Miller, then a graduate student at the University of Chicago, approached Harold Urey about doing an experiment to evaluate the possibility that organic compounds important for the origin of life may have been formed abiologically on the early Earth. The experiment was conducted using a custom-built glass apparatus ( Figure 1A ) designed to simulate the primitive Earth. Miller's experiment mimicked lightning by the action of an electric discharge on a mixture of gases representing the early atmosphere, in the presence of a liquid water reservoir, representing the early oceans. The apparatus also simulated evaporation and precipitation through the use of a heating mantle and a condenser, respectively. Specific details about the apparatus Miller used can be found elsewhere 4 . After a week of sparking, the contents in the flask were visibly transformed. The water turned a turbid, reddish color 5 and yellow-brown material accumulated on the electrodes 4 . This groundbreaking work is considered to be the first deliberate, efficient synthesis of biomolecules under simulated primitive Earth conditions.

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Figure 1. Comparison between the two types of apparatuses discussed in this paper. The classic apparatus used for the original Miller-Urey experiment ( A ) and the simplified apparatus used in the protocol outlined here ( B ). Click here to view larger image .

After the 1953 publication of results from Miller's classic experiment, numerous variations of the spark discharge experiment, for example using other gas mixtures, were performed to explore the plausibility of producing organic compounds important for life under a variety of possible early Earth conditions. For example, a CH 4 /H 2 O/NH 3 /H 2 S gas mixture was tested for its ability to produce the coded sulfur-containing α-amino acids, although these were not detected 6 . Gas chromatography-mass spectrometry (GC-MS) analysis of a CH 4 /NH 3 mixture subjected to an electric discharge showed the synthesis of α-aminonitriles, which are amino acid precursors 7 . In 1972, using a simpler apparatus, first introduced by Oró 8 ( Figure 1B ), Miller and colleagues demonstrated the synthesis of all of the coded α-amino acids 9 and nonprotein amino acids 10 that had been identified in the Murchison meteorite to date, by subjecting CH 4 , N 2 , and small amounts of NH 3 to an electric discharge. Later, using this same simplified experimental design, gas mixtures containing H 2 O, N 2 , and CH 4 , CO 2 , or CO were sparked to study the yield of hydrogen cyanide, formaldehyde, and amino acids as a function of the oxidation state of atmospheric carbon species 11 .

In addition to the exploration of alternative experimental designs over the years, significant analytical advances have occurred since Miller's classic experiment, which recently aided more probing investigations of electric discharge experimental samples archived by Miller, than would have been facilitated by the techniques Miller had access to in the 1950s. Miller's volcanic experiment 12 , first reported in 1955 4 , and a 1958 H 2 S-containing experiment 13 were shown to have formed a wider variety, and greater abundances, of numerous amino acids and amines than the classic experiment, including many of which that had not been previously identified in spark discharge experiments.

The experiment described in this paper can be conducted using a variety of gas mixtures. Typically, at the very least, such experiments will contain a C-bearing gas, an N-bearing gas, and water. With some planning, almost any mixture of gases can be explored, however, it is important to consider some chemical aspects of the system. For example, the pH of the aqueous phase can have a significant impact on the chemistry that occurs there 14 .

The method described here has been tailored to instruct researchers how to conduct spark discharge experiments that resemble the Miller-Urey experiment using a simplified 3 L reaction vessel, as described in Miller's 1972 publications 9,10 . Since this experiment involves a high voltage electric arc acting on inflammable gases, it is crucial to remove O 2 from the reaction flask to eliminate the risk of explosion, which can occur upon combustion of reduced carbon-bearing gases such as methane or carbon monoxide, or reaction of H 2 with oxygen.

There are additional details that should be kept in mind when preparing to conduct the experiment discussed here. First, whenever working with glass vacuum lines and pressurized gases, there exists the inherent danger of both implosion and over-pressuring. Therefore, safety glasses must be worn at all times. Second, the experiment is typically conducted at less than atmospheric pressure. This minimizes the risk of over-pressuring the manifold and reaction flask. Glassware may be rated at or above atmospheric pressure, however, pressures above 1 atm are not recommended. Pressures may increase in these experiments as water-insoluble H 2 is liberated from reduced gases (such as CH 4 and NH 3 ). Over-pressuring can lead to seal leakage, which can allow atmospheric O 2 to enter the reaction flask, making it possible to induce combustion, resulting in an explosion. Third, it should be borne in mind that modification of this protocol to conduct variations of the experiment requires careful planning to ensure unsafe conditions are not created. Fourth, it is highly recommended that the prospective experimenter read through the entire protocol carefully several times prior to attempting this experiment to be sure he or she is familiar with potential pitfalls and that all necessary hardware is available and in place. Lastly, conducting experiments involving combustible gases require compliance with the experimenter's host institution's Environmental Health and Safety departmental guidelines. Please observe these recommendations before proceeding with any experiments. All steps detailed in the protocol here are in compliance with the authors' host institutional Environmental Health and Safety guidelines.

1. Setting Up a Manifold/Vacuum System

Use ground glass joints and glass plugs with valves on the manifold. Ensure that all O-rings on the plugs are capable of making the necessary seals. If using glass joints, a sufficient amount of vacuum grease can be applied to help make a seal, if necessary. Silicon vacuum grease can be used to avoid potential organic contamination.
Use glass stopcocks on the manifold. Apply the minimum amount of vacuum grease necessary to make a seal.
Measure the manifold volume. This volume will be used for calculations related to final gas pressures in the 3 L reaction flask and should be known as precisely as possible.
Unless the manifold has enough connections to accommodate all gas cylinders simultaneously, connect one cylinder at a time to the manifold. Include in this connection, a tap allowing the manifold to be isolated from the ambient atmosphere.
Use suitable, clean, inert, and chemical and leak resistant tubing and ultratorr vacuum fittings to connect the gas cylinders to the manifold. Ultratorr fittings, where used, are to be finger-tightened.
To ensure rapid attainment of vacuum and to protect the pump, insert a trap between the manifold and the vacuum pump. A liquid nitrogen finger-trap is recommended as it will prevent volatiles such as NH 3 , CO 2 , and H 2 O from entering the pump. Care should be taken, as trapped volatiles, upon warming, may overpressure the manifold and result in glass rupture.
Connect to the manifold, a manometer or other vacuum gauge capable of 1 mmHg resolution or better. While various devices can be used, a mercury manometer, or MacLeod gauge, is preferable as mercury is fairly nonreactive.
Measure and record the ambient temperature using a suitable thermometer.

2. Preparation of Reaction Flask

Clean the tungsten electrodes by gently washing with clean laboratory wipes and methanol, and drying in air.
Introduce a precleaned and sterilized magnetic stir bar, which will ensure rapid dissolution of soluble gases and mixing of reactants during the experiment.
Attach the tungsten electrodes to the 3 L reaction flask using a minimal amount of vacuum grease, with tips separated by approximately 1 cm inside the flask. Fasten with clips.
Insert an adapter with a built-in stopcock into the neck of the 3 L reaction flask and secure with a clip.
Lightly grease all connections to ensure a good vacuum seal.
Open all valves and stopcocks on the manifold, except Valve 6 and Stopcock 1 ( Figure 4 ), and turn on the vacuum pump to evacuate the manifold. Once a stable vacuum reading of <1 mmHg has been attained, close Valve 1 and allow the manifold to sit for ~15 min to check for vacuum leaks. If none are detected, proceed to step 2.8. Otherwise troubleshoot the various connections until the leaks can be identified and fixed.
Apply magnetic stirring to the reaction vessel. Open Valve 1 and Stopcock 1 ( Figure 4 ) to evacuate the headspace of the 3 L reaction flask until the pressure has reached <1 mmHg.
Close Valve 1 ( Figure 4 ) and monitor the pressure inside the 3 L reaction flask. The measured pressure should increase to the vapor pressure of water. To ensure that no leaks exist, wait ~5 min at this stage. If the pressure (as read on the manometer) increases while Valve 1 is closed during this step, check for leaks in Stopcock 1 and the various reaction flask connections. If no leak is found, proceed to the next step.

3. Introduction of Gaseous NH 3

Calculate the necessary pressure of gaseous NH 3 to introduce into the manifold such that 200 mmHg of NH 3 will be introduced into the reaction flask. Details on how to do this are provided in the Discussion section.
Close Valves 1 and 6, and Stopcock 1 ( Figure 4 ) before introducing any gas into the manifold. Leave the other valves and stopcock open.
Introduce NH 3 into the manifold until a small pressure (approximately 10 mmHg) is reached and then evacuate the manifold to a pressure of <1 mmHg by opening Valve 1 ( Figure 4 ). Repeat 3x.
Introduce NH 3 into the manifold to reach the pressure determined in step 3.1.
Open Stopcock 1 ( Figure 4 ) to introduce 200 mmHg of NH 3 into the 3 L reaction flask. The NH 3 will dissolve in the water in the reaction flask and the pressure will fall slowly.
Once the pressure stops dropping, close Stopcock 1 ( Figure 4 ) and record the pressure read by the manometer. This value represents the pressure inside the flask and will be used to calculate the pressures for other gases that will be introduced into the manifold later.
Open Valve 1 ( Figure 4 ) to evacuate the manifold to a pressure of <1 mmHg.
Close Valve 2 ( Figure 4 ) and disconnect the NH 3 gas cylinder from the manifold.

4. Introduction of CH 4

Calculate the necessary pressure of CH 4 to be introduced into the manifold such that 200 mmHg of CH 4 will be introduced into the 3 L reaction flask. Example calculations are shown in the Discussion section.
Connect the CH 4 gas cylinder to the manifold.
Open all valves and stopcocks, except Valve 6 and Stopcock 1 ( Figure 4 ), and evacuate the manifold to a pressure of <1 mmHg.
Close Valve 1 once the manifold has been evacuated ( Figure 4 ).
Introduce CH 4 into the manifold until a small pressure (approximately 10 mmHg) is obtained. This purges the line of any contaminant gases from preceding steps. Open Valve 1 ( Figure 4 ) to evacuate the manifold to <1 mmHg. Repeat 2x more.
Introduce CH 4 into the manifold until the pressure calculated in step 4.1, is reached.
Open Stopcock 1 ( Figure 4 ) to introduce 200 mmHg of CH 4 into the 3 L reaction flask.
Close Stopcock 1 once the intended pressure of CH 4 has been introduced into the 3 L reaction flask ( Figure 4 ) and record the pressure measured by the manometer.
Open Valve 1 (Figure 4 ) to evacuate the manifold to <1 mmHg.
Close Valve 2 ( Figure 4 ) and disconnect the CH 4 cylinder from the manifold.

5. Introduction of Further Gases ( e.g. N 2 )

At this point, it is not necessary to introduce additional gases. However, if desired, it is recommended to add 100 mmHg of N 2 . In this case, calculate the necessary pressure of N 2 to be introduced into the manifold such that 100 mmHg of N 2 will be introduced into the 3 L reaction flask. Example calculations are shown in the Discussion section.
Connect the N 2 gas cylinder to the manifold.
Introduce N 2 into the manifold until a small pressure (approximately 10 mmHg) is obtained. Open Valve 1 ( Figure 4 ) to evacuate the manifold to <1 mmHg. Repeat 2x more.
Introduce N 2 into the manifold until the pressure calculated in step 5.1 is reached.
Open Stopcock 1 ( Figure 4 ) to introduce 100 mmHg of N 2 into the reaction flask.
Close Stopcock 1 once the intended pressure of N 2 has been introduced into the reaction flask, ( Figure 4 ) and record the pressure using the manometer.
Open Valve 1 ( Figure 4 ) to evacuate the manifold to <1 mmHg.
Close Valve 2 ( Figure 4 ) and disconnect the N 2 cylinder from the manifold.

6. Beginning the Experiment

Detach the reaction flask from the manifold by closing Stopcock 1 and Valve 1 ( Figure 4 ) once all gases have been introduced into the reaction flask, so that ambient air may enter the manifold and bring the manifold up to ambient pressure.
After carefully disconnecting the reaction flask from the manifold, set the flask somewhere it will not be disturbed ( e.g. inside an empty fume hood).
Disconnect the vacuum pump and carefully remove the cold trap and allow venting inside a fully operational fume hood.
Secure the Tesla coil connected to the high frequency spark generator.
Connect the opposite tungsten electrode to an electrical ground to enable the efficient passage of electrical current across the gap between the two electrodes.
Set the output voltage of the spark generator to approximately 30,000 V, as detailed by documents available from the manufacturer.
Prior to initiating the spark, close the fume hood sash, to serve as a safety shield between the apparatus and the experimenter. Turn the Tesla coil on to start the experiment, and allow sparking to continue for 2 weeks (or other desired period) in 1 hr on/off cycles.

7. End of Experiment

Stop the experiment by turning off the Tesla coil.
Open Stopcock 1 ( Figure 4 ) to slowly introduce ambient air into the reaction flask and facilitate the removal of the adapter and the tungsten electrodes so samples can be collected. If desired, a vacuum can be used to evacuate the reaction flask of noxious reaction gases.

8. Collecting Liquid Sample

Transfer the sample to a sterile plastic or glass receptacle. Plastic receptacles are less prone to cracking or breaking upon freezing, compared to glass receptacles.
Seal sample containers and store in a freezer capable of reaching temperatures of -20 °C or lower, as insoluble products may prevent the sample solution from freezing at 0 °C.

9. Cleaning the Apparatus

Use clean laboratory wipes to carefully remove vacuum grease from the neck of the apparatus, the adapter and stopcock, and the glass surrounding the tungsten electrodes.
Thoroughly clean the same surfaces described in step 9.1 with toluene to fully remove organic vacuum grease from the glassware. If using silicon grease, the high vacuum grease may remain on the glassware after pyrolysis, creating future problems, as detailed in the Discussion section.
Thoroughly clean the reaction flask with a brush and the following solvents in order: ultrapure water (18.2 MΩ cm, <5 ppb TOC), ultrapure water (18.2 MΩ cm, <5 ppb TOC) with 5% cleaning detergent, methanol, toluene, methanol, ultrapure water (18.2 MΩ cm, <5 ppb TOC) with 5% cleaning detergent, and finally ultrapure water (18.2 MΩ cm, <5 ppb TOC).
Cover all open orifices of the reaction flask with aluminum foil and wrap the adapter and its components in aluminum foil.
Once all the glassware has been wrapped in aluminum foil, pyrolyze for at least 3 hr in air at 500 °C.
Gently clean electrodes with methanol and let air dry.

