It was once believed that if you left food out to rot, living creatures like maggots and even rats would simply poof into existence. The idea was called Spontaneous Generation.
A series of experiments starting in the 1600s disproved this idea, and in the 1800s a new scientific law was proposed: Life only comes from life.
It’s true that rats, maggots, and even microbes are far too complex to simply poof into existence, but in 1859 English Naturalist Charles Darwin put forth the theory of evolution. In it he showed that under the right circumstances, relatively simple creatures can gradually give rise to more complex creatures. Given this information, serious thinkers began to wonder: Is it possible that simple life forms actually could come from non-living matter? Not by poofing into existence, but through a natural gradual process similar to what we see in biological evolution?
Darwin himself mentioned this idea when writing to friend, “But if (and oh what a big if)” he wrote, ‘we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, and so on present, that a protein compound was chemically formed ready to undergo still more complex changes...”
In 1924, Russian biochemist Alexander Oparin published a book which he titled The Origin of Life. In it he outlined his thoughts on a gradual progression from simple chemistry to living cells. He imagined the early ocean as a primordial soup - a rich collection of complex molecules produced by natural chemical reactions. In this soup, further reactions could take place, eventually producing living cells.
At the time, Darwin’s warm little pond, and Oparin's primordial soup were really just speculation. They were founded on a good understanding of chemistry and biology but they could not be considered legitimate scientific hypotheses because no one had found a way to test or observe them. Science, after all, is the study of observable facts and an ongoing conversation about how those facts can be best linked together.
Chemical reactions like those proposed by Darwin and Oparin, are not expected to leave an observable fossil record. Without either having fossils to examine or a time machine to travel back and observe what happened, how could scientists even begin to study the origin of life?
In the 1950s, Stanley Miller, then a graduate student at the University of Chicago, came up with an idea. We could simulate early Earth conditions in the lab, and then carefully watch what happens. If you can’t study fish in the sea, set up an aquarium.
Working with his professor, Harold Urey, Miller designed an apparatus to simulate the ancient water cycle.
Together they put in water to model the ancient ocean. It was gently boiled to mimic evaporation. Along with water vapor, for gasses of the atmosphere they chose methane, hydrogen, and ammonia. These are simple gases which scientists at the time thought were probably abundant on the ancient Earth. They added a condenser to cool the atmosphere, allowing water molecules to form drops and fall back into the ocean like rain.
The ancient Earth would have had many sources of energy: sunlight, geothermal heat, and even thunderstorms, so they added sparks to the atmosphere to simulate lightning.
The goal of the experiment was not to create life but to simply test the first step in Oparin’s model: Can simple chemicals naturally give rise to the complex molecules of life?
After running the experiment for just one week, their “ocean” became brownish black. Careful analysis revealed that through a series of reactions, many complex molecules had been produced. Among these were amino acids - special molecules of life that we once thought could only be built inside the bodies of living creatures.
This was a pivotal breakthrough in science! So significant in fact, that It gave rise to an entirely new field of research now known as Prebiotic Chemistry.
Scientists don't know for sure if the gasses used by Miller really were the most common gasses of the ancient Earth. Because of this many experiments have since been done, showing that the molecules of life can form in a wide variety of environments with different starting chemicals and different sources of energy.
Sugars, lipids and amino acids have even been found on meteorites, this suggests that the molecules of life formed all throughout the ancient solar system, and may be forming right now in other regions of our galaxy!
Together, these discoveries tell us that Oparin’s primordial soup, and Darwin’s warm little pond could have easily existed, in one form or another, on our ancient planet.
So to sum things up, what was the Miller-Urey experiment?
The Miller-Urey experiment was our first attempt at simulating ancient Earth conditions, in this case, the ancient Earth’s water cycle, for the purpose of testing ideas about the origin of life.
The Miller-Urey experiment is significant for two main reasons: First, though it was not a perfect simulation of the early Earth, it clearly demonstrated, for the first time, that biomolecules can form under ancient Earth-like conditions.
Second, the experiment took what was once mere speculation, (the idea that life may have emerged from chemistry) and transformed a portion of that speculation into legitimate, testable science!
Many questions remain to be answered about the origin of life, but scientists from many nations, and many fields of study, are now following Stanley Miller's lead – they’re finding ways to turn those questions about the origins of life into testable scientific hypotheses.
Simulation experiments cannot tell us exactly how life formed in the past, but if enough of them are done, they could eventually tell us if it’s possible for life to emerge from chemistry.