The Miller–Urey experiment, also called the Miller experiment, was a scientific test done in 1952 to study how organic compounds might have formed on early Earth before life existed. It showed that simple chemicals could combine to create more complex molecules, like amino acids, under conditions thought to be present in Earth’s early atmosphere. The experiment used methane (CH₄), ammonia (NH₃), hydrogen (H₂), and water (H₂O) in a ratio of 2:2:1. An electric arc, which mimics lightning, was used to create reactions that produced amino acids.
This experiment is considered a major scientific discovery and a key study in understanding how life might have begun (a process called abiogenesis). It was conducted by Stanley Miller, who worked under Harold Urey, a Nobel Prize winner, at the University of Chicago. The results were published in 1953. At the time, the experiment supported ideas by scientists Alexander Oparin and J. B. S. Haldane, who believed Earth’s early conditions could help form complex organic molecules from simpler ones.
After Stanley Miller passed away in 2007, scientists studied preserved samples from the original experiment and found that more amino acids were created than Miller had reported using paper chromatography. Although Earth’s early atmosphere may have had a different composition than the one used in the experiment, similar tests still produce mixtures of simple-to-complex organic compounds, including amino acids, under various conditions. Researchers also found that short-lived, hydrogen-rich atmospheres—similar to those in the Miller–Urey experiment—might have formed on early Earth after large asteroids hit the planet.
History
Until the 19th century, many people believed in the theory of spontaneous generation, which suggested that simple animals like insects or rodents could appear from decaying materials. However, experiments in the 1800s, especially Louis Pasteur’s swan neck flask experiment in 1859, showed that life does not come from decaying matter. In the same year, Charles Darwin published On the Origin of Species, explaining how living things change over time through evolution. Although Darwin did not write about the first living organism in his book, in a letter to Joseph Dalton Hooker, he imagined that life might have started in a warm pond with chemicals, light, and energy.
At this time, scientists already knew that organic molecules, such as proteins, could form from inorganic materials. Friedrich Wöhler discovered this in 1828 when he made urea from ammonium cyanate. Later, scientists like Alexander Butlerov and Adolph Strecker created sugars and amino acids using simple chemicals. In 1913, Walther Löb made amino acids by using electric sparks on formamide. These experiments showed that life’s building blocks could form from simpler materials, but they were not meant to explain how life began on Earth.
In the early 1900s, scientists proposed ideas about life’s origin. In 1903, physicist Svante Arrhenius suggested that life might have come to Earth from space, a theory called panspermia. In the 1920s, Leonard Troland thought a special enzyme could form in the ocean and help create life. Hermann J. Muller proposed that a gene with special abilities might have started evolution. Around the same time, Alexander Oparin and J. B. S. Haldane suggested that Earth’s early atmosphere, rich in chemicals and energy from lightning, could have helped form organic molecules in the ocean, leading to life. These ideas formed the basis for studying life’s origin, but by the mid-1900s, scientists had not yet tested them directly.
In 1952, Harold Urey, a famous chemist who won the Nobel Prize in 1934, proposed that Earth’s early atmosphere had methane, water, ammonia, and hydrogen. This environment, he thought, would support the "primordial soup" idea, where organic molecules could form. Stanley Miller, a student at the University of Chicago, became interested in testing this idea after hearing Urey speak. Although Urey was initially hesitant, he let Miller try an experiment. In 1953, Miller reported his results in a scientific journal. Urey did not want to be listed as a co-author because he believed Miller deserved full credit for the work. Despite this, the experiment is often called the Miller-Urey experiment. After a delay in publishing, Urey insisted the paper be published quickly. The results were finally shared in Science in May 1953.
Experiment
In the original 1952 experiment, methane (CH₄), ammonia (NH₃), and hydrogen (H₂) were mixed in a 2:2:1 ratio (1 part H₂) inside a sterile 5-liter glass flask connected to a 500-milliliter flask containing water (H₂O). The gas mixture represented Earth's early atmosphere, and the water simulated an ocean. Water from the smaller flask was boiled, creating vapor that entered the gas chamber and mixed with the gases. A continuous electrical spark was created between two electrodes in the larger flask, passing through the gas and water vapor to mimic lightning. A condenser below the gas chamber collected liquid into a U-shaped trap at the bottom, which was later tested.