10. Sample Analysis

Note: When preparing samples for analysis, the use of an acid hydrolysis protocol such as has been described in detail elsewhere 15 , is useful for obtaining more amino acids. Hydrolysis of a portion of the recovered sample provides the opportunity to analyze both free amino acids as well as their acid-labile precursors that are synthesized under abiotic conditions.

For amino acid analysis, use a suitable technique (such as liquid chromatography and mass spectrometry-based methods, or other appropriate approaches). Such analytical techniques include high performance liquid chromatography with fluorescence detection (HPLC-FD) 14 , and ultrahigh performance liquid chromatography with fluorescence detection in parallel with time-of-flight positive electrospray ionization mass spectrometry (UHPLC-FD/ToF-MS) 12,13 . This manuscript describes analysis using mass spectrometric analyses via a triple quadrupole mass spectrometer (QqQ-MS) in conjunction with HPLC-FD.

Representative Results

The products synthesized in electric discharge experiments can be quite complex, and there are numerous analytical approaches that can be used to study them. Some of the more commonly used techniques in the literature for analyzing amino acids are discussed here. Chromatographic and mass spectrometric methods are highly informative techniques for analyzing the complex chemical mixtures produced by Miller-Urey type spark discharge experiments. Amino acid analyses can be conducted using o -phthaldialdehyde/N-acetyl-L-cysteine (OPA/NAC) 16 , a chiral reagent pair that tags primary amino groups, yielding fluorescent diastereomer derivatives that can be separated on an achiral stationary phase. Figure 2 shows a chromatogram of an OPA/NAC-derivatized amino acid standard obtained by HPLC coupled to fluorescence detection and QqQ-MS. The amino acids contained in the standard include those typically produced in Miller-Urey type spark discharge experiments. The identities of these amino acids are listed in Table 1 . Representative fluorescence traces of a typical sample and analytical blank are shown in Figure 3 , demonstrating the molecular complexity of Miller-Urey type electric discharge samples. The sample chromatogram in Figure 3 was produced from a spark discharge experiment using the following starting conditions: 300 mmHg of CH 4 , 250 mmHg of NH 3 , and 250 ml of water.

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Figure 2. The 3-21 min region of the HPLC-FD/QqQ-MS chromatograms produced from the analysis of an OPA/NAC-derivatized amino acid standard . Amino acid peak identities are listed in Table 1 . The fluorescence trace is shown at the bottom and the corresponding extracted mass chromatograms are shown above. The electrospray ionization (ESI) QqQ-MS was operated in positive mode and monitored a mass range of 50-500 m/z. The ESI settings were: desolvation gas (N 2 ) temperature: 350 °C, 650 L/hr; capillary voltage: 3.8 kV; cone voltage: 30 V. The unlabeled peaks in the 367 extracted ion chromatogram are the 13 C 2 peaks from the 365 extracted ion chromatogram, as a result of the approximately 1% natural abundance of 13 C. Click here to view larger image .

Peak	Amino Acid
1	D-aspartic acid
2	L-aspartic acid
3	L-glutamic acid
4	D-glutamic acid
5	D-serine
6	L-serine
7	Glycine
8	b-Alanine
9	D-alanine
10	g-amino-n-butyric acid (g-ABA)
11	L-alanine
12	D-b-amino-n-butyric acid (D-b-ABA)
13	a-aminoisobutyric acid (a-AIB)
14	L-b-amino-n-butyric acid (L-b-ABA)
15	D/L-a-amino-n-butyric acid (D/L-a-ABA)
16	D-isovaline
17	L-isovaline
18	L-valine
19	e-amino-n-caproic acid (EACA)
20	D-valine
21	D-isoleucine
22	L-isoleucine
23	D/L-leucine

Table 1. Peak identities for amino acids detected in the standard and that are typically produced in Miller-Urey type spark discharge experiments.

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Figure 3. The 3-21 min region of the HPLC-FD chromatograms representative of Miller-Urey type spark discharge experiments. Peaks were identified and quantitated by retention time and mass analysis of target compounds compared to a standard and analytical blank. All target analytes with coeluting fluorescence retention times can be separated and quantitated using mass spectrometry, except for α-AIB and L-β-ABA (peaks 13 and 14), and D/L-norleucine, which coelutes with D/L-leucine (peak 23), under the chromatographic conditions used. D/L-norleucine was added as an internal standard to samples and analytical blanks during sample preparation. Amino acid separation was achieved using a 4.6 mm x 250 mm, 5 μm particle size Phenyl-Hexyl HPLC column. The mobile phase was composed of: A) ultrapure water (18.2 MΩ cm, <5 ppb TOC), B) methanol, and C) 50 mM ammonium formate with 8% methanol, at pH 8. The gradient used was: 0-5 min, 100% C; 5-15 min, 0-83% A, 0-12% B, 100-5% C; 15-22 min, 83-75% A, 12-20% B, 5% C; 22-35 min, 75-35% A, 20-60% B, 5% C; 35-37 min, 35-0% A, 60-100% B, 5-0% C; 37-45 min, 100% B; 45-46 min, 100-0% B, 0-100% C 46-55 min, 100% C. The flow rate was 1 ml/min. Click here to view larger image .

Numerous steps in the protocol described here are critical for conducting Miller-Urey type experiments safely and correctly. First, all glassware and sample handling tools that will come in contact with the reaction flask or sample need to be sterilized. Sterilization is achieved by thoroughly rinsing the items in question with ultrapure water (18.2 MΩ cm, <5 ppb TOC) and then wrapping them in aluminum foil, prior to pyrolyzing at 500 °C in air for at least 3 hr. Once the equipment has been pyrolyzed and while preparing samples for analysis, care must be taken to avoid organic contamination. The risk of contamination can be minimized by wearing nitrile gloves, a laboratory coat, and protective eyewear. Be sure to work with samples away from one's body as common sources of contamination include finger prints, skin, hair, and exhaled breath. Avoid contact with wet gloves and do not use any latex or Nylon materials. Second, thorough degassing of the reaction flask prior to gas addition into the reaction flask is critical. The presence of even small amounts of molecular oxygen in the reaction flask poses an explosion risk when the spark is discharged into inflammable gases such as CH 4 . While degassing the flask, the water inside the flask will boil, which will prevent a stable reading. At this stage there are two options: 1) degas the flask via freeze-thaw cycles (typically 3 are used), or 2) simply degas the liquid solution. In the latter case, some water will be lost, however, the amount will be relatively minor compared to the remaining volume. Third, a well-equipped and efficient setup must be carefully constructed to establish a consistent spark across the electrodes throughout the entirety of the experiment. BD-50E Tesla coils are not designed for prolonged operation, as they are intended for vacuum leak detection. Intermittent cooling of the Tesla coil is thus recommended for extended operational lifetime. There are multiple ways of achieving this. One simple way is to attach a timer in-line between the spark tester and its power supply and program the timer such that it alternates in 1 hour on/off cycles. Cooling the Tesla coil with a commercial fan may also be necessary to prolong the life of the Tesla coil. The Tesla coil tip should be touching or almost touching one of the tungsten electrodes; a distance between the two of approximately 1 mm or less. Additionally, an intense discharge can be achieved using a length of conductive metal wire with a loop in one end draped lightly over the electrode opposite the one touching the Tesla coil to avoid breaking the seal to the contents. It is also recommended to have a second spark generator available in case the primary spark generator fails due to extended use.

There are many additional notes worth keeping in mind when carrying out various steps in the protocol outlined here. When preparing the manifold system for an experiment and using a mercury manometer, it is generally conceded that a precision of 1 mmHg is the best achievable, due to the resolution of the human eye. Some gases may present conductivity problems with resistance-based gauges. Mercury manometers present potential spill hazards, which should be prepared for in advance.

While assembling the 3 L reaction flask, the use of silicon vacuum grease can mitigate potential organic contamination, but care should be taken to remove this thoroughly between runs. Failure to do so will result in the accumulation of silica deposits during high-temperature pyrolysis, which can interfere with vacuum seals. Additionally, the tungsten electrodes are commercially available as 2% thoriated tungsten and should be annealed into half-round ground glass fittings . Do not pyrolyze the glass-fitted tungsten electrodes in an oven. The coefficients of thermal expansion of tungsten and glass are different and heating above 100 °C may weaken the seal around the glass annealed electrodes and introduce leaks to the system. Also, ultrapure water can be introduced into the 3 L reaction flask by pouring, using care to avoid contact with any grease on the port used, or by pipetting, using a prepyrolyzed glass pipette. The aqueous phase in the reaction flask can be buffered, if desired. For example, Miller and colleagues 9 buffered the solution to pH ~8.7 with an NH 3 /NH 4 Cl buffer. To do this the aqueous phase is made 0.05 M in NH 4 Cl prior to introducing it into the reaction flask. NH 4 Cl of 99.5% purity, or greater, should be used. The remainder of the NH 3 is then added to the reaction flask as a gas.

In preparation for gas introduction into the 3 L reaction flask, the flask can be secured onto the manifold by placing the flask on a cork ring, set atop a lab jack and gently raising the flask assembly until a snug connection is achieved. When checking for leaks, it is worth noting that likely sources of leaks include poor seals at the junctions of the half-round ground glass joints, which attach the tungsten electrodes to the reaction flask, and the stopcock of the adapter attached to the neck of the 3 L reaction flask. If leaks from these sources are detected, carefully remove the 3 L reaction flask from the manifold, wipe these areas with clean laboratory tissue, reapply a fresh coating of vacuum grease and reattach the flask to the manifold to search for leaks. If no leaks are found, proceed to introduce gases into the reaction flask.

While introducing gases into the apparatus, gas cylinders should be securely fastened to a support. Care should be taken to introduce gases slowly. Valves on gas cylinders should be opened slowly and carefully while monitoring the manometer to avoid over-pressuring the glassware and attached fittings. It is important to note that while adding NH 3 into the reaction flask, because NH 3 is appreciably soluble in water below the pK a of NH 4 + (~9.2), essentially all of the NH 3 gas introduced into the manifold will dissolve in the aqueous phase, rendering the final pressure in the flask and manifold as the vapor pressure of water at the ambient temperature. Once this pressure is attained, one may assume the transfer is complete. The following are examples of the calculations that must be executed in order to precisely introduce gases into the reaction flask at their desired pressures:

Introduction of Gaseous NH 3

Due to the solubility of NH 3 , essentially all of it will transfer from the manifold to the reaction flask and dissolve in the aqueous phase as long as the NH 3 in the manifold is at a higher pressure than the vapor pressure of water in the reaction flask. Therefore, the ambient temperature should be noted and the vapor pressure of water at that temperature should be referenced prior to introducing NH 3 into the manifold. The target pressure of NH 3 to be introduced into the reaction flask should be equal to the target pressure of NH 3 in the 3 L reaction flask, plus the vapor pressure of water in the reaction flask, at the recorded ambient temperature. For example, at 25 °C, the vapor pressure of water is approximately 24 mmHg. Thus, in order to introduce 200 mmHg of NH 3 into the reaction flask, load roughly 225 mmHg of NH 3 into the manifold prior to transferring NH 3 from the manifold and into the reaction flask. This will result in approximately 200 mmHg of NH 3 being introduced into the reaction flask.

Introduction of CH 4

After NH 3 addition and its dissolution in the aqueous phase, the pressure in the headspace of the reaction flask will be equal to the vapor pressure of water at 25 °C, approximately 24 mmHg. This value will be used, in conjunction with the example manifold shown in Figure 4 , to carry out a calculation for how much CH 4 to introduce into the manifold such that 200 mmHg of CH 4 will be introduced into the reaction flask:

P 1 = total pressure desired throughout the entire system, including the reaction flask V 1 = total volume of the entire system, including the reaction flask

P 2 = pressure of CH 4 needed to fill manifold volume prior to introduction into reaction flask V 2 = volume of manifold used for gas introduction

P 3 = pressure already in the headspace of the reaction flask V 3 = volume of the reaction flask

P 1 = 200 mmHg of CH 4 + 24 mmHg of H 2 O = 224 mmHg V 1 = 3,000 ml + 100 ml + 300 ml + 40 ml + 20 ml + 3,000 ml + 40 ml + 500 ml = 7,000 ml

P 2 = pressure of CH 4 being calculated V 2 = 100 ml + 300 ml + 40 + 20 + 3,000 ml+ 40 ml + 500 ml = 4,000 ml

P 3 = 24 mmHg of H 2 O V 3 = 3,000 ml

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Introduction of N 2

After introduction of CH 4 , the headspace of the reaction flask is occupied by 200 mmHg of CH 4 and 24 mmHg of H 2 O for a total of 224 mmHg. This value will be used, along with the dimensions of the example manifold shown in Figure 4 , to calculate the N 2 pressure that needs to be introduced into the manifold such that 100 mmHg of N 2 will be introduced into the reaction flask:

P 2 = pressure of N 2 needed to fill manifold volume prior to introduction into reaction flask V 2 = volume of manifold used for gas introduction

P 1 = 24 mmHg of H 2 O + 200 mmHg of CH 4 + 100 mmHg of N 2 = 324 mmHg V 1 = 3,000 ml + 100 ml > + 300 ml + 40 ml + 20 ml + 3,000 ml + 40 ml + 500 ml = 7,000 ml

P 2 = pressure of N 2 being calculated V 2 = 100 ml + 300 ml + 40 ml + 20 ml + 3,000 ml + 40 ml + 500 ml = 4,000 ml

P 3 = 200 mmHg of CH 4 + 24 mmHg of H 2 O = 224 mmHg V 3 = 3,000 ml

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Figure 4. Manifold/vacuum system used to introduce gases into the 3 L reaction flask. Valves controlling gas flow are labeled as V 1 - V 8 , while stopcocks controlling gas flow are labeled as S 1 and S 2 . It is worth noting that while Valves 1, 2, and 6, and Stopcock 1 are referred to explicitly in the protocol, the other valves and stopcock in the manifold shown here are useful for adding or removing volume ( i.e. holding flasks) to or from the manifold. For example, when introducing gases into the manifold at relatively high pressures (approximately 500 mmHg or greater), it is advised that the experimenter makes use of all purge flasks attached to the manifold to increase the accessible volume of the manifold and help minimize the risk of over-pressuring the manifold.

