Following Charles Darwin's development of evolutionary theory, some scientists proposed that all life on Earth had originated spontaneously. The critics of evolution and the proponents of creationism claimed that this was outright impossible and that there was no way that living, or organic, chemicals could be constructed naturally from synthetic chemicals.
In 1953, graduate student Stanley Miller and his professor, Harold Urey, sought to prove that organic chemicals could be spontaneously formed in nature. They reproduced conditions present on the primitive Earth using a mixture of water, methane, ammonia, and hydrogen. All of these chemicals were placed in a sterile collection of glass tubes, connected in a loop so they could mix.
Inside of the loop, they also placed electrodes, which could be used to apply electricity and simulate the effect of lightning. After heating the water to the temperature of the ocean, electricity was discharged between the electrodes and then the water was cooled again, allowing the water to condense and mix with the other chemicals. This process of heating, discharging electricity, and cooling was repeated continuously over a period of one week.
At the end of the week, they analyzed the contents of the chemical system and observed that approximately 15% of the carbon atoms in the system had become part of organic compounds. Over two percent of the carbon atoms had become part of amino acids, with a total of 13 of the 22 amino acids found in living cells.
While the experiment did not create all of the components of a living cell, it did create many of the building blocks. It proved that nature could have potentially created the components needed to construct living cells from a collection of inorganic chemicals. The experiment has since been repeated and resulted in an even more diverse collection of organic molecules, further strengthening the theory that life was derived spontaneously from inorganic chemicals.
New evidence has also suggested that space might have played a role in the development of life in Earth. A meteorite that struck Australia on September 28, 1969 was found to contain 90 varieties of amino acids, nineteen of which are found in life on Earth.