After one day, the liquid in the trap turned pink. After one week, the solution became deep red and cloudy, which Miller linked to organic material absorbed onto colloidal silica. The boiling flask was removed, and mercuric chloride (a poison) was added to stop bacteria from growing. The reaction was stopped by adding barium hydroxide and sulfuric acid, and impurities were removed by evaporation. Using paper chromatography, Miller found five amino acids in the solution: glycine, α-alanine, and β-alanine were clearly identified, while aspartic acid and α-aminobutyric acid (AABA) were less certain due to faint spots on the test.
Materials and samples from the original experiments were still preserved in 2017 by Jeffrey Bada, a professor at the University of California, San Diego, and a researcher at the Scripps Institution of Oceanography. As of 2013, the equipment used in the experiment was on display at the Denver Museum of Nature and Science.
Chemistry of experiment
In 1957, Miller shared his research about the chemical reactions that happened in his experiment. Hydrogen cyanide (HCN) and aldehydes (such as formaldehyde) were shown to form early in the experiment because of the electric discharge. This matches what scientists now know about how chemicals behave in Earth’s atmosphere. For example, HCN can form when methane (CH₄) and nitrogen break apart in the atmosphere under ultraviolet (UV) light. Similarly, aldehydes can form when methane and water (H₂O) break apart, creating reactive molecules that lead to other substances like methanol. Energy sources such as lightning, UV light, and galactic cosmic rays can cause these reactions in planetary atmospheres, leading to the formation of HCN and aldehydes.
One example of a chemical process in the Miller–Urey atmosphere that creates formaldehyde is a series of reactions involving sunlight and other molecules. A path to forming HCN involves ammonia (NH₃) and methane (CH₄) reacting under specific conditions.
Other substances, like acetylene and cyanoacetylene, were found in the liquid part of Miller–Urey experiments. However, the quick formation of HCN and aldehydes, followed by the production of amino acids when HCN and aldehyde levels stayed steady, and the slower creation of amino acids as HCN and aldehydes decreased, showed that a process called Strecker synthesis was happening in the liquid.
Strecker synthesis is a reaction where an aldehyde, ammonia, and HCN combine to form a simple amino acid through a step involving a compound called aminoacetonitrile.
Additionally, water and formaldehyde can react in a process called Butlerov’s reaction to create sugars such as ribose.
The experiments proved that simple organic molecules, which are the basic parts of proteins and other large molecules, can form without life when gases are exposed to energy.
Related experiments and follow-up work
There were a few experiments similar to the Miller–Urey experiment happening at the same time. An article in The New York Times (March 8, 1953) titled "Looking Back Two Billion Years" described the work of Wollman M. MacNevin at Ohio State University before the Miller–Urey paper was published in May 1953. MacNevin used 100,000V sparks to pass through methane and water vapor, creating "resinous solids" that were too complex to analyze. Also, K. A. Wilde submitted a paper to Science on December 15, 1952, before Miller submitted his paper in February 1953. Wilde’s experiment used up to 600V on a mixture of carbon dioxide (CO₂) and water, but no significant chemical changes were reported. Some believe these earlier experiments explain why Urey pushed to publish Miller’s paper quickly and threatened to submit it to another journal.
The Miller–Urey experiment helped scientists study how life might have started. In 1961, Joan Oró created small amounts of adenine, a nucleobase, from a concentrated solution of hydrogen cyanide (HCN) and ammonia (NH₃) in water. Oró also found that amino acids formed under the same conditions. Later experiments showed that other nucleobases in RNA and DNA could be made using simulated prebiotic chemistry with a reducing atmosphere. Researchers also began using UV light in experiments, as early Earth had much stronger UV radiation. For example, UV light breaking down water vapor with carbon monoxide produced alcohols, aldehydes, and organic acids. In the 1970s, Carl Sagan used Miller–Urey-style reactions to make complex organic particles called "tholins," which may resemble particles in Titan’s atmosphere.