After initiating the experiment, the system must be checked on regularly to ensure the experiment is running properly. Things to check include: 1) the spark generator is producing a spark, and 2) the spark is being generated across the tungsten electrodes in a continuous manner. If the above conditions are not met, disconnect the Tesla coil from its power supply and replace it with the backup Tesla coil. Meanwhile, repairs to the malfunctioning Tesla coil can be made. Often times, the contact plates inside the spark generator housing can become corroded from extended use and should be polished, or replaced.

Upon completion of the experiment, the gases in the head-space may be irritating to the respiratory system. Harmful gases, such as hydrogen cyanide 4 can be produced by the experiment. If the experimenter is not collecting gas samples for analysis, it may be helpful to connect the apparatus to a water aspirator to evacuate volatiles for approximately one hour after completion of the experiment, while the apparatus remains in the fume hood, prior to collecting liquid samples. For safety reasons, it is advised that the apparatus is vented in a fully-operational fume hood. Sample collection should be performed in an operational fume hood and sample handling in a positive-pressure HEPA filtered flow bench is recommended.

Among the numerous types of products formed by spark discharge experiments, amino acids are of significance. Amino acids are synthesized readily via the Strecker synthesis 17 . The Strecker synthesis of amino acids involves the reaction of aldehydes or ketones and HCN generated by the action of electric discharge on the gases introduced into the reaction apparatus, which upon dissolving in the aqueous phase, may react with ammonia to form α-aminonitriles that undergo hydrolysis to yield amino acids. This is, of course, but one mechanism of synthesis, and others may also be operative, such as direct amination of precursors including acrylonitrile to give β-alanine precursors, or direct hydrolysis of higher molecular weight tholin-like material to give amino acids directly, by-passing the Strecker mechanism.

Amino acid contamination of the samples produced by Miller-Urey experiments can occur if the precautions mentioned earlier are not followed explicitly. During sample analysis, it is important to search for signs of terrestrial contamination that may have originated from sample handling or sample storage. The use of OPA/NAC 16 in conjunction with LC-FD techniques allows for the chromatographic separation of D- and L-enantiomers of amino acids with chiral centers and their respective, individual quantitation. Chiral amino acids synthesized by the experiment should be racemic. Acceptable experimental error during the synthesis of amino acids with chiral centers is generally considered to be approximately 10%. Therefore chiral amino acid D/L ratios suggestive of enrichment in one enantiomer by more than 10% is a good metric by which to determine if the sample has been contaminated.

The methods presented here are intended to instruct how to conduct a Miller-Urey type spark discharge experiment; however, there are limitations to the technique described here that should be noted. First, heating the single 3 L reaction flask ( Figure 1B ), will result in condensation of water vapor onto the tips of the electrodes, dampening the spark, and reducing the generation of radical species that drive much of the chemistry taking place within the experiment. Furthermore, the use of a heating mantle to heat the apparatus is not necessary to synthesize organic compounds, such as amino acids. This differs from Miller's original experiment where he used a more complex, custom-built, dual flask apparatus ( Figure 1A ) 5 and heated the small flask at the bottom of the apparatus, which had water in it ( Figure 1A ). Heating the apparatus helped with circulation of the starting materials and aimed to mimic evaporation in an early Earth system. Second, the protocol detailed here recommends a 1 hr on/off cycle when using the Tesla coil, which effectively doubles the amount of time an experiment takes to complete, compared to the experiments conducted by Miller, as he continuously discharged electricity into the system 4 . Third, as spark generators are not intended for long-term use, they are prone to malfunction during prolonged use and must be regularly maintained and sometimes replaced by a back-up unit, if the primary spark generator fails during the course of an experiment. Last, the protocol described here involves the use of glass stopcocks, which require high vacuum grease to make appropriate seals. If desired, polytetrafluoroethylene (PTFE) stopcocks can be used to avoid vacuum grease. However, if examining these stopcocks for potential leaks with a spark leak detector, be cautious to not overexpose the PTFE to the spark as this can compromise the integrity of the PTFE and lead to poor seals being made by these stopcocks.

The significance of the method reported here with respect to existing techniques, lies within its simplicity. It uses a commercially available 3 L flask, which is also considerably less fragile and easier to clean between experiments than the original design used by Miller 5 . Because the apparatus is less cumbersome, it is small enough to carry out an experiment inside a fume hood.

Once the technique outlined here has been mastered, it can be modified in a variety of ways to simulate numerous types of primitive terrestrial environments. For example, more oxidized gas mixtures can be used 14,18,19 . Furthermore, using modifications of the apparatus, the energy source can be changed, for example, by using a silent discharge 4 , ultraviolet light 20 , simulating volcanic systems 4,12,21 , imitating radioactivity from Earth's crust 22 , and mimicking energy produced by shockwaves from meteoritic impacts 23 , and also cosmic radiation 18,19 .

The classic Miller-Urey experiment demonstrated that amino acids, important building blocks of biological proteins, can be synthesized using simple starting materials under simulated prebiotic terrestrial conditions. The excitation of gaseous molecules by electric discharge leads to the production of organic compounds, including amino acids, under such conditions. While amino acids are important for contemporary biology, the Miller-Urey experiment only provides one possible mechanism for their abiotic synthesis, and does not explain the origin of life, as the processes that give rise to living organisms were likely more complex than the formation of simple organic molecules.

Disclosures

The authors declare no competing financial interests.

Acknowledgments

This work was jointly supported by the NSF and NASA Astrobiology Program, under the NSF Center for Chemical Evolution, CHE-1004570, and the Goddard Center for Astrobiology. E.T.P. would like to acknowledge additional funding provided by the NASA Planetary Biology Internship Program. The authors also want to thank Dr. Asiri Galhena for invaluable help in setting up the initial laboratory facilities.

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Miller–Urey Experiment

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That experiment is essential because it explained how some complex molecules were formed. If you are having trouble with this experiment, here is a very short, but very good youtube video about this theme:

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Oparin-Haldane theory

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Oparin-Haldane theory , idea that organic molecules could be formed from abiogenic materials in the presence of an external energy source—e.g., ultraviolet radiation —and that Earth’s primitive atmosphere was reducing (having very low amounts of free oxygen ) and contained ammonia and water vapour, among other gases. The theory emerged in the 1920s, when British scientist J.B.S. Haldane and Russian biochemist Aleksandr Oparin independently set forth similar ideas concerning the conditions required for the origin of life on Earth .

Haldane and Oparin both suspected that the first life-forms appeared in the warm, primitive ocean and were heterotrophic (obtaining preformed nutrients from the compounds in existence on early Earth) rather than autotrophic (generating food and nutrients from sunlight or inorganic materials). Oparin thought that life developed from coacervates, microscopic spontaneously formed spherical aggregates of lipid molecules that are held together by electrostatic forces and that may have been precursors of cells . Oparin’s work with coacervates confirmed that enzymes fundamental for the biochemical reactions of metabolism functioned more efficiently when contained within membrane-bound spheres than when free in aqueous solutions. Haldane, unfamiliar with Oparin’s coacervates, thought that simple organic molecules formed first and in the presence of ultraviolet light became increasingly complex, ultimately forming cells. Haldane and Oparin’s ideas formed the foundation for much of the research on abiogenesis that took place in later decades.

In 1953 American chemists Harold C. Urey and Stanley Miller tested the Oparin-Haldane theory and successfully produced organic molecules from some of the inorganic components thought to have been present on prebiotic Earth. This became known as the Miller-Urey experiment . Modern abiogenesis hypotheses are based largely on the same principles as the Oparin-Haldane theory and the Miller-Urey experiment. Subtle differences exist, however, between the several models that have been set forth, and explanations differ as to whether complex organic molecules first became self-replicating entities lacking metabolic functions or first became metabolizing protocells that then developed the ability to self-replicate.

Icon 1 — The Miller-Urey Experiment

The experiment itself

The understanding of the origin of life was largely speculative until the 1920s, when Oparin and Haldane, working independently, proposed a theoretical model for "chemical evolution." The Oparin-Haldane model suggested that under the strongly reducing conditions theorized to have been present in the atmosphere of the early earth (between 4.0 and 3.5 billion years ago), inorganic molecules would spontaneously form organic molecules (simple sugars and amino acids). In 1953, Stanley Miller, along with his graduate advisor Harold Urey, tested this hypothesis by constructing an apparatus that simulated the Oparin-Haldane "early earth." When a gas mixture based on predictions of the early atmosphere was heated and given an electrical charge, organic compounds were formed ( Miller, 1953 ; Miller and Urey, 1959 ). Thus, the Miller-Urey experiment demonstrated how some biological molecules, such as simple amino acids, could have arisen abiotically, that is through non-biological processes, under conditions thought to be similar to those of the early earth. This experiment provided the structure for later research into the origin of life. Despite many revisions and additions, the Oparin-Haldane scenario remains part of the model in use today. The Miller-Urey experiment is simply a part of the experimental program produced by this paradigm.

Wells boils off

Wells says that the Miller-Urey experiment should not be taught because the experiment used an atmospheric composition that is now known to be incorrect. Wells contends that textbooks don't discuss how the early atmosphere was probably different from the atmosphere hypothesized in the original experiment. Wells then claims that the actual atmosphere of the early earth makes the Miller-Urey type of chemical synthesis impossible, and asserts that the experiment does not work when an updated atmosphere is used. Therefore, textbooks should either discuss the experiment as an historically interesting yet flawed exercise or not discuss it at all. Wells concludes by saying that textbooks should replace their discussions of the Miller-Urey experiment with an "extensive discussion" of all the problems facing research into the origin of life.

These allegations might seem serious; however, Wells's knowledge of prebiotic chemistry is seriously flawed. First, Wells's claim that researchers are ignoring the new atmospheric data, and that experiments like the Miller-Urey experiment fail when the atmospheric composition reflects current theories, is simply false. The current literature shows that scientists working on the origin and early evolution of life are well aware of the current theories of the earth's early atmosphere and have found that the revisions have little effect on the results of various experiments in biochemical synthesis. Despite Wells's claims to the contrary, new experiments since the Miller-Urey ones have achieved similar results using various corrected atmospheric compositions ( Figure 1 ; Rode, 1999 ; Hanic et al., 2000 ). Further, although some authors have argued that electrical energy might not have efficiently produced organic molecules in the earth's early atmosphere, other energy sources such as cosmic radiation (e.g., Kobayashi et al., 1998 ), high temperature impact events (e.g., Miyakawa et al., 2000 ), and even the action of waves on a beach ( Commeyras, et al., 2002 ) would have been quite effective.

Even if Wells had been correct about the Miller-Urey experiment, he does not explain that our theories about the origin of organic "building blocks" do not depend on that experiment alone ( Orgel, 1998a ). There are other sources for organic "building blocks," such as meteorites, comets, and hydrothermal vents. All of these alternate sources for organic materials and their synthesis are extensively discussed in the literature about the origin of life, a literature that Wells does not acknowledge. In fact, what is most striking about Wells's extensive reference list is the literature that he has left out. Wells does not mention extraterrestrial sources of organic molecules, which have been widely discussed in the literature since 1961 (see Oró, 1961 ; Whittet, 1997 ; Irvine, 1998 ). Wells apparently missed the vast body of literature on organic compounds in comets (e.g. Oró, 1961 ; Anders, 1989 ; Irvine, 1998 ), carbonaceous meteorites (e.g. Kaplan et al., 1963 ; Hayes, 1967 ; Chang, 1994; Maurette, 1998 ; Cooper et al., 2001 ), and conditions conducive to the formation of organic compounds that exist in interstellar dust clouds ( Whittet, 1997 ).

Wells also fails to cite the scientific literature on other terrestrial conditions under which organic compounds could have formed. These non-atmospheric sources include the synthesis of organic compounds in a reducing ocean (e.g., Chang, 1994 ), at hydrothermal vents (e.g., Andersson, 1999 ; Ogata et al., 2000 ), and in volcanic aquifers ( Washington, 2000 ). A cursory review of the literature finds more than 40 papers on terrestrial prebiotic chemical synthesis published since 1997 in the journal Origins of life and the evolution of the biosphere alone. Contrary to Wells's presentation, there appears to be no shortage of potential sources for organic "building blocks" on the early earth.

Instead of discussing this literature, Wells raises a false "controversy" about the low amount of free oxygen in the early atmosphere. Claiming that this precludes the spontaneous origin of life, he concludes that "[d]ogma had taken the place of empirical science" ( Wells 2000 :18). In truth, nearly all researchers who work on the early atmosphere hold that oxygen was essentially absent during the period in which life originated ( Copley, 2001 ) and therefore oxygen could not have played a role in preventing chemical synthesis. This conclusion is based on many sources of data , not "dogma." Sources of data include fluvial uraninite sand deposits ( Rasmussen and Buick, 1999 ) and banded iron formations ( Nunn, 1998 ; Copley, 2001 ), which could not have been deposited under oxidizing conditions. Wells also neglects the data from paleosols (ancient soils) which, because they form at the atmosphere-ground interface, are an excellent source to determine atmospheric composition ( Holland, 1994 ). Reduced paleosols suggest that oxygen levels were very low before 2.1 billion years ago ( Rye and Holland, 1998 ). There are also data from mantle chemistry that suggest that oxygen was essentially absent from the earliest atmosphere ( Kump et al. 2001 ). Wells misrepresents the debate as over whether oxygen levels were 5/100 of 1%, which Wells calls "low," or 45/100 of 1%, which Wells calls "significant." But the controversy is really over why it took so long for oxygen levels to start to rise. Current data show that oxygen levels did not start to rise significantly until nearly 1.5 billion years after life originated ( Rye and Holland, 1998 ; Copley, 2001 ). Wells strategically fails to clarify what he means by "early" when he discusses the amount of oxygen in the "early" atmosphere. In his discussion he cites research about the chemistry of the atmosphere without distinguishing whether the authors are referring to times before, during, or after the period when life is thought to have originated. Nearly all of the papers he cites deal with oxygen levels after 3.0 billion years ago. They are irrelevant, as chemical data suggest that life arose 3.8 billion years ago ( Chang, 1994 ; Orgel, 1998b ), well before there was enough free oxygen in the earth's atmosphere to prevent Miller-Urey-type chemical synthesis.

