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NCERT gives Stanley Miller's experiment four lines and a diagram. The original paper is only two pages, and reading it clears up several things that students often get wrong, including what the flask contained and what the experiment proved. This post goes through the paper, the people behind it, a calculation from its numbers and what has changed since.
Who was Stanley Miller?
His National Academy of Sciences memoir, by Jeffrey Bada and Antonio Lazcano, says he was born on 7 March 1930 in Oakland, California, and died on 20 May 2007. He studied chemistry at the University of California, Berkeley, and arrived at the University of Chicago in September 1951 as a graduate student with a teaching assistantship.
In his first term he heard the Nobel laureate Harold Urey argue that the early Earth's atmosphere was strongly reducing, rich in methane, ammonia and hydrogen, and that organic compounds might have formed in it. Miller first worked with Edward Teller on how elements form in stars. When Teller left Chicago, Miller went back to Urey's idea. Urey was not keen: graduate students, he felt, should do experiments likely to work. He agreed only on the condition that there must be signs of success within a year.
What was in the flask?
Miller's paper, A production of amino acids under possible primitive Earth conditions, published in Science on 15 May 1953, describes the apparatus. Water boiled in a small flask, mixed with the gases in a 5-litre flask, circulated past electrodes, condensed and flowed back. A small corona discharge from an induction coil supplied the energy.
The procedure was to add 200 ml of water, remove the air, then add 10 cm pressure of hydrogen, 20 cm of methane and 20 cm of ammonia. The water was boiled and the discharge ran continuously for a week. The water turned pink after the first day and deep red by the end. To make sure no living organisms caused the result, Miller added mercuric chloride and pointed out that boiling during the run would also prevent their growth.
Worked example: the gas mixture
The paper gives pressures in centimetres of mercury. Turn them into a composition.
Total pressure: 10 + 20 + 20 = 50 cm Hg. Since 1 atm = 76 cm Hg, that is 50 / 76 = 0.66 atm.
Mole fractions: for ideal gases, mole fraction equals partial pressure divided by total pressure. Hydrogen 10/50 = 0.2, methane 20/50 = 0.4 and ammonia 20/50 = 0.4.
Check: 0.2 + 0.4 + 0.4 = 1.0.
So the starting gas was two-fifths methane, two-fifths ammonia and one-fifth hydrogen, plus water vapour from the boiling flask, which the pressures do not include.
Common mistakes:
Adding water vapour to the 50 cm. The pressures were set before boiling. Water vapour is extra.
Thinking the experiment made life. It made small molecules. Miller found amino acids, not cells.
Saying it proved what the early atmosphere was. It tested what one assumed atmosphere could produce.
What came out?
Using paper chromatography, Miller identified glycine, alpha-alanine and beta-alanine. Aspartic acid and alpha-amino-n-butyric acid were less certain, and two spots were unidentified. He estimated the total yield of amino acids "in the milligram range". He wrote that the aim was to imitate a primitive atmosphere, "and not to obtain the optimum conditions".
The memoir adds what happened next. After more careful work, nine different amino acids were positively identified, along with hydroxy acids such as lactic acid. Miller showed that hydrogen cyanide and aldehydes form during the run, which supported a known reaction, the Strecker synthesis, as the route to amino acids. When his group repeated the experiment in the 1970s with an automatic amino acid analyser, they found 33 amino acids.
What has changed since 1953?
Two things. First, the atmosphere. Cleaves and colleagues (2008), writing with Miller, state that "it is now generally held that the early Earth's atmosphere was likely not reducing, but was dominated by N2 and CO2". They reported that such neutral mixtures still give significant amounts of amino acids when oxidation during analysis is prevented.
Second, the old samples. Vials from Miller's 1950s experiments were found among his materials. Johnson and colleagues (2008) reanalysed extracts from a second "volcanic" apparatus that sprayed steam into the spark, and found it produced a wider variety of amino acids than the classic set-up.
The NCERT Class 12 Evolution chapter sums up chemical evolution as "more or less accepted", while saying there is no idea how the first self-replicating capsule of life arose. Our evolution guide for NEET covers what comes after this step.
What can a student take from his way of working?
Pick a bold question, then make it testable. Miller turned an idea from a lecture into a flask you could seal and run.
Rule out the obvious objection. He killed any possible microbes so living things could not explain the result.
Say exactly what you tried to do. His paper states it imitated an atmosphere rather than optimising yield.
Keep your samples. Labelled vials let others reanalyse his work fifty years later.
Getting the chemistry straight
If the Strecker route or mole fractions feel shaky, fix that one idea before moving on. The Fermi Concept Library has every concept in chemistry and biology with its own lesson, so you can study one gap and return to the chapter.
Try it now: redo the mole-fraction calculation for a mixture of 15 cm hydrogen, 15 cm methane and 20 cm ammonia.