Since the 1950s, scientists have tested how Miller–Urey chemistry works in different environments. In 1983, Miller and another researcher repeated experiments with different atmospheric gases, including hydrogen (H₂), water (H₂O), nitrogen (N₂), carbon dioxide (CO₂), and methane (CH₄), sometimes with ammonia (NH₃). They found that ammonia levels did not greatly affect amino acid production because ammonia was created from nitrogen during the experiment. Methane was important for high amino acid yields, likely because it helps form HCN. Lower yields occurred with more oxidized carbon, but similar results were seen with high hydrogen-to-carbon dioxide ratios. This shows Miller–Urey reactions can work in atmospheres with different gas mixtures. Later, Jeffrey Bada and H. James Cleaves, who studied under Miller, suggested that nitrites in CO₂- and N₂-rich atmospheres might reduce amino acid production. In experiments with a less-reducing atmosphere (CO₂ + N₂ + H₂O), adding calcium carbonate and ascorbic acid increased amino acid yields, showing that amino acids can form in neutral atmospheres under certain conditions. They argued that early seawater might have been buffered, and ferrous iron could have prevented oxidation.
In 1999, after Miller had a stroke, he gave his lab to Bada. Bada found unanalyzed samples from Miller’s 1950s experiments in an old cardboard box. Using modern tools like high-performance liquid chromatography and mass spectrometry, Bada’s team analyzed samples from an experiment where Miller used a "volcanic" apparatus with a steam jet. They found higher amino acid yields and more types of amino acids. Bada suggested that steam injection might have split water into hydrogen and hydroxyl radicals, leading to more hydroxylated amino acids during Strecker synthesis. In another experiment, Miller added hydrogen sulfide (H₂S) to the atmosphere, and Bada found much higher yields, including amino acids with sulfur.
A 2021 study highlighted the role of high-energy free electrons in the experiment. These electrons create ions and radicals, an aspect that needs further study. A 2021 paper also compared experiments using borosilicate glassware and Teflon equipment. It suggested that glassware acts as a mineral catalyst, implying that silicate rocks may have played a key role in prebiotic Miller–Urey reactions.
Early Earth's prebiotic atmosphere
While scientists have not yet gathered enough chemical data to determine the exact makeup of Earth's early atmosphere, recent models suggest it was likely "weakly reducing." This means the atmosphere probably contained mostly carbon dioxide (CO₂) and nitrogen (N₂), rather than methane (CH₄) and ammonia (NH₃), which were used in the original Miller–Urey experiment. This idea is supported by the composition of gases released from modern volcanoes. Geologist William Rubey studied these gases and found they are rich in CO₂, water (H₂O), and nitrogen (N₂), with smaller amounts of hydrogen (H₂), sulfur dioxide (SO₂), and hydrogen sulfide (H₂S). If the chemical balance of Earth's mantle—determined by volcanic gas release—has remained the same since Earth formed, then the early atmosphere was likely weakly reducing. However, some studies suggest the atmosphere may have been more reducing for the first few hundred million years.
Although the early atmosphere may have differed from the one used in the Miller–Urey experiment, experiments modified to reflect less-reducing conditions have shown that amino acids can still form without life. Additionally, research based on Urey's idea of a "post-impact" reducing atmosphere suggests that an iron-rich object with a mass between 4×10⁻⁵ and 5×10⁻⁵ kg could have temporarily made the early atmosphere rich in hydrogen (H₂), methane (CH₄), and ammonia (NH₃), similar to the Miller–Urey setup. Studies of Earth's mantle and lunar craters estimate that four to seven such objects may have struck Earth during the Hadean eon.
