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Miller–Urey experiment
1953· Event· Microbes & cells
Well establishedStrong fossil, genetic or geological evidence.
A 1953 experiment in which electric sparks through gases and water produced amino acids, the building blocks of proteins.
See it on the Life map →What happened
At the University of Chicago, graduate student Stanley Miller and his adviser Harold Urey sealed water, methane, ammonia and hydrogen, then thought to resemble early Earth's air, in glass flasks. Boiling water supplied vapor, and electric sparks mimicked lightning. Within a week the water turned brown with organic compounds, including amino acids such as glycine and alanine. A 2008 reanalysis of Miller's saved samples found more than 20 amino acids in one version of the experiment.
Why it matters
It made the origin of life an experimental science, showing that key ingredients of life form easily from simple chemicals plus energy.
The evidence (fossils, genes, rocks)
Miller's 1953 paper in Science, countless repeats, and amino acids later found in meteorites such as Murchison (1969), showing similar chemistry happens naturally in space.
Still debated
Early Earth's air was probably mostly carbon dioxide and nitrogen, which yields fewer organics; and the leap from amino acids to self-copying life remains unsolved.
Connections (2)
▸Miller–Urey experiment tested Primordial soup (Oparin–Haldane hypothesis)Well established
The connection
Oparin and Haldane's soup idea had never been tested. Harold Urey argued that early Earth's air was rich in hydrogen, methane and ammonia, and Stanley Miller built an apparatus to see whether lightning in such air would make organic molecules. Amino acids appeared within days, giving the soup hypothesis its first experimental support.
When and how
University of Chicago, 1952; published in Science in May 1953 by Stanley Miller, then a graduate student, working with Harold Urey.
The evidence
Miller's paper framed the experiment as a test of Oparin's ideas, and the result has been reproduced many times, including from his own saved samples in 2008.
Debates and open questions
Whether early Earth's air was ever that hydrogen-rich, and whether a dilute ocean-wide soup could concentrate chemicals enough to assemble anything lifelike.
▸Miller–Urey experiment simulated early chemistry for Origin of lifeWell established
The connection
By showing that protein building blocks form readily from simple gases and energy, the experiment made life's origin a lab problem. It launched prebiotic chemistry: in 1961 Joan Oró made adenine, one of DNA's letters, from hydrogen cyanide, and later work made sugars and RNA pieces under plausible early-Earth conditions.
When and how
Chicago, 1953 onward; followed by labs worldwide, from Oró in Houston in 1961 to John Sutherland's group in England, which made RNA building blocks in 2009.
The evidence
Prebiotic experiments repeatedly produce amino acids, sugars and pieces of nucleotides, and meteorites carry many of the same molecules, so this chemistry clearly happens naturally.
Debates and open questions
No experiment has yet produced a self-copying system from scratch, and researchers disagree whether life started in ponds, at deep-sea vents or somewhere else.