Finally, the Miller-Urey experiment tells us nothing about the other stages in the origin of life, including the formation of a simple genetic code (PNA or "peptide"-based codes and RNA-based codes) or the origin of cellular membranes (liposomes), some of which are discussed in all the textbooks that Wells reviewed. The Miller-Urey experiment only showed one possible route by which the basic components necessary for the origin of life could have been created, not how life came to be. Other theories have been proposed to bridge the gap between the organic "building blocks" and life. The "liposome" theory deals with the origin of cellular membranes, the RNA-world hypothesis deals with the origin of a simple genetic code, and the PNA (peptide-based genetics) theory proposes an even simpler potential genetic code ( Rode, 1999 ). Wells doesn't really mention any of this except to suggest that the "RNA world" hypothesis was proposed to "rescue" the Miller-Urey experiment. No one familiar with the field or the evidence could make such a fatuous and inaccurate statement. The Miller-Urey experiment is not relevant to the RNA world, because RNA was constructed from organic "building blocks" irrespective of how those compounds came into existence ( Zubay and Mui, 2001 ). The evolution of RNA is a wholly different chapter in the story of the origin of life, one to which the validity of the Miller-Urey experiment is irrelevant.

What the textbooks say

All of the textbooks reviewed contain a section on the Miller-Urey experiment. This is not surprising given the experiment's historic role in the understanding of the origin of life. The experiment is usually discussed over a couple of paragraphs (see Figure 2 ), a small proportion (roughly 20%) of the total discussion of the origin and early evolution of life. Commonly, the first paragraph discusses the Oparin-Haldane scenario, and then a second outlines the Miller-Urey test of that scenario. All textbooks contain either a drawing or a picture of the experimental apparatus and state that it was used to demonstrate that some complex organic molecules (e.g., simple sugars and amino acids, frequently called "building blocks") could have formed spontaneously in the atmosphere of the early earth. Textbooks vary in their descriptions of the atmospheric composition of the early earth. Five books present the strongly reducing atmosphere of the Miller-Urey experiment, whereas the other five mention that the current geochemical evidence points to a slightly reducing atmosphere. All textbooks state that oxygen was essentially absent during the period in which life arose. Four textbooks mention that the experiment has been repeated successfully under updated conditions. Three textbooks also mention the possibility of organic molecules arriving from space or forming at deep-sea hydrothermal vents ( Figure 2 ). No textbook claims that these experiments conclusively show how life originated; and all textbooks state that the results of these experiments are tentative.

It is true that some textbooks do not mention that our knowledge of the composition of the atmosphere has changed. However, this does not mean that textbooks are "misleading" students, because there is more to the origin of life than just the Miller-Urey experiment. Most textbooks already discuss this fact. The textbooks reviewed treat the origin of life with varying levels of detail and length in "Origin of life" or "History of life" chapters. These chapters are from 6 to 24 pages in length. In this relatively short space, it is hard for a textbook, particularly for an introductory class like high school biology, to address all of the details and intricacies of origin-of-life research that Wells seems to demand. Nearly all texts begin their origin of life sections with a brief description of the origin of the universe and the solar system; a couple of books use a discussion of Pasteur and spontaneous generation instead (and one discusses both). Two textbooks discuss how life might be defined. Nearly all textbooks open their discussion of the origin of life with qualifications about how the study of the origin of life is largely hypothetical and that there is much about it that we do not know.

Wells's evaluation

As we will see in his treatment of the other "icons," Wells's criteria for judging textbooks stack the deck against them, ensuring failure. No textbook receives better than a D for this "icon" in Wells's evaluation, and 6 of the 10 receive an F. This is largely a result of the construction of the grading criteria. Under Wells's criteria (Wells 2000:251-252), any textbook containing a picture of the Miller-Urey apparatus could receive no better than a C, unless the caption of the picture explicitly says that the experiment is irrelevant, in which case the book would receive a B. Therefore, the use of a picture is the major deciding factor on which Wells evaluated the books, for it decides the grade irrespective of the information contained in the text! A grade of D is given even if the text explicitly points out that the experiment used an incorrect atmosphere, as long as it shows a picture. Wells pillories Miller and Levine for exactly that, complaining that they bury the correction in the text. This is absurd: almost all textbooks contain pictures of experimental apparatus for any experiment they discuss. It is the text that is important pedagogically, not the pictures. Wells's criteria would require that even the intelligent design "textbook" Of Pandas and People would receive a C for its treatment of the Miller-Urey experiment.

In order to receive an A, a textbook must first omit the picture of the Miller-Urey apparatus (or state explicitly in the caption that it was a failure), discuss the experiment, but then state that it is irrelevant to the origin of life. This type of textbook would be not only scientifically inaccurate but pedagogically deficient.

Why we should still teach Miller-Urey

The Miller-Urey experiment represents one of the research programs spawned by the Oparin-Haldane hypothesis. Even though details of our model for the origin of life have changed, this has not affected the basic scenario of Oparin-Haldane. The first stage in the origin of life was chemical evolution. This involves the formation of organic compounds from inorganic molecules already present in the atmosphere and in the water of the early earth. This spontaneous organization of chemicals was spawned by some external energy source. Lightning (as Oparin and Haldane thought), proton radiation, ultraviolet radiation, and geothermal or impact-generated heat are all possibilities.

The Miller-Urey experiment represents a major advance in the study of the origin of life. In fact, it marks the beginning of experimental research into the origin of life. Before Miller-Urey, the study of the origin of life was merely theoretical. With the advent of "spark experiments" such as Miller conducted, our understanding of the origin of life gained its first experimental program. Therefore, the Miller-Urey experiment is important from an historical perspective alone. Presenting history is good pedagogy because students understand scientific theories better through narratives. The importance of the experiment is more than just historical, however. The apparatus Miller and Urey designed became the basis for many subsequent "spark experiments" and laid a groundwork that is still in use today. Thus it is also a good teaching example because it shows how experimental science works. It teaches students how scientists use experiments to test ideas about prehistoric, unobserved events such as the origin of life. It is also an interesting experiment that is simple enough for most students to grasp. It tested a hypothesis, was reproduced by other researchers, and provided new information that led to the advancement of scientific understanding of the origin of life. This is the kind of "good science" that we want to teach students.

Finally, the Miller-Urey experiment should still be taught because the basic results are still valid. The experiments show that organic molecules can form under abiotic conditions. Later experiments have used more accurate atmospheric compositions and achieved similar results. Even though origin-of-life research has moved beyond Miller and Urey, their experiments should be taught. We still teach Newton even though we have moved beyond his work in our knowledge of planetary mechanics. Regardless of whether any of our current theories about the origin of life turn out to be completely accurate, we currently have models for the processes and a research program that works at testing the models.

How textbooks could improve their presentations of the origin of life

Textbooks can always improve discussions of their topics with more up-to-date information. Textbooks that have not already done so should explicitly correct the estimate of atmospheric composition, and accompany the Miller-Urey experiment with a clarification of the fact that the corrected atmospheres yield similar results. Further, the wealth of new data on extraterrestrial and hydrothermal sources of biological material should be discussed. Finally, textbooks ideally should expand their discussions of other stages in the origin of life to include PNA and some of the newer research on self-replicating proteins. Wells, however, does not suggest that textbooks should correct the presentation of the origin of life. Rather, he wants textbooks to present this "icon" and then denigrate it, in order to reduce the confidence of students in the possibility that scientific research can ever establish a plausible explanation for the origin of life or anything else for that matter. If Wells's recommendations are followed, students will be taught that because one experiment is not completely accurate (albeit in hindsight), everything else is wrong as well. This is not good science or science teaching.

Icon 2 — Darwin's Tree of Life
Icon 3 — Homology
Icon 4 — Haeckel's Embryos
Icon 5 — Archaeopteryx
Icon 6 — Peppered Moths
Icon 7 — Darwin's Finches
Icons of Evolution? Conclusion
Icons of Evolution? Figures
Icons of Evolution? References
"Icons" Critique — pdf versions
Fatally Flawed Iconoclasm
10 Answers to Jonathan Wells's "10 Questions"

Chapter 18: Life On Earth

Chapter 1 how science works.

The Scientific Method
Measurements
Units and the Metric System
Measurement Errors
Mass, Length, and Time
Observations and Uncertainty
Precision and Significant Figures
Errors and Statistics
Scientific Notation
Ways of Representing Data
Mathematics
Testing a Hypothesis
Case Study of Life on Mars
Systems of Knowledge
The Culture of Science
Computer Simulations
Modern Scientific Research
The Scope of Astronomy
Astronomy as a Science
A Scale Model of Space
A Scale Model of Time

Chapter 2 Early Astronomy

The Night Sky
Motions in the Sky
Constellations and Seasons
Cause of the Seasons
The Magnitude System
Angular Size and Linear Size
Phases of the Moon
Dividing Time
Solar and Lunar Calendars
History of Astronomy
Ancient Observatories
Counting and Measurement
Greek Astronomy
Aristotle and Geocentric Cosmology
Aristarchus and Heliocentric Cosmology
The Dark Ages
Arab Astronomy
Indian Astronomy
Chinese Astronomy
Mayan Astronomy

Chapter 3 The Copernican Revolution

Ptolemy and the Geocentric Model
The Renaissance
Copernicus and the Heliocentric Model
Tycho Brahe
Johannes Kepler
Elliptical Orbits
Kepler's Laws
Galileo Galilei
The Trial of Galileo
Isaac Newton
Newton's Law of Gravity
The Plurality of Worlds
The Birth of Modern Science
Layout of the Solar System
Scale of the Solar System
The Idea of Space Exploration
History of Space Exploration
Moon Landings
International Space Station
Manned versus Robotic Missions
Commercial Space Flight
Future of Space Exploration
Living in Space
Moon, Mars, and Beyond
Societies in Space

Chapter 4 Matter and Energy in the Universe

Matter and Energy
Rutherford and Atomic Structure
Early Greek Physics
Dalton and Atoms
The Periodic Table
Structure of the Atom
Heat and Temperature
Potential and Kinetic Energy
Conservation of Energy
Velocity of Gas Particles
States of Matter
Thermodynamics
Laws of Thermodynamics
Heat Transfer
Thermal Radiation
Radiation from Planets and Stars
Internal Heat in Planets and Stars
Periodic Processes
Random Processes

Chapter 5 The Earth-Moon System

Earth and Moon
Early Estimates of Earth's Age
How the Earth Cooled
Ages Using Radioactivity
Radioactive Half-Life
Ages of the Earth and Moon
Geological Activity
Internal Structure of the Earth and Moon
Basic Rock Types
Layers of the Earth and Moon
Origin of Water on Earth
The Evolving Earth
Plate Tectonics
Geological Processes
Impact Craters
The Geological Timescale
Mass Extinctions
Evolution and the Cosmic Environment
Earth's Atmosphere and Oceans
Weather Circulation
Environmental Change on Earth
The Earth-Moon System
Geological History of the Moon
Tidal Forces
Effects of Tidal Forces
Historical Studies of the Moon
Lunar Surface
Ice on the Moon
Origin of the Moon
Humans on the Moon

Chapter 6 The Terrestrial Planets

Studying Other Planets
The Planets
The Terrestrial Planets
Mercury's Orbit
Mercury's Surface
Volcanism on Venus
Venus and the Greenhouse Effect
Tectonics on Venus
Exploring Venus
Mars in Myth and Legend
Early Studies of Mars
Mars Close-Up
Modern Views of Mars
Missions to Mars
Geology of Mars
Water on Mars
Polar Caps of Mars
Climate Change on Mars
Terraforming Mars
Life on Mars
The Moons of Mars
Martian Meteorites
Comparative Planetology
Incidence of Craters
Counting Craters
Counting Statistics
Internal Heat and Geological Activity
Magnetic Fields of the Terrestrial Planets
Mountains and Rifts
Radar Studies of Planetary Surfaces
Laser Ranging and Altimetry
Gravity and Atmospheres
Normal Atmospheric Composition
The Significance of Oxygen

Chapter 7 The Giant Planets and Their Moons

The Gas Giant Planets
Atmospheres of the Gas Giant Planets
Clouds and Weather on Gas Giant Planets
Internal Structure of the Gas Giant Planets
Thermal Radiation from Gas Giant Planets
Life on Gas Giant Planets?
Why Giant Planets are Giant
Ring Systems of the Giant Planets
Structure Within Ring Systems
The Origin of Ring Particles
The Roche Limit
Resonance and Harmonics
Tidal Forces in the Solar System
Moons of Gas Giant Planets
Geology of Large Moons
The Voyager Missions
Jupiter's Galilean Moons
Jupiter's Ganymede
Jupiter's Europa
Jupiter's Callisto
Jupiter's Io
Volcanoes on Io
Cassini Mission to Saturn
Saturn's Titan
Saturn's Enceladus
Discovery of Uranus and Neptune
Uranus' Miranda
Neptune's Triton
The Discovery of Pluto
Pluto as a Dwarf Planet
Dwarf Planets

Chapter 8 Interplanetary Bodies

Interplanetary Bodies
Early Observations of Comets
Structure of the Comet Nucleus
Comet Chemistry
Oort Cloud and Kuiper Belt
Kuiper Belt
Comet Orbits
Life Story of Comets
The Largest Kuiper Belt Objects
Meteors and Meteor Showers
Gravitational Perturbations
Surveys for Earth Crossing Asteroids
Asteroid Shapes
Composition of Asteroids
Introduction to Meteorites
Origin of Meteorites
Types of Meteorites
The Tunguska Event
The Threat from Space
Probability and Impacts
Impact on Jupiter
Interplanetary Opportunity

