Early Life Theories: Primordial Soup

Miller–Urey experiment
Miller–Urey experiment. (Carny/Wikimedia Commons/CC BY 2.5)

The early atmosphere of the Earth was a reducing atmosphere, meaning there was little to no oxygen. The gases that mostly made up the atmosphere were thought to include methane, hydrogen, water vapor, and ammonia. The mixture of these gases included many important elements, like carbon and nitrogen, that could be rearranged to make amino acids. Since amino acids are the building blocks of proteins, scientists believe that combining these very primitive ingredients could have possibly lead to organic molecules coming together on Earth.

Those would be the precursors to life. Many scientists worked to prove this was the case.


The "primordial soup" idea came about when Russian scientist Alexandr Oparin and English geneticist John Haldane each came up with the idea independently. It had been theorized that life started in the oceans. Oparin and Haldane thought that with the mix of gases in the atmosphere and the energy from lightning strikes, amino acids could spontaneously form in the oceans. This idea is now known as "primordial soup."


In 1953, American scientists Stanley Miller and Harold Urey decided to test this theory. They combined the reducing atmosphere gases in the amounts that were hypothesized the early Earth's atmosphere was thought to have and simulated an ocean in a closed apparatus. With constant lightning shocks simulated using electric sparks, they were able to create organic compounds, including amino acids.

In fact, almost 15% of the carbon in the modeled atmosphere had turned into various organic building blocks in only a week. This ground breaking experiment seemed to have proven that life on Earth could have spontaneously formed from non-organic ingredients.


However, as years passed, there is now doubt about some parts of the experiment.

First, the Miller-Urey experiment required constant lightning strikes. While lightning was very common on early Earth, it wasn't constant. This means that although making amino acids and organic molecules was possible, it most likely did not happen as quickly or in the large amounts the experiment showed. This still did not disprove the hypothesis right away. Just because the process would have taken longer than the lab simulation does not negate the fact building blocks could have been made. It may not have happened in a week, but the Earth was around for more than a billion years before known life was formed. That was certainly within the time limit for the creation of life.

Another possible issue with the Miller-Urey Primordial Soup experiment is that scientists are finding evidence that the atmosphere was not quite the reducing atmosphere that was used in the experiment. In fact, there seems to have been much less methane than previously thought. Since the methane was the source of carbon in the simulated atmosphere, that would reduce the numbers of organic molecules even further. 

Even though these roadblocks have been discovered and Primordial Soup may not have been exactly the way early Earth was, it was still a very significant experiment.

It proved that the organic molecules that are the building blocks of life can be made from inorganic materials. This is very important in figuring out how life did begin on Earth.

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Scoville, Heather. "Early Life Theories: Primordial Soup." ThoughtCo, Jul. 16, 2017, thoughtco.com/early-life-theory-of-primordial-soup-1224531. Scoville, Heather. (2017, July 16). Early Life Theories: Primordial Soup. Retrieved from https://www.thoughtco.com/early-life-theory-of-primordial-soup-1224531 Scoville, Heather. "Early Life Theories: Primordial Soup." ThoughtCo. https://www.thoughtco.com/early-life-theory-of-primordial-soup-1224531 (accessed January 17, 2018).