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124 ORIGIN OF LIFE life depends A detailed experimental and theoretical study of the origin of life was now possible The Earliest Life Sedimentary rocks of about 3.5 Ga old are found in the Pilbara region of Western Australia and in the Barberton Mountain Land in South Africa; these are the oldest such rocks that are sufficiently well preserved to contain fossils These rocks contain structures that are interpreted as fossil stromatolites (layered structures built by colonies of micro-organisms) as well as filamentous microstructures interpreted to be microfossils of bacteria (see Biosediments and Biofilms) The biogenicity of many of these early fossils is controversial, but, if accepted, their presence indicates that life was well established on Earth at 3.5 Ga It is even more difficult to say much about the nature of this early life Some of the Pilbara microfossils have been suggested to resemble present-day cyanobacteria, but recent reinterpretation of the stratigraphy of the region suggests that the rocks originated in a hydrothermal environment Evidence for life has also been claimed in rocks of about 3.8 Ga from Greenland These rocks are too metamorphosed to preserve physical fossils, but they contain carbon depleted in 13C, which is consistent with the occurrence of biochemical reactions These claims are also controversial The Tree of Life The early history of life can also be studied through the techniques of molecular biology It has been found that all life on Earth shares the same fundamental chemical processes All life uses DNA as its basic genetic material The DNA message is read by copying it onto RNA (transcription), and this RNA message is used to synthesize proteins (translation) by means of an RNA–protein complex called the ribosome (Figure 1) The resulting proteins are most frequently used as catalysts (enzymes) to drive the many chemical processes in the cell The genetic code – which translates three-‘letter’ sequences of the four DNA bases (A, C, G, and T) into the corresponding amino acid to be added to a protein – is almost universal (with a few minor variations) The common chemical basis for life strongly supports the idea that all life on Earth is related and descended from a single universal ancestor, sometimes known as LUCA (last universal common ancestor) Moreover, by comparing the sequences of genes involved in these fundamental processes common to all life, we can construct a ‘family tree’ showing the relatedness of all groups of life Such molecular phylogenetic analysis has led to the ‘three domain’ taxonomy, in which life is divided into the domains Bacteria, Archaea, and Eucarya (Figure 2) Figure Summary of the key molecular processes common to all life Genetic information is carried by DNA and can be copied (replication) onto new DNA molecules The genetic message is read by first copying onto RNA (transcription) and then using the RNA message to synthesize a protein (translation) Three letter sequences of the RNA bases (uracil (U), cytosine (C), adenine (A), and guanine (G)) are translated into a corresponding amino acid to include in a protein according to the genetic code shown on the right
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