“The idea that the organic compounds that serve as the basis of life were formed when the earth had an atmosphere of methane, ammonia, water and hydrogen instead of carbon dioxide, nitrogen, oxygen and water was suggested by [Alexander] Oparin and has been given emphasis by [Harold] Urey and [John Desmond] Bernal. In order to test this hypothesis…”
These were the opening words of Stanley Miller’s 800-odd word paper that brought significance to the idea that life as we know it now could have stemmed from the primordial soup. The primordial soup is a term coined by Oparin, referring to a solution in the primitive oceans of earth from which life is thought to have originated.
The experiment that Miller performed with Urey, his advisor at the University of Chicago, demonstrated that amino acids needed for life could be made in a flask by a simple experiment. Though the Miller-Urey experiment has assumed tremendous significance over time, it was rejected by one of the reviewers back then, who did not believe in the results. This meant that though the paper was submitted by mid-December 1952, it was published only on May 15, 1953.
What was the experiment? Very little is known definitively about how our earth might have been four billion years ago. To test the idea stated in the opening paragraph, Miller used a setup similar to the one shown in the figure.
The apparatus was to mimic conditions of the primitive atmosphere of the earth. While one of the flasks was filled with methane (natural gas), hydrogen and ammonia (believed to be part of the soup) the other provided for a miniature pond of water, model of an early ocean. Lightning was simulated using flashes of voltage. Spurred on by curiosity, he left the pot to cook overnight, firmly believing that the odds were stacked against him. So when the water in the flask turned pink at the end of the first day, and deep red and turbid by the end of the week, Miller was pleasantly surprised. He was more devoted to the how and why of the experiment, rather than the possible ramifications.
What were the findings? On analysing the water that was taken out, he identified a number of amino acids (glycine being the most abundant among them) that make proteins in living cells, and hand-labelled them on the chromatograph. Most of life’s cellular machinery — including DNA that acts our genetic code — are strings of amino acids.
After Miller’s death in 2007, scientists examined sealed vials from the original experiment using the latest technologies and were able to discern more amino acids than originally reported, and more amino acids than naturally found. The Miller-Urey experiment has become a classic, and is often repeated in school laboratories.
The decades since, however, have enabled a better understanding of early earth, including a key finding. The simplest hydrogen might have been in short supply back then, without which triggering the right organic chemistry is impossible. Evaporation and a colliding comet (large seeding event) have been suggested as possible workarounds to get the necessary chemical composition for self-assembly.
All this implies that we are still a long distance away from confirming the initial hypothesis, which however, still remains powerful and possible. Irrespective of how things develop, this experiment surely did forever change the face of a field - studying the origin of life.
