The Miller–Urey experiment, also called the Miller experiment, was a chemical synthesis experiment conducted in 1952. It recreated conditions believed to exist in the atmosphere of early Earth before life began. This experiment was one of the first to successfully show how organic compounds could form from inorganic materials in a scenario where life originated. The experiment used methane (CH₄), ammonia (NH₃), hydrogen (H₂), and water (H₂O) in a ratio of 2:2:1. When an electric arc (simulating lightning) was applied, amino acids were produced.
This experiment is considered groundbreaking and is often viewed as the classic study of how life might have begun (abiogenesis). Stanley Miller performed the experiment in 1952 under the guidance of Nobel laureate Harold Urey at the University of Chicago. The results were published the following year. At the time, the experiment supported the ideas of Alexander Oparin and J. B. S. Haldane, who suggested that early Earth’s conditions encouraged chemical reactions that created complex organic compounds from simpler inorganic materials.
After Stanley Miller passed away in 2007, scientists examined sealed vials from the original experiment and found that more amino acids were produced than Miller had reported using paper chromatography. While evidence suggests that Earth’s early atmosphere may have had a different composition than the gases used in the Miller experiment, experiments today still produce racemic mixtures of simple-to-complex organic compounds, including amino acids, under various conditions. Researchers have also shown that short-lived, hydrogen-rich atmospheres—similar to those in the Miller–Urey experiment—could have occurred on early Earth after large asteroid impacts.
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 material. However, experiments in the 1800s, especially Louis Pasteur’s swan neck flask experiment in 1859, showed that life does not arise 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 life form, he once wrote in a letter that he thought life might have started in a way not yet understood.
By the early 1800s, scientists had learned that organic molecules could form from non-living materials. For example, Friedrich Wöhler created urea from ammonium cyanate in 1828. Later, Alexander Butlerov made sugars from formaldehyde, and Adolph Strecker produced the amino acid alanine from acetaldehyde, ammonia, and hydrogen cyanide. 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 chemicals, 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 in a theory called panspermia. In the 1920s, Leonard Troland thought a special enzyme might have formed in Earth’s early oceans, and Hermann J. Muller proposed that a molecule with special properties could have started evolution. Around the same time, Alexander Oparin and J. B. S. Haldane suggested that Earth’s early atmosphere, rich in chemicals like methane and ammonia, might have helped create organic molecules in the ocean. These ideas helped scientists think about how life might have begun, but no experiments had proven them by the mid-1900s.
In 1952, Harold Urey, a famous chemist who won the Nobel Prize in 1934 and worked on the Manhattan Project, proposed that Earth’s early atmosphere had methane, water, ammonia, and hydrogen. These conditions, he thought, could support the "primordial soup" idea. Stanley Miller, a student at the University of Chicago, became interested in testing this theory after hearing Urey speak. Though Urey was hesitant at first, he let Miller try an experiment. Miller created a setup that mimicked Earth’s early atmosphere and used electric sparks to simulate lightning. In 1953, he published his results in Science, with Urey not listed as a co-author because he wanted to give Miller full credit. The experiment showed that amino acids, which are essential for life, could form under these conditions. This discovery helped scientists better understand how life might have started.
Experiment
In the 1952 experiment, methane (CH₄), ammonia (NH₃), and hydrogen (H₂) were mixed in a 2:2:1 ratio (1 part H₂) inside a clean, 5-liter glass flask connected to a smaller 500-milliliter flask filled halfway with water (H₂O). The gas mixture represented Earth's early atmosphere before life existed, while the water in the smaller flask symbolized an ocean. The water was boiled to create steam, which entered the gas chamber and mixed with the gases. A continuous electrical spark was sent between two electrodes in the larger flask, passing through the gas and steam to mimic lightning. A condenser below the gas chamber collected liquid droplets, which gathered in a U-shaped trap at the bottom of the setup for testing.
After one day, the liquid in the trap turned pink. After one week, the liquid became deep red and cloudy. Miller believed the color change was caused by organic materials sticking to tiny particles of silica. To stop germs from growing, mercuric chloride (a poison) was added to the boiling flask. The reaction was stopped by adding barium hydroxide and sulfuric acid, and then the mixture was heated to remove impurities. 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 harder to confirm because their spots were faint.
In 2017, materials and samples from the original experiment were still kept by Jeffrey Bada, a professor at the University of California, San Diego, and a researcher at the Scripps Institution of Oceanography. He was a former student of Miller. 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 changes that happened in his experiment. Hydrogen cyanide (HCN) and aldehydes (such as formaldehyde) were shown to form early in the experiment because of electric sparks. 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 due to ultraviolet (UV) light. Similarly, aldehydes can form when methane and water (H₂O) break apart, creating reactive molecules that lead to aldehydes. Energy sources like lightning, UV light, and cosmic rays can cause these reactions in planetary atmospheres.
For example, here is a set of chemical reactions that can happen in the Miller–Urey atmosphere, leading to formaldehyde:
A path to HCN from ammonia (NH₃) and methane (CH₄) involves several steps:
Other chemicals, such as acetylene and cyanoacetylene, have been found in water solutions from experiments like Miller’s. 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 production of amino acids as HCN and aldehydes decreased, showed that a process called Strecker synthesis was happening in the water solution.
Strecker synthesis is a process where an aldehyde, ammonia, and HCN combine to form a simple amino acid through an intermediate called aminoacetonitrile.
Additionally, water and formaldehyde can react through a process called Butlerov’s reaction to create sugars like ribose.
The experiments showed that simple organic compounds, which are the building blocks of proteins and other large molecules, can form without life when gases are exposed to energy.
Related experiments and follow-up work
In the 1950s, several scientists conducted experiments similar to the Miller–Urey experiment. Before Miller’s famous study was published in May 1953, an article in The New York Times (March 8, 1953) described work by Wollman M. MacNevin at Ohio State University. MacNevin used 100,000V sparks to pass through methane and water vapor, creating "resinous solids" that were too complex to analyze. Earlier, in December 1952, K. A. Wilde submitted a paper to Science about using 600V sparks on a mixture of carbon dioxide and water. Wilde’s experiment did not produce significant chemical changes. Some scientists believe these earlier studies may have influenced Urey’s urgency to publish Miller’s results quickly.
The Miller–Urey experiment helped scientists study how life might have begun. In 1961, Joan Oró created small amounts of adenine, a key chemical in DNA, using hydrogen cyanide and ammonia in water. Oró also found that amino acids formed under similar conditions. Later experiments showed that other DNA and RNA building blocks could form in simulated early Earth environments with reducing gases. Scientists also used UV light, which was stronger on early Earth, to break down water vapor and carbon monoxide, creating alcohols, aldehydes, and acids. In the 1970s, Carl Sagan used Miller–Urey methods to make complex organic particles called "tholins," which may resemble materials in Titan’s atmosphere.
Since the 1950s, scientists have tested how Miller–Urey chemistry works in different environments. In 1983, Miller and a colleague repeated the experiment with various gases, including hydrogen, water, nitrogen, carbon dioxide, and methane. They found that ammonia levels did not greatly affect amino acid production because ammonia formed from nitrogen during the experiment. Methane was important for high amino acid yields, likely because it helped create hydrogen cyanide. Lower yields occurred with more oxidized carbon, but high hydrogen-to-carbon dioxide ratios could produce similar results. In 2000s studies, Jeffrey Bada and H. James Cleaves found that adding calcium carbonate and ascorbic acid to a less-reducing atmosphere (with carbon dioxide, nitrogen, and water) increased amino acid production, suggesting that early Earth’s seawater and iron could help form amino acids.
In 1999, after Miller had a stroke, his lab was donated to Bada. Bada found unanalyzed samples from Miller’s 1950s experiments. Using modern tools, Bada’s team found higher amino acid yields and more types of amino acids in experiments where steam was injected into the reaction chamber. Bada suggested that steam might have split water into radicals, helping create more hydroxylated amino acids. In other experiments, adding hydrogen sulfide to the atmosphere led to much higher amino acid yields, including some with sulfur.
A 2021 study highlighted the role of high-energy electrons in the Miller–Urey experiment, which create ions and radicals. Another 2021 study found that glass containers used in the experiments acted as mineral catalysts, suggesting that silicate rocks may have helped chemical reactions occur on early Earth.
Early Earth's prebiotic atmosphere
Scientists do not have enough data to determine the exact chemical makeup of Earth's early atmosphere before life began. However, recent models suggest that the early atmosphere was likely "weakly reducing," meaning it 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 chemical composition of gases released during volcanic activity. Geologist William Rubey studied gases from modern volcanoes and found they contain large amounts of CO₂, water (H₂O), and nitrogen (N₂), along with smaller amounts of hydrogen (H₂), sulfur dioxide (SO₂), and hydrogen sulfide (H₂S). If the chemical makeup of Earth's mantle—responsible for the gases released during volcanic activity—has remained the same since Earth formed, then the early atmosphere was likely weakly reducing. Some studies, however, suggest that the early atmosphere may have been more reducing for the first few hundred million years.
