The way in which living organisms have developed from inorganic matter. It is generally believed that the earth is about 4600 million years old and that the earliest evidence of life dates from 3000-3500 million years ago. Fossils of this age are interpreted as bacteria-like organisms. It is thought that in the early stages the earth's atmosphere was chemically reducing, composed of such gases as hydrogen, nitrogen, methane, ammonia, and water vapour. Such an atmosphere would be more favourable for the for mation of complex organic molecule than the present oxidizing atmospher (largely nitrogen and oxygen). Labora tory experiments in which mixtures ο such gases are subjected to electri sparks or ultraviolet radiation show tha simple organic molecules (e.g. amino acids and nitrogenous bases) can be formed. Most theories on the origin of life involve starting with a 'primaeval soup' containing such compounds. Possibly long-chain molecules (e.g. proteins and carbohydrates) were formed by catalytic reactions in which simple molecules adsorbed on regular mineral surfaces (e.g. mica clays). The way living organisms evolved from such molecules is still in question. Possibly it first involved the formation of primitive cells (*protobionts
) leading to simple heterotrophic prokaryotic organisms. From these came photosynthetic organisms resembling the blue-green algae, which gradually formed an oxidizing atmosphere enabling aerobic organisms to develop. Aerobic eukaryotic organisms then appeared, possibly arising as symbiotic associations of prokaryotes (see serial endosymbiotic theory
), and from these came multicellular organisms.