Chapter 9 Planet Formation and Exoplanets

Formation of the Solar System
Early History of the Solar System
Conservation of Angular Momentum
Angular Momentum in a Collapsing Cloud
Helmholtz Contraction
Safronov and Planet Formation
Collapse of the Solar Nebula
Why the Solar System Collapsed
From Planetesimals to Planets
Accretion and Solar System Bodies
Differentiation
Planetary Magnetic Fields
The Origin of Satellites
Solar System Debris and Formation
Gradual Evolution and a Few Catastrophies
Chaos and Determinism
Extrasolar Planets
Discoveries of Exoplanets
Doppler Detection of Exoplanets
Transit Detection of Exoplanets
The Kepler Mission
Direct Detection of Exoplanets
Properties of Exoplanets
Implications of Exoplanet Surveys
Future Detection of Exoplanets

Chapter 10 Detecting Radiation from Space

Observing the Universe
Radiation and the Universe
The Nature of Light
The Electromagnetic Spectrum
Properties of Waves
Waves and Particles
How Radiation Travels
Properties of Electromagnetic Radiation
The Doppler Effect
Invisible Radiation
Thermal Spectra
The Quantum Theory
The Uncertainty Principle
Spectral Lines
Emission Lines and Bands
Absorption and Emission Spectra
Kirchoff's Laws
Astronomical Detection of Radiation
The Telescope
Optical Telescopes
Optical Detectors
Adaptive Optics
Image Processing
Digital Information
Radio Telescopes
Telescopes in Space
Hubble Space Telescope
Interferometry
Collecting Area and Resolution
Frontier Observatories

Chapter 14 The Milky Way

The Distribution of Stars in Space
Stellar Companions
Binary Star Systems
Binary and Multiple Stars
Mass Transfer in Binaries
Binaries and Stellar Mass
Nova and Supernova
Exotic Binary Systems
Gamma Ray Bursts
How Multiple Stars Form
Environments of Stars
The Interstellar Medium
Effects of Interstellar Material on Starlight
Structure of the Interstellar Medium
Dust Extinction and Reddening
Groups of Stars
Open Star Clusters
Globular Star Clusters
Distances to Groups of Stars
Ages of Groups of Stars
Layout of the Milky Way
William Herschel
Isotropy and Anisotropy
Mapping the Milky Way

Chapter 15 Galaxies

The Milky Way Galaxy
Mapping the Galaxy Disk
Spiral Structure in Galaxies
Mass of the Milky Way
Dark Matter in the Milky Way
Galaxy Mass
The Galactic Center
Black Hole in the Galactic Center
Stellar Populations
Formation of the Milky Way
The Shapley-Curtis Debate
Edwin Hubble
Distances to Galaxies
Classifying Galaxies
Spiral Galaxies
Elliptical Galaxies
Lenticular Galaxies
Dwarf and Irregular Galaxies
Overview of Galaxy Structures
The Local Group
Light Travel Time
Galaxy Size and Luminosity
Mass to Light Ratios
Dark Matter in Galaxies
Gravity of Many Bodies
Galaxy Evolution
Galaxy Interactions
Galaxy Formation

Chapter 16 The Expanding Universe

Galaxy Redshifts
The Expanding Universe
Cosmological Redshifts
The Hubble Relation
Relating Redshift and Distance
Galaxy Distance Indicators
Size and Age of the Universe
The Hubble Constant
Large Scale Structure
Galaxy Clustering
Clusters of Galaxies
Overview of Large Scale Structure
Dark Matter on the Largest Scales
The Most Distant Galaxies
Black Holes in Nearby Galaxies
Active Galaxies
Radio Galaxies
The Discovery of Quasars
Types of Gravitational Lensing
Properties of Quasars
The Quasar Power Source
Quasars as Probes of the Universe
Star Formation History of the Universe
Expansion History of the Universe

Chapter 17 Cosmology

Early Cosmologies
Relativity and Cosmology
The Big Bang Model
The Cosmological Principle
Universal Expansion
Cosmic Nucleosynthesis
Cosmic Microwave Background Radiation
Discovery of the Microwave Background Radiation
Measuring Space Curvature
Cosmic Evolution
Evolution of Structure
Mean Cosmic Density
Critical Density
Dark Matter and Dark Energy
Age of the Universe
Precision Cosmology
The Future of the Contents of the Universe
Fate of the Universe
Alternatives to the Big Bang Model
Particles and Radiation
The Very Early Universe
Mass and Energy in the Early Universe
Matter and Antimatter
The Forces of Nature
Fine-Tuning in Cosmology
The Anthropic Principle in Cosmology
String Theory and Cosmology
The Multiverse
The Limits of Knowledge

Chapter 18 Life On Earth

Nature of Life
Chemistry of Life
Molecules of Life
The Origin of Life on Earth
Origin of Complex Molecules

Miller-Urey Experiment

Pre-RNA World
From Molecules to Cells
Extremophiles
Thermophiles
Psychrophiles
Acidophiles
Alkaliphiles
Radiation Resistant Biology
Importance of Water for Life
Hydrothermal Systems
Silicon Versus Carbon
DNA and Heredity
Life as Digital Information
Synthetic Biology
Life in a Computer
Natural Selection
Tree Of Life
Evolution and Intelligence
Culture and Technology
The Gaia Hypothesis
Life and the Cosmic Environment

Chapter 19 Life in the Universe

Life in the Universe
Astrobiology
Life Beyond Earth
Sites for Life
Complex Molecules in Space
Life in the Solar System
Lowell and Canals on Mars
Implications of Life on Mars
Extreme Environments in the Solar System
Rare Earth Hypothesis
Are We Alone?
Unidentified Flying Objects or UFOs
The Search for Extraterrestrial Intelligence
The Drake Equation
The History of SETI
Recent SETI Projects
Recognizing a Message
The Best Way to Communicate
The Fermi Question
The Anthropic Principle
Where Are They?

Hardly a conversation can be had about the origin of life on Earth without mention of the Miller-Urey experiment. Very little is known about the conditions on Earth during the time that life would have been forming. Harold Urey and his then-graduate student Stanley Miller were amongst the first scientists to postulate about early conditions. They conducted an experiment that has been repeated in its original and altered form for over five decades. Their work has become seminal for those studying the chemistry of the origin of life on Earth.

Although scientists continue to collect new data that sheds light on the subject, there is still quite a bit of debate over the composition of early Earth's atmosphere . What does the atmosphere have to do with the origin of life, you might ask? Well, the chemical composition of the atmosphere strongly influences the types of chemical reactions occurring at the surface of the planet and consequently impacts the conditions under which life would have originated. Based on work published in The Origin of Life by the Russian scientist Alexander Oparin in 1938, Miller suggested that life was forming during a time when Earth's atmosphere consisted of methane, ammonia, water, and hydrogen. This chemical makeup is quite different from our modern atmosphere of nitrogen, oxygen, and other gases. Miller introduced these molecules into a sealed flask, applied an electric discharge, and allowed the system to cycle for a week. What he discovered has impacted origins research for over fifty years.

The original apparatus used by Miller and Urey was quite simple compared to today's standards. It essentially consisted of two glass flasks connected by glass tubing. One flask served as the boiling flask, where gases and other molecules could accumulate in a water phase. The other flask (located above the boiling flask) served as a place where gases could accumulate and mix together. An electrical discharge, meant to simulate lightning to produce free radicals, was provided by using an induction coil.

The experimental procedure was also straightforward. Water was first added to the boiling flask. Then the apparatus was evacuated completely of air. Once the air had been removed, hydrogen gas (H 2 ), methane (CH 4 ), and ammonia (NH 3 ) were pumped into the apparatus. Finally, the water in the flask was boiled and the electrical discharge was started. The entire system was allowed to run continuously for a week.

Like any good scientist, Miller took copious notes of what happened inside the apparatus during the weeklong experiment. After the first day, the water in the flask turned distinctly pink. As the week progressed, the solution inside became redder and redder and also a bit cloudy. Once the experiment was complete, Miller and Urey determined that the cloudiness, or turbidity, of the solution was due to silica from the glass. The reddish color, however, resulted from organic compounds that "stuck" to the silica. Although difficult to see at first, Miller also noted yellow organic molecules.

At the end of the week, Miller collected the contents of the apparatus and tested the contents for amino acids using chromatography. Initial tests confirmed the presence of glycine, alpha-alanine, and beta-alanine and suggested that aspartic acid and alpha-amino-n-butyric acid had also been produced. This list of amino acids falls miles short of the 20 amino acids commonly used by life on Earth. However, Miller and Urey both suspected that other amino acids were also present, but in such small amounts that their detection was difficult to impossible.

The intent of Miller was not to try and produce amino acids. Rather, his intent was to explore the early conditions on Earth and what the naturally occurring results would be. What he discovered was that, although the conditions he proposed are not optimum, organic molecule synthesis could have been a natural consequence in Earth's history. More importantly, Miller and Urey went on to explore amino acid synthesis by developing a more efficient apparatus and altering the initial atmospheric conditions in the simulated environment.

Scientists studying the atmosphere of early Earth now believe that the primary atmospheric constituents were different from those first proposed by Oparin and later tested by Urey. James Kasting at Pennsylvania State University has suggested that the atmosphere on Earth just after the succession of heavy bombardment would have been dominated by carbon dioxide and nitrogen and contained small amounts of carbon monoxide, hydrogen gas, and reduced sulfur gases. Now instead of a simple set of glass flasks connected by tubes and sealed, scientists use complex computer models and mathematical equations to simulate the conditions of early Earth. Unless we develop a time machine, we will never know exactly what the planet was like. But through good observations and critical analysis by all scientists in the field, we will definitely arrive at feasible theories about the beginnings of Earth. The Miller-Urey experiment traveled only the first tentative steps along the road from simple molecules to a cell. But it showed that some of life's core ingredients can form quickly and naturally and that concentrating chemicals and adding energy can lead to a progression from simplicity to complexity.

The origin of life on Earth, explained

The origin of life on Earth stands as one of the great mysteries of science. Various answers have been proposed, all of which remain unverified. To find out if we are alone in the galaxy, we will need to better understand what geochemical conditions nurtured the first life forms. What water, chemistry and temperature cycles fostered the chemical reactions that allowed life to emerge on our planet? Because life arose in the largely unknown surface conditions of Earth’s early history, answering these and other questions remains a challenge.

Several seminal experiments in this topic have been conducted at the University of Chicago, including the Miller-Urey experiment that suggested how the building blocks of life could form in a primordial soup.

Jump to a section:

When did life on Earth begin?

Where did life on Earth begin?

What are the ingredients of life on earth, what are the major scientific theories for how life emerged, what is chirality and why is it biologically important, what research are uchicago scientists currently conducting on the origins of life, when did life on earth begin .

Earth is about 4.5 billion years old. Scientists think that by 4.3 billion years ago, Earth may have developed conditions suitable to support life. The oldest known fossils, however, are only 3.7 billion years old. During that 600 million-year window, life may have emerged repeatedly, only to be snuffed out by catastrophic collisions with asteroids and comets.

The details of those early events are not well preserved in Earth’s oldest rocks. Some hints come from the oldest zircons, highly durable minerals that formed in magma. Scientists have found traces of a form of carbon—an important element in living organisms— in one such 4.1 billion-year-old zircon . However, it does not provide enough evidence to prove life’s existence at that early date.

Two possibilities are in volcanically active hydrothermal environments on land and at sea.

Some microorganisms thrive in the scalding, highly acidic hot springs environments like those found today in Iceland, Norway and Yellowstone National Park. The same goes for deep-sea hydrothermal vents. These chimney-like vents form where seawater comes into contact with magma on the ocean floor, resulting in streams of superheated plumes. The microorganisms that live near such plumes have led some scientists to suggest them as the birthplaces of Earth’s first life forms.

Organic molecules may also have formed in certain types of clay minerals that could have offered favorable conditions for protection and preservation. This could have happened on Earth during its early history, or on comets and asteroids that later brought them to Earth in collisions. This would suggest that the same process could have seeded life on planets elsewhere in the universe.

The recipe consists of a steady energy source, organic compounds and water.

Sunlight provides the energy source at the surface, which drives photosynthesis. On the ocean floor, geothermal energy supplies the chemical nutrients that organisms need to live.

Also crucial are the elements important to life . For us, these are carbon, hydrogen, oxygen, nitrogen, and phosphorus. But there are several scientific mysteries about how these elements wound up together on Earth. For example, scientists would not expect a planet that formed so close to the sun to naturally incorporate carbon and nitrogen. These elements become solid only under very cold temperatures, such as exist in the outer solar system, not nearer to the sun where Earth is. Also, carbon, like gold, is rare at the Earth’s surface. That’s because carbon chemically bonds more often with iron than rock. Gold also bonds more often with metal, so most of it ends up in the Earth’s core. So, how did the small amounts found at the surface get there? Could a similar process also have unfolded on other planets?

The last ingredient is water. Water now covers about 70% of Earth’s surface, but how much sat on the surface 4 billion years ago? Like carbon and nitrogen, water is much more likely to become a part of solid objects that formed at a greater distance from the sun. To explain its presence on Earth, one theory proposes that a class of meteorites called carbonaceous chondrites formed far enough from the sun to have served as a water-delivery system.

There are several theories for how life came to be on Earth. These include:

Life emerged from a primordial soup

As a University of Chicago graduate student in 1952, Stanley Miller performed a famous experiment with Harold Urey, a Nobel laureate in chemistry. Their results explored the idea that life formed in a primordial soup.

Miller and Urey injected ammonia, methane and water vapor into an enclosed glass container to simulate what were then believed to be the conditions of Earth’s early atmosphere. Then they passed electrical sparks through the container to simulate lightning. Amino acids, the building blocks of proteins, soon formed. Miller and Urey realized that this process could have paved the way for the molecules needed to produce life.

Scientists now believe that Earth’s early atmosphere had a different chemical makeup from Miller and Urey’s recipe. Even so, the experiment gave rise to a new scientific field called prebiotic or abiotic chemistry, the chemistry that preceded the origin of life. This is the opposite of biogenesis, the idea that only a living organism can beget another living organism.

