Origin of life

Fossil traces of living cells have been found going back more than 3.8 billion years—to perhaps as little as 700 million years after the formation of the Earth. There were presumably simpler forms of life that preceded the advent of recognizable cells, and even if life arose more than once it is unlikely that evidence of this would remain. (One sees many branches in the fossil record—such as organisms with dominant symmetries other than fivefold—but all seem to have the same ancestry.)

From antiquity until the 1700s it was widely believed that smaller living organisms arise spontaneously in substances like mud, and this was not finally disproved until the 1860s. Controversy surrounding the theory of biological evolution in the late 1800s dissuaded investigation of non-biological origins for life, and at the end of the 1800s it was for example suggested that life on Earth might have arisen from spores of extraterrestrial origin. In the 1920s the idea developed that electrical storms in the atmosphere of the early Earth could lead to production of molecules seen in living systems—and this was confirmed by the experiment of Stanley Miller and Harold Urey in 1953. The molecules obtained were nevertheless still fairly simple—and as it turns out most of them have now also been found in interstellar space. Starting in the 1960s suggestions were made for the chemical and other roles of constituents of the crust as well as atmosphere. Schemes for early forms of self-replication were invented based on molecules such as RNA and on patterns in clay-like materials. (The smallest known system that independently replicates itself is a mycoplasma bacterium with about 580,000 base pairs and perhaps 470 genes. Viroids can be as small as 10,000 atoms but require a host for replication.) In the 1970s it then became popular to investigate complicated cycles of chemical reactions that seemed analogous to ones found in living systems. But with the advent of widespread computer simulations in the 1980s it became clear that all sorts of features normally associated with life were actually rather easy to obtain. (See note above.)