Early Life Theories: Primordial Soup

A 1950s experiment may show how life formed on Earth

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 led to organic molecules coming together on Earth. Those would be the precursors to life. Many scientists have worked to prove this theory.

Primordial Soup

The "primordial soup" idea came about when Russian scientist Alexander 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."

The Miller-Urey Experiment

In 1953, American scientists Stanley Miller and Harold Urey tested the theory. They combined the atmospheric gases in the amounts that early Earth's atmosphere was thought to contain. They then 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 percent of the carbon in the modeled atmosphere turned into various organic building blocks in only a week. This groundbreaking experiment seemed to prove that life on Earth could have spontaneously formed from nonorganic ingredients.

Scientific Skepticism

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 that the experiment showed. This does not, in itself, disprove the hypothesis. Just because the process would have taken longer than the lab simulation suggests 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 timeframe for the creation of life.

A more serious possible issue with the Miller-Urey primordial soup experiment is that scientists are now finding evidence that the atmosphere of early Earth was not exactly the same as Miller and Urey simulated in their experiment. There was likely much less methane in the atmosphere during Earth's early years than previously thought. Since methane was the source of carbon in the simulated atmosphere, that would reduce the number of organic molecules even further.

Significant Step

Even though primordial soup in ancient Earth may not have been exactly the same as in the Miller-Urey experiment, their effort was still very significant. Their primordial soup experiment proved that organic molecules—the building blocks of life—can be made from inorganic materials. This is an important step in figuring out how life began on Earth.