Seeded by comets or meteors

Some scientists think that some of the molecules important to life may be produced outside the Earth. Instead, they suggest that these ingredients came from meteorites or comets.

“A colleague once told me, ‘It’s a lot easier to build a house out of Legos when they’re falling from the sky,’” said Fred Ciesla, a geophysical sciences professor at UChicago. Ciesla and that colleague, Scott Sandford of the NASA Ames Research Center, published research showing that complex organic compounds were readily produced under conditions that likely prevailed in the early solar system when many meteorites formed.

Meteorites then might have served as the cosmic Mayflowers that transported molecular seeds to Earth. In 1969, the Murchison meteorite that fell in Australia contained dozens of different amino acids—the building blocks of life.

Comets may also have offered a ride to Earth-bound hitchhiking molecules, according to experimental results published in 2001 by a team of researchers from Argonne National Laboratory, the University of California Berkeley, and Lawrence Berkeley National Laboratory. By showing that amino acids could survive a fiery comet collision with Earth, the team bolstered the idea that life’s raw materials came from space.

In 2019, a team of researchers in France and Italy reported finding extraterrestrial organic material preserved in the 3.3 billion-year-old sediments of Barberton, South Africa. The team suggested micrometeorites as the material’s likely source. Further such evidence came in 2022 from samples of asteroid Ryugu returned to Earth by Japan’s Hayabusa2 mission. The count of amino acids found in the Ryugu samples now exceeds 20 different types .

In 1953, UChicago researchers published a landmark paper in the Journal of Biological Chemistry that marked the discovery of the pro-chirality concept , which pervades modern chemistry and biology. The paper described an experiment showing that the chirality of molecules—or “handedness,” much the way the right and left hands differ from one another—drives all life processes. Without chirality, large biological molecules such as proteins would be unable to form structures that could be reproduced.

Today, research on the origin of life at UChicago is expanding. As scientists have been able to find more and more exoplanets—that is, planets around stars elsewhere in the galaxy—the question of what the essential ingredients for life are and how to look for signs of them has heated up.

Nobel laureate Jack Szostak joined the UChicago faculty as University Professor in Chemistry in 2022 and will lead the University’s new interdisciplinary Origins of Life Initiative to coordinate research efforts into the origin of life on Earth. Scientists from several departments of the Physical Sciences Division are joining the initiative, including specialists in chemistry, astronomy, geology and geophysics.

“Right now we are getting truly unprecedented amounts of data coming in: Missions like Hayabusa and OSIRIS-REx are bringing us pieces of asteroids, which helps us understand the conditions that form planets, and NASA’s new JWST telescope is taking astounding data on the solar system and the planets around us ,” said Prof. Ciesla. “I think we’re going to make huge progress on this question.”

Last updated Sept. 19, 2022.

Faculty Experts

Clara Blättler

Fred Ciesla

Jack Szostak

Additional Resources

The Origins of Life Speaker Series

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Big Brains podcast: Unraveling the mystery of life’s origins on Earth

Discovering the Missing Link with Neil Shubin (Ep. 1)

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The Miller-Urey Experiment – Chemical Evolution

The Miller-Urey experiment was a simulation of conditions on the early Earth testing the idea that life, or more specifically organic molecules, could have formed by nothing more than simple chemical reactions. Miller’s success validated the theoretical ideas of A.I. Oparin and is considered to be the classic experiment investigating the concept of abiogenesis.

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What Was The Miller‑Urey Experiment?

If the origin of life happened before scientists were around to observe it, how can our origin be studied with the scientific method? This question is answered in our overview of the Miller-Urey experiment.

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What is the rna world hypothesis, can science explain the origin of life.

This animation was sponsored by the Center for Chemical Evolution , NSF, and NASA:

The Miller-Urey experiment was the first attempt to scientifically explore ideas about the origin of life. Stanley Miller simulated conditions thought be common on the ancient Earth. The purpose was to test the idea that the complex molecules of life (in this case, amino acids) could have arisen on our young planet through simple, natural chemical reactions.

The experiment was a success in that amino acids, the building blocks of life, were produced during the simulation. The finding was so significant that it kick-started an entirely new field of study: Prebiotic Chemistry.

Scientists now have reason to believe that the gases used in the Miller-Urey simulation were not actually the same as those of the ancient atmosphere. Because of this, many experiments have since been done, testing a wide variety of atmospheres and different environmental conditions. The results are overwhelming: the molecules of life can form under a wide variety of ancient Earth-like conditions.

Many questions about the origin of life remain to be answered but these findings give strong support to the idea that the first living cells on Earth may have emerged from natural chemical reactions.

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The Miller-Urey Experiment:

Original Paper from 1953 (free access to full paper)

Biomolecules found in meteorites:

Research overview by NASA
DNA molecules in meteorites
Distinguishing actual space bio-molecules from Earth contamination

Biomolecules form in asteroid simulations:

Scientific Paper (free access to abstract only)

Biomolecules form in Volcanic Simulations:

Scientific Paper (free access to full paper)

Biomolecules (amino acids) form in non-reducing atmosphere simulations:

It was once believed that if you left food out to rot, living creatures like maggots and even rats would simply poof into existence. The idea was called Spontaneous Generation.

A series of experiments starting in the 1600s disproved this idea, and in the 1800s a new scientific law was proposed: Life only comes from life.

It’s true that rats, maggots, and even microbes are far too complex to simply poof into existence, but in 1859 English Naturalist Charles Darwin put forth the theory of evolution. In it he showed that under the right circumstances, relatively simple creatures can gradually give rise to more complex creatures. Given this information, serious thinkers began to wonder: Is it possible that simple life forms actually could come from non-living matter? Not by poofing into existence, but through a natural gradual process similar to what we see in biological evolution?

Darwin himself mentioned this idea when writing to friend, “But if (and oh what a big if)” he wrote, ‘we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, and so on present, that a protein compound was chemically formed ready to undergo still more complex changes…”

In 1924, Russian biochemist Alexander Oparin published a book which he titled The Origin of Life. In it he outlined his thoughts on a gradual progression from simple chemistry to living cells. He imagined the early ocean as a primordial soup – a rich collection of complex molecules produced by natural chemical reactions. In this soup, further reactions could take place, eventually producing living cells.

At the time, Darwin’s warm little pond, and Oparin’s primordial soup were really just speculation. They were founded on a good understanding of chemistry and biology but they could not be considered legitimate scientific hypotheses because no one had found a way to test or observe them. Science, after all, is the study of observable facts and an ongoing conversation about how those facts can be best linked together.

Chemical reactions like those proposed by Darwin and Oparin, are not expected to leave an observable fossil record. Without either having fossils to examine or a time machine to travel back and observe what happened, how could scientists even begin to study the origin of life?

In the 1950s, Stanley Miller, then a graduate student at the University of Chicago, came up with an idea. We could simulate early Earth conditions in the lab, and then carefully watch what happens. If you can’t study fish in the sea, set up an aquarium.

Working with his professor, Harold Urey, Miller designed an apparatus to simulate the ancient water cycle.

Together they put in water to model the ancient ocean. It was gently boiled to mimic evaporation. Along with water vapor, for gasses of the atmosphere they chose methane, hydrogen, and ammonia. These are simple gases which scientists at the time thought were probably abundant on the ancient Earth. They added a condenser to cool the atmosphere, allowing water molecules to form drops and fall back into the ocean like rain.

The ancient Earth would have had many sources of energy: sunlight, geothermal heat, and even thunderstorms, so they added sparks to the atmosphere to simulate lightning.

The goal of the experiment was not to create life but to simply test the first step in Oparin’s model: Can simple chemicals naturally give rise to the complex molecules of life?

After running the experiment for just one week, their “ocean” became brownish black. Careful analysis revealed that through a series of reactions, many complex molecules had been produced. Among these were amino acids – special molecules of life that we once thought could only be built inside the bodies of living creatures.

This was a pivotal breakthrough in science! So significant in fact, that It gave rise to an entirely new field of research now known as Prebiotic Chemistry.

Scientists don’t know for sure if the gasses used by Miller really were the most common gasses of the ancient Earth. Because of this many experiments have since been done, showing that the molecules of life can form in a wide variety of environments with different starting chemicals and different sources of energy.

Sugars, lipids and amino acids have even been found on meteorites, this suggests that the molecules of life formed all throughout the ancient solar system, and may be forming right now in other regions of our galaxy!

Together, these discoveries tell us that Oparin’s primordial soup, and Darwin’s warm little pond could have easily existed, in one form or another, on our ancient planet.

So to sum things up, what was the Miller-Urey experiment?

The Miller-Urey experiment was our first attempt at simulating ancient Earth conditions, in this case, the ancient Earth’s water cycle, for the purpose of testing ideas about the origin of life.

The Miller-Urey experiment is significant for two main reasons: First, though it was not a perfect simulation of the early Earth, it clearly demonstrated, for the first time, that biomolecules can form under ancient Earth-like conditions.

Second, the experiment took what was once mere speculation, (the idea that life may have emerged from chemistry) and transformed a portion of that speculation into legitimate, testable science!

Many questions remain to be answered about the origin of life, but scientists from many nations, and many fields of study, are now following Stanley Miller’s lead – they’re finding ways to turn those questions about the origins of life into testable scientific hypotheses.

Simulation experiments cannot tell us exactly how life formed in the past, but if enough of them are done, they could eventually tell us if it’s possible for life to emerge from chemistry.

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A is for abiogenesis —

Scientists recreated classic origin-of-life experiment and made a new discovery, 1952 miller-urey experiment showed organic molecules forming from inorganic precursors..

Jennifer Ouellette - Oct 28, 2021 6:59 pm UTC

Stanley Miller with the original laboratory equipment used in the 1952 Miller-Urey Experiment, which gave credence to the idea that organic molecules could have been created by the conditions of the early Earth's atmosphere.

In 1952, a University of Chicago chemist named Stanley Miller and his adviser, Harold Urey, conducted a famous experiment . Their results, published the following year, provided the first evidence that the complex organic molecules necessary for the emergence of life ( abiogenesis ) could be formed using simpler inorganic precursors, essentially founding the field of prebiotic chemistry. Now a team of Spanish and Italian scientists has recreated that seminal experiment and discovered a contributing factor that Miller and Urey missed. According to a new paper published in the journal Scientific Reports, minerals in the borosilicate glass used to make the tubes and flasks for the experiment speed up the rate at which organic molecules form.

In 1924 and 1929, respectively, Alexander Oparin and J.B.S. Haldane had hypothesized that the conditions on our primitive Earth would have favored the kind of chemical reactions that could synthesize complex organic molecules from simple inorganic precursors—sometimes known as the " primordial soup " hypothesis. Amino acids formed first, becoming the building blocks that, when combined, made more complex polymers.

Miller set up an apparatus to test that hypothesis by simulating what scientists at the time believed Earth's original atmosphere might have been. He sealed methane, ammonia, and hydrogen inside a sterile 5-liter borosilicate glass flask, connected to a second 500-ml flask half-filled with water. Then Miller heated the water, producing vapor, which in turn passed into the larger flask filled with chemicals, creating a mini-primordial atmosphere. There were also continuous electric sparks firing between two electrodes to simulate lighting. Then the "atmosphere" was cooled down, causing the vapor to condense back into water. The water trickled down into a trap at the bottom of the apparatus.

That solution turned pink after one day and deep red after a week. At that point, Miller removed the boiling flask and added barium hydroxide and sulfuric acid to stop the reaction. After evaporating the solution to remove any impurities, Miller tested what remained via paper chromatography. All known life consists of just 20 amino acids. Miller's experiment produced five amino acids, although he was less certain about the results for two of them.

Diagram of the 1952 Miller-Urey experiment.

When Miller showed his results to Urey, the latter suggested a paper should be published as soon as possible. (Urey was senior but generously declined to be listed as co-author, lest this lead to Miller getting little to no credit for the work.) The paper appeared in 1953 in the journal Science. "Just turning on the spark in a basic pre-biotic experiment will yield 11 out of 20 amino acids," Miller said in a 1996 interview . The original apparatus has been on display at the Denver Museum of Nature and Science since 2013.

Miller died in 2007. Shortly before he passed, one of his students, Jeffrey Bada, now at the University of San Diego, inherited all his mentor's original equipment. This included several boxes filled with vials of dried residues from the original experiment. Those 1952 samples were re-analyzed the following year using the latest chromatography methods, revealing that the original experiment actually produced even more compounds (25) than had been reported at the time.

Miller had also performed additional experiments simulating conditions similar to those of a water-vapor-rich volcanic eruption, which involved spraying steam from a nozzle at the spark discharge. Bada and several colleagues re-analyzed the original samples from those experiments, too, and found this environment produced 22 amino acids, five amines, and several hydroxylated molecules. So the original experiments were even more successful than Miller and Urey realized.

There have been many, many more experiments on abiogenesis over the ensuing decades, but co-author Joaquin Criado-Reyes of the Universidad de Granada in Spain and his collaborators thought that one potential factor had been overlooked: the role of the borosilicate glass that comprised the flasks and tubes Miller had used. They noted that Miller's simulated atmosphere was highly alkaline, which should cause the silica to dissolve. "Therefore, it could be expected that upon contact of the alkaline water with the inner wall of the borosilicate flask, even this reinforced glass will slightly dissolve, releasing silica and traces of other metal oxides [into the vapor]," the authors wrote.

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RESEARCH HIGHLIGHT
22 November 2021

Message in a bottle: revisiting the origin of life

Andrea Gentile

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Artist's impression of the early Earth conditions that the Miller-Urey experiment tried to recreate. Credit: CSIC.

Some of the basic ingredients for life are well known: a dash of water, methane, ammonia, hydrogen and a spark. But a pinch of minerals is also needed, according to a new study 1 by Italian and Spanish researchers that recreated an experiment from 1952, paying attention to a detail that had been overlooked for all these years: the glass pot in which it was performed.

