It is impossible to know for certain how life on Earth began. Possibly, living organisms arrived from an extraterrestrial source, such as Mars, traveling on a meteor. Alternatively, the conditions on the early Earth might have allowed the formation of the large molecules unique to living things. There is some evidence for both the extraterrestrial-origin hypothesis, collected from meteorites that have landed on Earth, as well as the chemical evolution hypothesis.
In the 1950s, Stanley Miller and Harold Urey performed an experiment that was profoundly important in giving weight to speculations about the chemical origin of life on Earth and elsewhere in the universe. They set up an apparatus to create conditions that mimicked those thought to exist on the primitive Earth. They filled a chamber with gases such as methane, ammonia, hydrogen, and water vapor. Then they generated electrical sparks to simulate lightning.
Decades of experimental work and critical evaluation followed Miller and Urey's experiments. The later experiments showed that, under the conditions used by Miller and Urey, many small molecular building blocks of life could be formed:
However, the 5-carbon sugar ribose was not produced in these experiments.
The results of the Miller–Urey experiments have undergone several refinements. The amino acids in living things are always L-isomers. But a mixture of D- and L-isomers appeared in the amino acids formed in the Miller–Urey experiments. Recent experiments show that natural processes could have selected the L-amino acids from the mixture. Some minerals, especially calcite-based rocks, have unique crystal structures that selectively bind to D- or L-amino acids, separating the two. Such rocks were abundant on early Earth. This suggests that while both kinds of amino acid structures were made, binding to certain rocks may have eliminated the D-amino acids.
Ideas about Earth's original atmosphere have changed since Miller and Urey did their experiments. There is abundant evidence indicating that major volcanic eruptions occurred 4 billion years ago, which would have released carbon dioxide (CO2), nitrogen (N2), hydrogen sulfide (H2S), and sulfur dioxide (SO2) into the atmosphere. Experiments using these gases in addition to the ones in the original experiment have produced more diverse molecules, including: