The Miller–Urey experiment, also called the Miller experiment, was a scientific study conducted in 1952 to recreate the conditions believed to exist in Earth’s early atmosphere before life began. This experiment demonstrated how simple, non-living materials could form complex organic molecules, such as amino acids, which are essential for life. The experiment used methane (CH₄), ammonia (NH₃), hydrogen (H₂), and water (H₂O) in a ratio of 2:2:1. An electric arc, which mimicked lightning, was applied to these gases, leading to the formation of amino acids.
This experiment is considered a major scientific breakthrough and a classic example of research into how life might have originated from non-living matter (a process called abiogenesis). It was conducted by Stanley Miller, who was guided by Nobel Prize winner Harold Urey at the University of Chicago. The results were published in 1953. At the time, the experiment supported the ideas of scientists Alexander Oparin and J. B. S. Haldane, who proposed that Earth’s early atmosphere could have encouraged chemical reactions that created complex organic molecules from simpler ones.
After Stanley Miller passed away in 2007, scientists examined sealed containers from the original experiment and found that more amino acids were produced than Miller had initially reported using a technique called paper chromatography. Although later research suggests Earth’s early atmosphere may have had a different composition than the one used in the Miller–Urey experiment, scientists continue to find that simple organic compounds, including amino acids, can form under various conditions. Additionally, studies show that short-lived, hydrogen-rich atmospheres—similar to those in the Miller–Urey experiment—may have existed on early Earth after large asteroids struck 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 matter. However, experiments in the 1800s, especially Louis Pasteur’s swan neck flask experiment in 1859, showed that the theory was incorrect. In the same year, Charles Darwin published On the Origin of Species, explaining how living things change over time through evolution. Although Darwin never wrote about the first life form in his book, he mentioned in a letter to a scientist that he thought life might have started in a different way.
By the early 1800s, scientists had discovered that organic molecules could be made from inorganic materials. For example, Friedrich Wöhler created urea from ammonium cyanate in 1828. Later, Alexander Butlerov made sugars from formaldehyde, and Adolph Strecker created the amino acid alanine from acetaldehyde, ammonia, and hydrogen cyanide. In 1913, Walther Löb made amino acids by using electric sparks on formamide, showing that life’s building blocks could form from simple molecules. However, these experiments were not meant to explain how life began on Earth.
In the early 1900s, scientists proposed theories 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 have formed in the early ocean and helped start chemical reactions. 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, which had no oxygen, might have helped create organic molecules through sunlight or lightning, leading to life. These ideas formed the basis for studying life’s origin, but no experiments had direct proof by the mid-1900s.
In 1952, Harold Urey, a professor of chemistry at the University of Chicago, proposed that Earth’s early atmosphere had methane, water, ammonia, and hydrogen, creating conditions needed for life to begin. Stanley Miller, a student at the university, wanted to test this idea. After hearing Urey’s lecture, Miller asked to conduct an experiment. Urey allowed him to try for a year. In 1953, Miller sent a report of his experiment to Science magazine. Urey did not want to be listed as a co-author because he believed it would reduce recognition for Miller’s work. Despite this, the experiment is now most commonly called the Miller-Urey experiment. After a delay, Science published Miller’s findings in May 1953.
Experiment
In the original 1952 experiment, methane (CH₄), ammonia (NH₃), and hydrogen (H₂) were placed in a 2:2:1 ratio (1 part H₂) inside a clean, 5-liter glass flask connected to a 500-milliliter flask that was half-filled with water (H₂O). The gas mixture represented Earth's early atmosphere before life existed, and the water in the smaller flask acted as a model for the ocean. Water from the smaller flask was boiled, creating steam that entered the gas chamber and mixed with the gases. A continuous electrical spark was created between two metal rods 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 of continuous operation, the liquid became deep red and cloudy, which Miller explained was caused by organic material sticking to tiny particles of silica. The boiling flask was then removed, and mercuric chloride (a toxic chemical) was added to stop bacteria from growing. The reaction was stopped by adding barium hydroxide and sulfuric acid, and 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 markings were faint.
Materials and samples from the original experiment were still being kept in 2017 by Jeffrey Bada, a professor at the University of California, San Diego, who also studies how life began. As of 2013, the equipment used in the experiment was displayed 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, like formaldehyde, formed as early steps 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 under ultraviolet (UV) light. Aldehydes can also form when methane and water (H₂O) break apart, creating other chemicals like methanol. Energy sources such as lightning, UV light, and cosmic rays from space can cause these reactions in planetary atmospheres.
One example of chemical reactions in the Miller–Urey experiment shows how formaldehyde can form:
A chemical pathway to HCN involves ammonia (NH₃) and methane (CH₄):
Other chemicals, such as acetylene and cyanoacetylene, were found in water solutions from similar experiments. However, the quick formation of HCN and aldehydes, followed by a steady level of these chemicals and the later production of amino acids, showed that a process called Strecker synthesis was happening in the water solution.
Strecker synthesis is a reaction where an aldehyde, ammonia, and HCN combine to form simple amino acids through an intermediate step called aminoacetonitrile.
Additionally, water and formaldehyde can react in a process called Butlerov’s reaction to create sugars like ribose.
The experiments proved that simple organic compounds, such as those needed to build proteins and other large molecules, can form without living organisms when gases are exposed to energy.
Related experiments and follow-up work
There were other experiments similar to the Miller–Urey experiment happening around the same time. An article in The New York Times (March 8, 1953) titled "Looking Back Two Billion Years" described work by 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 written report 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 and water but found no major chemical changes. Some scientists believe these earlier experiments explain why Urey pushed to publish Miller’s paper quickly and threatened to send it to another journal.
The Miller–Urey experiment created a method to study how life might have begun before Earth’s earliest life forms. In 1961, Joan Oró made small amounts of adenine, a building block of DNA, by mixing hydrogen cyanide and ammonia in water. Oró also found amino acids formed under the same conditions. Later experiments showed other DNA and RNA building blocks could form using similar methods with a reducing atmosphere. Researchers also used UV light in experiments, as early Earth had strong UV radiation. For example, UV light breaking down water vapor and carbon monoxide produced alcohols, aldehydes, and organic acids. In the 1970s, Carl Sagan used Miller–Urey-style reactions to create complex organic particles called "tholins," which may resemble particles in Titan’s atmosphere.
Since the 1950s, scientists have studied how Miller–Urey chemistry works in different environments. In 1983, Miller and another researcher tested different gas mixtures, including hydrogen, water, nitrogen, carbon dioxide, and methane. They found that ammonia levels did not greatly affect amino acid production because ammonia was created during the experiment. Methane was important for high amino acid yields, likely because it helps form hydrogen cyanide. Lower yields occurred with more oxidized carbon compounds, but similar results were achieved with high hydrogen and carbon dioxide ratios. This shows Miller–Urey reactions can work in different atmospheres depending on gas mixtures. In 2002, Jeffrey Bada and H. James Cleaves, who studied with Miller, suggested that nitrites in carbon dioxide and nitrogen-rich atmospheres might reduce amino acid production. In experiments with a less-reducing atmosphere (carbon dioxide, nitrogen, and water), adding calcium carbonate and ascorbic acid increased amino acid yields, showing amino acids could still form in neutral conditions with the right chemical balance. They argued that early Earth’s seawater might have acted as a buffer, and 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 box. Using modern tools like liquid chromatography and mass spectrometry, Bada’s team found higher amino acid yields and more types of amino acids in experiments where Miller used a "volcanic" setup with a steam jet. Bada suggested that steam might have split water into hydrogen and hydroxyl radicals, increasing hydroxylated amino acids during a chemical process called Strecker synthesis. In another set of experiments, Miller added hydrogen sulfide to the atmosphere, and Bada’s analysis showed much higher yields, including amino acids with sulfur.
A 2021 study emphasized the role of high-energy free electrons in the experiment. These electrons create ions and radicals, a part of the process that needs more research. A 2021 paper also compared experiments using glass and Teflon equipment. It found that glassware might act as a mineral catalyst, suggesting silicate rocks could have helped chemical reactions in early Earth’s environment.
Early Earth's prebiotic atmosphere
There are not enough scientific observations to know the exact composition 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), not methane (CH₄) and ammonia (NH₃) as used in the original Miller–Urey experiment. This is partly explained by the gases released during volcanic activity. Geologist William Rubey studied gases from modern volcanoes and found they are rich in CO₂, water (H₂O), and likely N₂, with smaller amounts of hydrogen (H₂), sulfur dioxide (SO₂), and hydrogen sulfide (H₂S). If the chemical balance of Earth's mantle—determining the gases released—has remained the same since Earth formed, then the early atmosphere was likely weakly reducing. However, some scientists argue that the atmosphere may have been more reducing for the first few hundred million years.
Although the prebiotic atmosphere may have had a different chemical balance than the one used in the Miller–Urey experiment, modified versions of the experiment showed that amino acids can still form without life under specific conditions. Referring back to Urey's original idea of a "post-impact" reducing atmosphere, a recent study found that an iron-rich object with a mass of about 4×10 – 5×10 kg could temporarily make the early atmosphere more reducing, creating conditions similar to those in the Miller–Urey experiment. Scientists estimate that between four and seven such objects may have hit Earth during the Hadean period.
A major factor influencing the chemical balance of Earth's early atmosphere is how quickly hydrogen (H₂) escaped into space after Earth formed. Atmospheric escape occurs when gas molecules gain enough energy to escape Earth's gravity. Scientists generally agree that hydrogen made up less than 1% of the prebiotic atmosphere, but a 2005 model predicted much slower escape rates, suggesting hydrogen could have been up to 30% of the atmosphere. A hydrogen-rich atmosphere would have major effects on early chemical processes, but later studies found issues with the model's accuracy. During atmospheric escape, lighter molecules like hydrogen can pull heavier molecules with them. Recent research on xenon escape suggests hydrogen levels may have been at least 1% or higher during the Archean period.
Overall, the idea that Earth's early atmosphere was weakly reducing, with brief periods of stronger reducing conditions after large impacts, is widely supported.
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 to power chemical reactions. The Murchison meteorite, which fell near Murchison, Victoria, Australia in 1969, was found to have amino acids that closely match those produced in the Miller–Urey experiments. Analysis of the meteorite’s organic material using a special type of mass spectrometry identified more than 10,000 different compounds, even though they were present in very small amounts (measured in parts per billion to parts per million). This discovery supports the idea that chemical processes like those in the Miller–Urey experiments 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, created through processes similar to the Miller–Urey setup. These compounds may darken the surfaces of these objects. Some scientists suggest that comets hitting early Earth might have delivered large amounts of complex organic molecules, along with water and other volatile substances. However, the very low levels of materials important for life, combined with uncertainty about whether organic matter survives 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 began. Studies of proteins from the last universal common ancestor (LUCA), which is the shared ancestor of all living species today, found that these proteins had more simple amino acids. These simple amino acids were available in Earth’s early environment, as shown by the Miller–Urey experiment. This suggests that the genetic code, which all life uses, originally included 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 scientific view of how life began.
Another common criticism is that the Miller–Urey experiment produced a mix of left-handed and right-handed amino acids, but life today uses only left-handed ones. While it is true that the experiment made this mix, the question of why life uses only one type of amino acid is a separate area of research. Recent studies show that magnetic minerals, such as magnetite, can help create a preference for one type of amino acid through a process called the chiral-induced spin selectivity (CISS) effect. Once this preference starts, 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 in Earth’s environment do so.
The Miller–Urey experiment and similar studies focus on creating simple building blocks like amino acids. The next step in prebiotic chemistry is to combine these building blocks into more complex structures, such as peptides. This process, called polymerization, requires reactions that remove water, which are difficult to happen naturally in water. Scientists like John Desmond Bernal suggested that clay surfaces might help by concentrating these building blocks. Ideas like the layers of certain minerals, such as green rust, and wet-dry cycles have been proposed as ways to help form peptides. Some methods, like the interaction between amino acid precursors at the air-water boundary or a new process involving sulfide, can form peptides even in water. Research into how life’s building blocks combined into larger molecules is an ongoing area of study in prebiotic chemistry.
Amino acids identified
The table below shows the amino acids found in the "classic" 1952 experiment, as studied by Miller in 1952 and later by Bada and others using modern tools like mass spectrometry. It also includes results from a 2008 review of samples from the volcanic spark discharge experiment and a 2010 review of samples from the H₂S-rich spark discharge experiment. Not all amino acids used to build proteins have been created in these experiments. However, it is widely believed that early life forms relied on a simpler group of amino acids that were naturally available before life began.