The origins and evolution of prokaryotes and eukaryotes : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Molecular BioSciences at Massey University
For most biologists, the big picture regarding the origin and evolution of prokaryotes and eukaryotes is not at issue, and recent evidence only serves to back up the intuitively obvious: complex eukaryotes evolved from simpler prokaryotic ancestors. In the standard account, prokaryotes predated eukaryotes by at least 800 million years, as evidenced by cyanobacterial microfossils dating back 3.5 billion years [e.g. Schopf & Packer 1987, Walsh 1992]. (The finding of molecular markers of eukaryote metabolism by Brocks et al.  has pushed back the emergence of the earliest eukaryotes from 2.1 billion years to 2.7 billion years.) Establishing the root of the tree of life has shown that prokaryotes in fact consist of two domains, the archaea and bacteria, that the Last Universal Common Ancestor (LUCA) of all extant life lived at extremely high temperatures and that the eukaryotes emerged from the archaea [Woese & Fox 1977, Woese 1987, Woese et al. 1990]. Prior to the emergence of cyanobacteria, life arose from prebiotic conditions on the early earth, and at some stage, possessed an RNA-rich metabolism. This period, dubbed the RNA world [Gilbert 1986, Benner et al. 1989], predated both the emergence of genetically-encoded proteins and of DNA as genetic storage molecule. The standard picture is therefore that, after the period of heavy bombardment that is suggested to have vapourised the oceans on Earth perhaps as recently as 3.8 billion years ago [reviewed in Nisbet & Sleep 2001], life emerged, went through an RNA world period, a thermophilic prokaryote LUCA, and developed into cyanobacteria in an astonshingly short period of time - perhaps 300 million years [Lazcano & Miller 1994]. Indeed, life may have arisen in an even shorter timeframe than this. Among the oldest rocks are those from the Isua belt of Southwest Greenland, which arguably date back around 3.85 billion years. Enrichment of the 13C isotope of carbon in these rocks have been argued to betray evidence of biological carbon fixation [Mojzsis et al. 1996].
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