Primordial soup is a theoretical mixture of organic compounds which may have given rise to life on Earth. The primordial soup theory is used to explain how living organisms appeared on Earth, and it appears to be the most plausible scientific explanation arrived at thus far.
When the Earth first formed, it did not contain any organic material. Yet, here we are, billions of years later, demonstrating that at some point, inorganic material turned into organic material. This process is known as "abiogenesis," and it would theoretically be extremely difficult.
Conditions on Earth in its early years were very different than they are now. The atmosphere contained no oxygen, and it was rich in things like hydrogen, ammonia, methane, and water. According to the primordial soup theory, these substances were primed to produce amino acids, which would have been able to combine to create organic material which could have eventually given rise to life. In order for this to occur, there needed to be a catalyst; amino acids did not emerge spontaneously.
Research suggests that lightning or heat may have triggered the formation of amino acids from the inorganic compounds in the Earth's environment, generating primordial soup and setting the stage for the development of living organisms. In 1953, the publication of the famous Miller-Urey Experiment, in which researchers replicated the conditions and managed to produce amino acids, generated a great deal of support for this theory. The researchers found that when they sealed the components of the Earth's early atmosphere in glass flasks, heated the materials, and generated electric shocks, amino acids did indeed emerge.
Life on Earth didn't leap from primordial soup to complex organisms. The process would have been very gradual, as the acids increased in complexity and joined with each other in new ways. As living organisms emerged, they also had a direct impact on the environment and subsequently their own evolution, especially when organisms started photosynthesizing and producing oxygen as a byproduct. The production of oxygen changed the atmosphere so radically that the conditions which first created life on Earth would never be able to be replicated in the modern natural environment.
Support for the concept of a primordial soup which served as an incubator for life comes from several experiments, along with extensive research on geologic samples which can provide clues to what the Earth's atmosphere was like in the planet's early years. As researchers have discovered, amino acids are precursors to nucleic acids, which act as the building blocks of life. Nucleic acids are constantly changing and adapting, and periodically they recombine to form something entirely new.