Even though the early atmosphere may have had a different chemical balance than the one used in the Miller–Urey experiment, modified versions of the experiment show that amino acids can still form without methane and ammonia under certain conditions. Scientists have also studied the idea that a large, iron-rich object hitting Earth could temporarily change the atmosphere. A recent study found that an object with a mass of at least 4×10⁻⁵ to 5×10⁻⁵ kg could have created a short-lived, highly reducing atmosphere similar to the one in the Miller–Urey experiment. Research on Earth's mantle and lunar craters suggests that Earth may have experienced four to seven such impacts during its earliest history.
The amount of hydrogen (H₂) escaping into space after Earth formed greatly influenced the early atmosphere's chemical balance. On young planets, gases can escape into space if they move fast enough to overcome gravity. It is generally believed that hydrogen made up less than 1% of the early atmosphere, but a 2005 study predicted much slower escape rates, allowing hydrogen to remain at about 30% of the atmosphere. A hydrogen-rich atmosphere would have affected the Miller–Urey experiment, but later research found possible flaws in the 2005 model. During hydrogen escape, lighter molecules like hydrogen can carry heavier gases with them through collisions. Studies on xenon escape suggest that hydrogen levels may have reached at least 1% or higher during the Archean period.
Overall, evidence supports the idea that Earth's early atmosphere was weakly reducing, with brief periods of highly reducing conditions caused by large impacts.
Extraterrestrial sources of amino acids
Conditions similar to those in the Miller–Urey experiments exist 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, contains amino acids that closely match those produced in Miller–Urey experiments. Analysis of the meteorite’s organic material using specialized equipment found over 10,000 unique compounds, though these were present in very low levels (parts per billion to parts per million). This discovery suggests that Miller–Urey-like chemical processes may have occurred beyond Earth.
Comets and other icy bodies in the outer Solar System are believed to hold large amounts of complex carbon compounds, such as tholins, formed through processes similar to Miller–Urey setups. These compounds can darken the surfaces of these objects. Some scientists think comets striking early Earth may have delivered complex organic molecules, water, and other substances. However, the extremely low amounts of biologically important materials and uncertainty about whether organic matter survives impacts make it difficult to confirm this idea.
Relevance to the origin of life
The Miller–Urey experiment showed that the basic parts of life could form naturally from gases without the help of living things. It also created a new way to study how life began. Studies of proteins from the last universal common ancestor (LUCA), which is the shared ancestor of all living things today, found that simple amino acids were more common. These amino acids were available in the environment before life began, as shown by the Miller–Urey experiment. This suggests that the genetic code used by all life today evolved from a smaller set of amino acids. Even though some people argue that the Miller–Urey experiment did not create all 22 amino acids used in life today, this does not contradict the scientific view of how life began.
Another common criticism is that the Miller–Urey experiment produced a mix of two types of amino acids, called L and D enantiomers. However, life on Earth uses almost only L-amino acids. While the experiment did create this mix, the reason life uses only one type of amino acid is a separate topic studied by scientists.
Recent research shows that certain magnetic minerals, like magnetite, can help create a preference for one type of amino acid over another. This happens through a process called chiral-induced spin selectivity. Once this preference is created, it can spread throughout living systems. This means the Miller–Urey experiment does not need to produce only one type of amino acid if other natural processes helped create this preference.
The Miller–Urey experiment and similar studies focus on creating simple building blocks of life, such as amino acids. The next step is combining these building blocks to form more complex structures, like peptides. This process requires reactions that remove water, which is hard to do in water-based environments. Scientists, like John Desmond Bernal, suggested that clay surfaces might help with this process by concentrating the building blocks. Ideas like the layers of certain minerals, such as green rust, and other methods, like forming peptides at the surface where water meets air, have been proposed. Scientists are still studying how these building blocks could have joined together to form the complex molecules needed for life.
Amino acids identified
The table below lists amino acids found in the "classic" 1952 experiment, as studied by Miller in 1952 and later by Bada and other scientists using modern tools like mass spectrometry. It also includes results from a 2008 re-examination of samples from the volcanic spark discharge experiment and a 2010 re-examination of samples from the H₂S-rich spark discharge experiment. Not all amino acids used by living organisms have been created in these experiments. However, scientists generally agree that early life likely relied on a simpler group of amino acids that were naturally available in the environment before life began.