“In science you should take nothing for granted,” says Raffaele Saladino, a professor at Tuscia University and president of the Italian Society of Astrobiology. “Nobody would have guessed that a setting tested hundreds of times could tell us anything more.” In 1952 at the University of Chicago, Stanley Miller and Harold Urey simulated the Earth’s environment 4.6 billion years ago to study abiogenesis, the natural synthesis of organic molecules such as amino acids and nucleobases (the building blocks of proteins and DNA/RNA respectively). In a sealed flask, they recreated the primordial atmosphere along with water, while a spark simulated lightning. Later, they found several amino acids, demonstrating how the precursors of life could emerge in a prebiotic soup. “In some experiments Miller also noted the presence of silica [the main component of glass and some rocks],” says Saladino, “but he didn’t pay much attention to it.” And nobody else investigated its role until now.

In previous studies, the team found that silica and its minerals in a solution similar to Miller’s could facilitate the process. So they decided to test the idea that, in the original experiment, they had been diluted from the flask because of the causticity of the mixture. They repeated the experiment using three containers made of materials with different pHs: borosilicate glass or Pyrex (the same material used by Miller), Teflon, which is an inert material, and Teflon with some borosilicate bits in the solution. The results confirmed that organic matter emerged in every flask independently of the pH, but the Teflon container had the fewest products, followed by the one with glass pieces. The abundance of organic molecules in the Pyrex container – 56 different kinds, amino acids and nucleobases included – was staggering, with some molecules appearing only in the borosilicate glass, revealing the importance of minerals as hidden ingredients for the precursors of life. “It makes sense, if we want to simulate a realistic scenario,” explains Saladino, “because we would have the atmosphere, water, lightning, but what we missed was the rock containing the water.”

A renewed interest in abiogenesis could help the search for life on other planets. “The complexity of a molecule doesn’t guarantee that it was produced by biological processes,” notes Saladino. “If we were able to create such molecular richness with a single experiment, then finding molecules like glycine or phosphine on other planets wouldn’t necessarily imply that they were synthesized by a living organism.” Future studies will test which molecules can emerge in a Miller-Urey setting using different minerals and alien atmospheres. Then, when looking for life on different planets we will better know what molecules to expect and, more importantly, those that are truly unexpected.

doi: https://doi.org/10.1038/d43978-021-00144-0

Criado-Reyes J, Bizzarri BM, García-Ruiz JM, Saladino R & Di Mauro E, Sci. Rep. 11 , 21009 (2021)

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What the famous Miller-Urey experiment got wrong

The famous experiment showed that a mixture of gases and water could produce amino acids and other biomolecular precursors.
However, new research shows that an unexpected factor may have played a major part in the result: glassware.
Complex experiments need good controls, and the Miller-Urey experiment failed in this regard.

Science in the early 20th century was undergoing many simultaneous revolutions . Radiological dating numbered the years of Earth’s existence in the billions, and eons of sediment demonstrated its geological evolution. The biological theory of evolution had become accepted, but mysteries remained about its selection mechanism and the molecular biology of genetics. Remnants of life dated far, far back, beginning with simple organisms. These ideas came to a head with the question of abiogenesis : could the first life have sprung from non-living matter?

In 1952, a graduate student named Stanley Miller, just 22 years old, designed an experiment to test whether the amino acids that form proteins could be created under the conditions thought to exist on the primordial Earth. Working with his Nobel Prize-winning advisor Harold Urey, he performed the experiment, which is now told time and again in textbooks all over the world.

The experiment mixed water and simple gases — methane, ammonia, and hydrogen — and shocked them with artificial lightning within a sealed glass apparatus . Within days, a thick colored substance built up at the bottom of the apparatus. This detritus contained five of the basic molecules common to living creatures. Revising this experiment over the years, Miller claimed to find as many as 11 amino acids. Subsequent work varying the electrical spark, the gases, and the apparatus itself created another dozen or so. After Miller’s death in 2007, the remains of his original experiments were re-examined by his former student . There may have been as many as 20-25 amino acids created even in that primitive original experiment.

The Miller-Urey experiment is a daring example of testing a complex hypothesis. It is also a lesson in drawing more than the most cautious and limited conclusions from it.

Did anyone consider the glassware?

In the years following the original work, several limitations curbed excitement over its result . The simple amino acids did not combine to form more complex proteins or anything resembling primitive life. Further, the exact composition of the young Earth did not match Miller’s conditions. And small details of the setup appear to have affected the results. A new study published last month in Scientific Reports investigates one of those nagging details. It finds that the precise composition of the apparatus housing the experiment is crucial to amino acid formation.

The highly alkaline chemical broth dissolves a small amount of the borosilicate glass reactor vessel used in the original and subsequent experiments. Dissolved bits of silica permeate the liquid, likely creating and catalyzing reactions . The eroded walls of the glass may also boost catalysis of various reactions. This increases total amino acid production and allows the formation of some chemicals which are not created when the experiment is repeated in an apparatus made of Teflon. But, running the experiment in a Teflon apparatus deliberately contaminated with borosilicate recovered some of the lost amino acid production.

Complex questions need carefully designed experiments

The Miller-Urey experiment was based on a complicated system. Over the years, many variables were tweaked, such as the concentration and composition of gases. For the purpose of demonstrating what might be plausible — that is, whether biomolecules can be created from inorganic materials — it was stunningly successful. But there wasn’t a good control. We now see that might have been a pretty big mistake.

One of the elements of art in science is to divine which of innumerable complexities matter and which do not. Which variables can be accounted for or understood without testing, and which ones can be cleverly elided by experimental design? This is a borderland between hard science and intuitive art. It is certainly not obvious that glass would play a role in the outcome, but it apparently does.

A more certain and careful form of science is to conduct an experiment that varies one and only one variable at a time. This is a slow and laborious process. It can be prohibitively difficult for testing complex hypotheses like, “Could life evolve from non-life on the early Earth?” The authors of the new work performed just such a single-variable test. They ran the entire Miller-Urey experiment multiple times, varying only the presence of silicate glass. The runs performed in as glass vessel produced one set of results, while those using a Teflon apparatus produced another.

Systematically marching through each potential variable, one at a time, might be called “brute force.” But there is art here too, namely, in deciding which single variable out of many possibilities to test and in what way. In this case, we learned that glass silicates played an important role in the Miller-Urey experiment. Perhaps this means that silicate rock formations on the early Earth were necessary to produce life. Maybe.

NOT EXACTLY ROCKET SCIENCE

Scientists finish a 53-year-old classic experiment on the origins of life

In 1958, a young scientist called Stanley Miller electrified a mixture of simple gases, designed to mimic the atmosphere of our primordial lifeless planet. It was a sequel to one of the most evocative experiments in history, one that Miller himself had carried five years earlier. But for some reason, he never finished his follow-up. He dutifully collected his samples and stored them in vials but, whether for ill health or dissatisfaction, he never analysed them.

The vials languished in obscurity, sitting unopened in a cardboard box in Miller’s office. But possessed by the meticulousness of a scientist, he never threw them away. In 1999, the vials changed owners. Miller had suffered a stroke and bequeathed his old equipment, archives and notebooks to Jeffrey Bada , one of his former students. Bada only twigged to the historical treasures that he had inherited in 2007. “Inside, were all these tiny glass vials carefully labeled, with page numbers referring Stanley’s laboratory notes. I was dumbstruck. We were looking at history,” he said in a New York Times interview .

By then, Miller was completely incapacitated. He died of heart failure shortly after, but his legacy continues. Bada’s own student Eric Parker has finally analysed Miller’s samples using modern technology and published the results, completing an experiment that began 53 years earlier.

Miller conducted his original 1953 experiment as a graduate student, working with his mentor Harold Urey. It was one of the first to tackle the seemingly insurmountable question of how life began. In their laboratory, the pair tried to recreate the conditions on early lifeless Earth, with an atmosphere full of simple gases and laced with lightning storms. They filled a flask with water, methane, ammonia and hydrogen and sent sparks of electricity through them.

The result, both literally and figuratively, was lightning in a bottle. When Miller looked at the samples from the flask, he found five different amino acids – the building blocks of proteins and essential components of life.

The relevance of these results to the origins of life is debatable, but there’s no denying their influence. They kicked off an entire field of research, graced the cover of Time magazine and made a celebrity of Miller. Nick Lane beautifully describes the reaction to the experiment in his book, Life Ascending : “Miller electrified a simple mixture of gases, and the basic building blocks of life all congealed out of the mix. It was as if they were waiting to be bidden into existence. Suddenly the origin of life looked easy.”

Over the next decade, Miller repeated his original experiment with several twists. He injected hot steam into the electrified chamber to simulate an erupting volcano, another mainstay of our primordial planet. The samples from this experiment were among the unexamined vials that Bada inherited. In 2008, Bada’s student Adam Johnson showed that the vials contained a wider range of amino acids than Miller had originally reported in 1953.

Miller also tweaked the gases in his electrified flasks. He tried the experiment again with two newcomers – hydrogen sulphide and carbon dioxide – joining ammonia and methane. It would be all too easy to repeat the same experiment now. But Parker and Bada wanted to look at the original samples that Miller had himself collected, if only for their “considerable historical interest”.

Using modern techniques, around a billion times more sensitive than those Miller would have used, Parker identified 23 different amino acids in the vials, far more than the five that Miller had originally described. Seven of these contained sulphur, which is either a first for science or old news, depending on how you look at it. Other scientists have since produced sulphurous amino acids in similar experiments, including Carl Sagan . But unbeknownst to all of them, Miller had beaten them to it by several years. He had even scooped himself – it took him till 1972 to publish results where he produced sulphur amino acids!

The amino acids in Miller’s vials all come in an equal mix of two forms, each the mirror image of the other. You only see that in laboratory reactions – in nature, amino acids come almost entirely in one version. As such, Parker, like Miller before him, was sure that the amino acids hadn’t come from a contaminating source, like a stray bacterium that had crept into the vials.

Imagine then, a young and violent planet, wracked with exploding volcanoes, noxious gases and lightning strikes. These ingredients combined to brew a “primordial soup”, fashioning the precursors of life in pools of water. On top of that, meteorites raining down from space could have added to the accumulating molecules. After all, Parker found that the amino acid cocktail in Miller’s samples is very similar to that found on the Murchison meteorite , which landed in Australia in 1969.

These are powerful images, so why aren’t people more excited? Echoing many sources I spoke to, Jim Kasting , who studies the evolution of Earth’s atmosphere, said, “I am underwhelmed by it.” The main problem with the study is that Miller was probably wrong about the conditions on early Earth.

By analysing ancient rocks, scientists have since found that Earth was never particularly teeming in hydrogen-rich gases like methane, hydrogen sulphide or hydrogen itself. If you repeat Miller’s experiment with a more realistic mixture – heavy in carbon dioxide and nitrogen, with just trace amounts of other gases – you’d have a hard time finding amino acids in the resulting brew.

Parker accepts the problem, but he suggests that a few specific places on the planet may have had the right conditions. Exploding volcanoes, for example, throw up masses of sulphurous compounds, as well as methane and ammonia. These gases, belched forth into lightning storms , could have produced amino acids that rained out and gathered in tidal pools. But Kasting still isn’t convinced. “Even then the reduced gases would not be as concentrated as they are in this experiment.”

Even if our young planet had the right conditions to produce amino acids, that’s a less impressive feat than it appeared in the 1950s. “Amino acids are old hat and are a million miles from life,” says Nick Lane. Indeed, as Miller’s experiments showed, it’s not difficult to create amino acids. The far bigger challenge is to create nucleic acids – the building blocks of molecules like RNA and DNA. The origin of life lies in the origin of these “replicators”, molecules that can make copies of themselves. Lane says, “Even if you can make amino acids (and nucleic acids) under soup conditions, it has little if any bearing on the origin of life.”

The problem is that replicators don’t spontaneously emerge from a mixture of their building blocks, just as you wouldn’t hope to build a car by throwing some parts into a swimming pool. Nucleic acids are innately “shy”. They need to be strong-armed into forming more complex molecules, and it’s unlikely that the odd bolt of lightning would have been enough. The molecules must have been concentrated in the same place, with a constant supply of energy and catalysts to speed things up. “Without that lot, life will never get started, and a soup can’t provide much if any of that,” says Lane.

Deep-sea vents are a better location for the origins of life. Deep under the ocean’s surface, these rocky chimneys spew out superheated water and hydrogen-rich gases. Their rocky structures contain a labyrinth of small compartments that could have concentrated life’s building blocks into dense crowds, and minerals that would have catalysed their get-togethers. Far away from visions of languid soups, these churning environments are the current best guess for the site of life’s hatchery.

So Miller’s iconic experiment, and its now-completed follow-ups, probably won’t lay out the first steps of life. As Adam Rutherford, who is writing a book on the origin of life, says, “It’s really a historical piece, like finding that Darwin had described a Velociraptor in one of his notebooks.”

If anything, the analysis of Miller’s vials is a testament to the value of meticulous scientific work. Here was a man who prepared his samples so cleanly, who recorded his notes so thoroughly, and who stored everything so carefully, that his contemporaries could pick up where he left off five decades later.

Reference: Parker, Cleaves, Dworkin, Glavin, Callahan, Aubrey, Lazcano & Bada. 2011. Primordial synthesis of amines and amino acids in a 1958 Miller H2S-rich spark discharge experiment. PNAS http://dx.doi.org/10.1073/pnas.1019191108

Photos by Carlos Gutierrez and Marco Fulle

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Origins and experimental evidence

Experiments can help scientists figure out how the molecules involved in the RNA world arose. These experiments serve as “proofs of concept” for hypotheses about steps in the origin of life — in other words, if a particular chemical reaction happens in a modern lab under conditions similar to those on early Earth, the same reaction could have happened on early Earth and could have played a role in the origin of life. The 1953 Miller-Urey experiment, for example, simulated early Earth’s atmosphere with nothing more than water, hydrogen, ammonia, and methane and an electrical charge standing in for lightning, and produced complex organic compounds like amino acids. Now, scientists have learned more about the environmental and atmospheric conditions on early Earth and no longer think that the conditions used by Miller and Urey were quite right. However, since Miller and Urey, many scientists have performed experiments using more accurate environmental conditions and exploring alternate scenarios for these reactions. These experiments yielded similar results – complex molecules could have formed in the conditions on early Earth.

