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.
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 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.
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.