A major factor influencing the chemical balance of Earth's early atmosphere is the rate at which hydrogen (H₂) escaped into space after Earth formed. Atmospheric escape occurs when gas particles gain enough energy to escape Earth's gravity. Scientists generally agree that hydrogen made up less than 1% of the early atmosphere, but a 2005 computer model predicted slower escape rates, suggesting hydrogen could have been as much as 30% of the atmosphere. A hydrogen-rich atmosphere would have significant effects on the Miller–Urey synthesis process, but later studies found possible flaws in the model's assumptions. However, during hydrogen escape, lighter molecules like hydrogen can carry heavier molecules with them through collisions. Recent models of xenon escape suggest hydrogen levels may have been at least 1% or higher during the Archean eon.
Overall, the idea that Earth's early atmosphere was weakly reducing, with brief periods of highly reducing conditions after major impacts, is widely supported by scientific evidence.
Extraterrestrial sources of amino acids
Conditions similar to those in the Miller–Urey experiments are found in other parts of the Solar System, often using ultraviolet light instead of lightning as the energy source for chemical reactions. The Murchison meteorite, which fell near Murchison, Victoria, Australia in 1969, was discovered to contain amino acids that closely match those produced in Miller–Urey experiments. Using advanced mass spectrometry, scientists found over 10,000 different compounds in the meteorite’s organic material, although these compounds were present in very small amounts (measured in parts per billion to parts per million). This discovery supports the idea that chemical processes similar to the Miller–Urey synthesis may have occurred beyond Earth.
Comets and other icy objects in the outer Solar System are believed to contain large amounts of complex carbon compounds, such as tholins, formed through processes similar to those in the Miller–Urey experiments. These compounds may darken the surfaces of these bodies. Some scientists suggest that comets striking early Earth might have delivered large amounts of complex organic molecules along with water and other volatile substances. However, the very low levels of biologically important materials and uncertainty about whether organic compounds survive impacts make it difficult to confirm this theory.
Relevance to the origin of life
The Miller–Urey experiment showed that the basic parts of life could be created without living things using gases from Earth’s early atmosphere. This experiment helped scientists develop a new way to study how life might have started. Studies of proteins from the last universal common ancestor (LUCA), which is the shared ancestor of all living things today, suggest that simpler amino acids were more common in Earth’s early environment, as predicted by Miller–Urey chemistry. This implies that the genetic code, which all life uses, originally relied on fewer types of amino acids than are used today. While some people argue that the Miller–Urey experiment did not create all 22 amino acids used in life today, this does not contradict the idea that life evolved from simpler chemical processes.
A common criticism is that the Miller–Urey experiment produced a mix of left- and right-handed amino acids, but life today uses only left-handed versions. Although this is true, the reason life uses only left-handed amino acids is a separate topic in origin-of-life research.
Recent studies show that certain minerals, like magnetite, can help create a preference for one type of amino acid over the other. This process, called the chiral-induced spin selectivity (CISS) effect, allows for the formation of left-handed molecules. Once this preference begins, it can spread through biological systems. This means that the Miller–Urey experiment does not need to produce only left-handed amino acids, as other natural processes might have created this preference.
The Miller–Urey experiment and similar studies focus on creating simple building blocks like amino acids. However, forming more complex structures, such as peptides, requires combining these building blocks through chemical reactions that are difficult in water. Scientists have long thought that minerals like clay might help with this process by concentrating the building blocks. Recent research suggests that minerals such as green rust, found in wet and dry cycles, could help form peptides. Other methods, like reactions at the air-water boundary or a new process involving sulfide, may also work in water. Creating complex molecules from life’s building blocks is an ongoing area of study in prebiotic chemistry.
Amino acids identified
The table below lists amino acids found and recorded in the "classic" 1952 experiment. These results were studied by Miller in 1952 and later re-examined by Bada and other scientists using modern tools like mass spectrometry. In 2008, scientists analyzed vials from the volcanic spark discharge experiment, and in 2010, they studied vials from the H₂S-rich spark discharge experiment. Although not all amino acids used to build proteins were created in these experiments, scientists believe early life likely used a smaller group of amino acids that were naturally available before life began.