This experimental approach can also help scientists study the functioning of the RNA world itself. For example, origins biochemist, Andy Ellington, hypothesizes that in the early RNA world, RNA copied itself, not by matching individual units of the molecules (as in modern DNA), but by matching short strings of units — it’s a bit like assembling a house from prefabricated walls instead of brick by brick. He is studying this hypothesis by performing experiments to search for molecules that copy themselves like this and to study how they evolve.

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November 26, 2021

Redo of a Famous Experiment on the Origins of Life Reveals Critical Detail Missed for Decades

The Miller-Urey experiment showed that the conditions of early Earth could be simulated in a glass flask. New research finds the flask itself played an underappreciated, though outsize, role.

By Sarah Vitak

A man looks into a laboratory glass flask that's glowing with purple light.

Chemist Stanley Miller re-creates the Miller-Urey experiment using the original laboratory equipment in the 1980s.

Roger Ressmeyer/Corbis/VCG/Getty Images

Illustration of a Bohr atom model spinning around the words Science Quickly with various science and medicine related icons around the text

Sarah Vitak: This is Scientific American ’s 60 Second Science. I’m Sarah Vitak.

The question of how life came to be has captivated humans for millennia. The prevailing theory now is that, on a highly volatile early Earth, lightning struck mineral-rich waters and that the energy from lighting strikes turned those minerals into the building blocks of life: organic compounds like amino acids—something we often refer to as the “primordial soup.” The wide acceptance of this theory is in large part due to the very famous Miller-Urey experiment. You surely encountered this in a science textbook at some point. But to refresh your memory: in 1952 Stanley Miller and Harold Urey simulated the conditions of early Earth by sealing water, methane, ammonia and hydrogen in a glass flask. Then they applied electrical sparks to the mixture. Miraculously, amino acids came into existence amid the roiling mixture. It was a big deal.

But recently a team of researchers realized that—much like that first primordial soup sitting in a bowl of Earth—the experiment’s container played an underappreciated role—that perhaps it was also critical to the creation of organic building blocks inside their laboratory life soup.

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I talked to someone from the team.

Saladino: I am Raffaele Saladino from University of Tuscia in Italy.

Vitak: Then, much like today, when a researcher goes to start an experiment, often one of the first things they do is reach for their glassware. Well, today, actually, we use a lot of plastic as well.

Saladino: But 20 years ago in the lab, only glass containers because, in the mind of the researcher, glass is inert.

Vitak: He said inert, meaning that it doesn’t react with the chemicals you put inside it. But in reality, that is not necessarily always the case.

Most of the time glass is pretty inert. When you’re baking with Pyrex (which is made of borosilicate glass, the same type of glass most labware is made out of) the cookware isn’t going into your brownies. But when you’re baking, whatever is in the pan is usually mostly water, so it will come at a pH of around 7 or so.

But the pH of the Miller-Urey experiment is much higher. In the original experiments, they used a pH of 8.7, which is more alkaline, or basic.

Saladino: Why alkaline environment is an important topic? Since under alkaline condition borosilicate can be impacted through blinds in the reaction menu, it is not inert it became a reagent.

Vitak: In fact, this was actually noted by Miller in his original experiments--that the alkaline conditions caused the silica to dissolve. But it was overshadowed by the discovery of the synthesis of organic compounds. And as future researchers carried on they missed that point in Miller’s notes.

Saladino: The attention was concentrated on modifying the atmosphere, on modifying the energy, the intensity, and modifying the analytical tools.

Vitak: And the role of the silica got forgotten entirely.

Dr. Saladino’s team wanted to see if the glass was doing anything in the reaction. To test this they set up three different versions of the original experiment where everything was the same except the containers. For comparison they chose teflon which does not dissolve when holding an alkaline solution, the way the glass does.

Saladino: There is the experiment only glass, the experiment only Teflon, and in the middle, there is the experiment in teflon with some pieces of glass added inside.

Vitak: Then they used a technique called mass spectrometry to analyze what each reaction produced. Mass spectrometry is great for figuring out what kinds of molecules are in a complex mixture.

They found that teflon produced very few organic compounds. There were more compounds in the teflon with glass pieces. But the glass container, by far, created the greatest number and largest variety of organic molecules.

The mechanism of exactly how the silica helps catalyze the reaction is not clear yet--but it is very clearly does.

The obvious question then is: Was there silica available in the early earth environment?

Saladino: The water is not suspended in a vacuum. No? The water is in geochemistry, it is surrounded by minerals. Borosilicate and silica are the most abundant minerals surrounding the water.

Vitak: The team has two next major objectives in mind. First, to try updating the experiment to model more closely the amount of silica that would have been available in the early Earth.

Second, they want to try replacing the silica with extraterrestrial minerals like, pieces of meteorite or rocks from other planets. Apart from just being very cool, that could give a more concrete idea of how to look for life in space.

But here on Earth, coming one step closer to fully understanding why we exist is that much more satisfying. Even after nearly 70 years, a key discovery in our complex origin story still carries new revelations. As the authors say in the paper: "The role of the rocks was hidden in the walls of the reactors."

Thanks for listening. For Scientific American’s 60 Second Science, I’m Sarah Vitak.

[ The above text is a transcript of this podcast .]

COMMENTS

Miller-Urey experiment
The Miller-Urey experiment was an experimental simulation conducted in 1953 that tested whether organic molecules could be formed from chemical reactions occurring between inorganic molecules thought to have been present early in Earth's history. To test Oparin and Haldane's ideas, Miller and Urey designed a closed experiment in a laboratory.
Miller-Urey experiment
The Miller-Urey experiment was a synthesis of small organic molecules in a mixture of simple gases in a thermal gradient created by heating (right) and cooling (left) the mixture at the same time, with electrical discharges. The Miller-Urey experiment[ 1] (or Miller experiment[ 2]) was an experiment in chemical synthesis carried out in 1952 ...
Miller-Urey Experiment
The Miller-Urey Experiment was a landmark experiment to investigate the chemical conditions that might have led to the origin of life on Earth. The scientist Stanley Miller, under the supervision of the Nobel laureate scientist Harold Urey conducted it in 1952 at the University of Chicago. They tried to recreate the conditions that could have ...
Miller Urey Experiment: Hypothesis, Steps, Conclusions, and Limitations
The Miller-Urey experiment, conducted in 1953 by Stanley L. Miller and Harold C. Urey, aimed to simulate early Earth's conditions and test the Oparin-Haldane Hypothesis. Here are the key steps of the experiment: Simulating Early Earth's Atmosphere: The researchers recreated early Earth's atmosphere in a closed system using a mixture of ...
A Brief Explanation Of Miller Urey Experiment
Miller and Urey Experiment. Stanley L. Muller and Harold C. Urey performed an experiment to describe the origin of life on earth. They were of the idea that the early earth's atmosphere was able to produce amino acids from inorganic matter. The two biologists made use of methane, water, hydrogen, and ammonia which they considered were found ...
Miller-urey Experiment
A classic experiment in molecular biology and genetics, the Miller-Urey experiment, established that the conditions that existed in Earth ' s primitive atmosphere were able to produce amino acids, the subunits of proteins (complex carbon-containing molecules required by all living organisms). The Miller-Urey experiment fundamentally ...
Conducting Miller-Urey Experiments
In 1952, Stanley Miller, then a graduate student at the University of Chicago, approached Harold Urey about doing an experiment to evaluate the possibility that organic compounds important for the origin of life may have been formed abiologically on the early Earth. The experiment was conducted using a custom-built glass apparatus ( Figure 1A ...
Miller-Urey Experiment
Why are the miller-urey experiments essential to the theory of evolution? That experiment is essential because it explained how some complex molecules were formed. If you are having trouble with this experiment, here is a very short, but very good youtube video about this theme: This video is private. Deevona · 7 · Jun 28 2014.
Oparin-Haldane theory
In 1953 American chemists Harold C. Urey and Stanley Miller tested the Oparin-Haldane theory and successfully produced organic molecules from some of the inorganic components thought to have been present on prebiotic Earth. This became known as the Miller-Urey experiment.Modern abiogenesis hypotheses are based largely on the same principles as the Oparin-Haldane theory and the Miller-Urey ...
Primordial Soup's On: Scientists Repeat Evolution's Most Famous Experiment
Bada was revisiting the famous experiment first done by his mentor, chemist Stanley Miller, at the University of Chicago in 1953. Miller, along with his colleague Harold Urey, used a sparking ...
Miller-Urey Experiment
The Miller-Urey experiment initially created at least five amino acids and some carbohydrates, which are both complex organic compounds. These compounds emerged out of less complex and inorganic ...
Icon 1
The Miller-Urey experiment is not relevant to the RNA world, because RNA was constructed from organic "building blocks" irrespective of how those compounds came into existence (Zubay and Mui, 2001). The evolution of RNA is a wholly different chapter in the story of the origin of life, one to which the validity of the Miller-Urey experiment is ...
Teach Astronomy
The Miller-Urey experiment. The original apparatus used by Miller and Urey was quite simple compared to today's standards. It essentially consisted of two glass flasks connected by glass tubing. One flask served as the boiling flask, where gases and other molecules could accumulate in a water phase. The other flask (located above the boiling ...
The origin of life on Earth, explained
The origin of life on Earth stands as one of the great mysteries of science. To find out if we are alone in the galaxy, we will need to better understand what geochemical conditions nurtured the first life forms. Several seminal experiments in this topic have been conducted at the University of Chicago, including the Miller-Urey experiment that suggested how the building blocks of life could ...
The Miller-Urey Experiment
The Miller-Urey experiment was a simulation of conditions on the early Earth testing the idea that life, or more specifically organic molecules, could have formed by nothing more than simple chemical reactions. Miller's success validated the theoretical ideas of A.I. Oparin and is considered to be the classic experiment investigating the concept of abiogenesis.
What Was The Miller-Urey Experiment? • Stated Clearly
The Miller-Urey experiment was the first attempt to scientifically explore ideas about the origin of life. Stanley Miller simulated conditions thought be common on the ancient Earth. The purpose was to test the idea that the complex molecules of life (in this case, amino acids) could have arisen on our young planet through simple, natural ...
Scientists recreated classic origin-of-life experiment and made a new
Miller's experiment produced five amino acids, although he was less certain about the results for two of them. Enlarge / Diagram of the 1952 Miller-Urey experiment. iStock / Getty Images Plus
Message in a bottle: revisiting the origin of life
A new version of the famous 1952 Miller-Urey experiment suggests that the glass of the flask may have been a key ingredient for its chemical reactions. A new version of the famous 1952 Miller-Urey ...
Prebiotic Soup--Revisiting the Miller Experiment
The origin of Miller's experiment can be traced to 1950, when Nobel laureate Harold C. Urey, who had studied the origin of the solar system and the chemical events associated with this process, began to consider the emergence of life in the context of his proposal of a highly reducing terrestrial atmosphere. ... Please describe the competing ...
What the famous Miller-Urey experiment got wrong
The Miller-Urey experiment is a daring example of testing a complex hypothesis. It is also a lesson in drawing more than the most cautious and limited conclusions from it.
Scientists finish a 53-year-old classic experiment on the origins of life
Miller conducted his original 1953 experiment as a graduate student, working with his mentor Harold Urey. It was one of the first to tackle the seemingly insurmountable question of how life began ...
Origins and experimental evidence
The 1953 Miller-Urey experiment, for example, simulated early Earth's atmosphere with nothing more than water, hydrogen, ammonia, and methane and an electrical charge standing in for lightning, and produced complex organic compounds like amino acids. Now, scientists have learned more about the environmental and atmospheric conditions on early ...
Redo of a Famous Experiment on the Origins of Life Reveals Critical
The Miller-Urey experiment showed that the conditions of early Earth could be simulated in a glass flask. New research finds the flask itself played an underappreciated, though outsize, role.

BIOLOGY Related Links

Miller-Urey Experiment

Miller-Urey Experiment And The Primordial Soup Theory

The Miller-Urey Experiment In Support Of Abiogenesis

Miller-Urey Experiment Apparatus and Procedure

Miller-Urey Experiment Animation

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Miller Urey Experiment: Hypothesis, Steps, Conclusions, and Limitations

Oparin-Haldane Hypothesis

Steps of the Miller Urey Experiment

Conclusions of the Miller Urey Experiment

Limitations of the Miller Urey Experiment

Ongoing Debates and Significance

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Miller Urey Experiment

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Conducting Miller-Urey Experiments

James H. Cleaves

Aaron S. Burton

Daniel P. Glavin

Jason P. Dworkin

Facundo M. Fernández

Introduction

1. Setting Up a Manifold/Vacuum System

2. Preparation of Reaction Flask

3. Introduction of Gaseous NH 3

4. Introduction of CH 4

5. Introduction of Further Gases ( e.g. N 2 )

6. Beginning the Experiment

7. End of Experiment

8. Collecting Liquid Sample

9. Cleaning the Apparatus

10. Sample Analysis

Representative Results

Disclosures

Acknowledgments

... and beyond

Key Questions

Oparin-Haldane theory

The experiment itself

Wells boils off

What the textbooks say

Wells's evaluation

Why we should still teach Miller-Urey

How textbooks could improve their presentations of the origin of life

Table of Contents

Chapter 18: Life On Earth

Chapter 2 Early Astronomy

Chapter 3 The Copernican Revolution

Chapter 4 Matter and Energy in the Universe

Chapter 5 The Earth-Moon System

Chapter 6 The Terrestrial Planets

Chapter 7 The Giant Planets and Their Moons

Chapter 8 Interplanetary Bodies

Chapter 9 Planet Formation and Exoplanets

Chapter 10 Detecting Radiation from Space

Chapter 11 Our Sun: The Nearest Star

Chapter 12 Properties of Stars

Chapter 13 Star Birth and Death

Chapter 14 The Milky Way

Chapter 15 Galaxies

Chapter 16 The Expanding Universe

Chapter 17 Cosmology

Chapter 18 Life On Earth

Miller-Urey Experiment

Chapter 19 Life in the Universe

The origin of life on Earth, explained

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