Universal Ancestor

To say that life on Earth began exactly once does not mean that there is a Universal Ancestor from whom all other hving forms are derived. Life has no obvious starting point. The key classes of molecules of hfe were originally created abiotically. 

The universal phylogenetic tree, therefore, is not an organismal tree at its base but gradually becomes one as its peripheral branchings emerge. The Universal Ancestor is not a distinct entity. It is, rather, a diverse community of cells that survives and evolves as... 

The primary lines of descent and the universal ancestor. In Evolution from Molecules to Man, ed. D. S. Bendall. Cambridge Universiy Press, pp. 209-33. 

C. Woese, The universal ancestor, Proc. Natl. Acad. Sci. USA 1998, 95, 6854-6859. 

Wallin IE (1927) Symbionticism and the origin of species. Williams and Wilkins, Baltimore Watasd S (1893) On the nature of cell organization. Woods Hole Biol Lect 83-103 Wilson EB (1896) The cell in development and inheritance. Macmillan, New York Wilson EB (1925) The cell in development and heredity, 3rd edn. Macmillan, New York Woese CR (1977) Endosymbionts and mitochondrial origins. J Mol Evol 10 93-96 Woese CR (1998) The universal ancestor. Proc Natl Acad Sci 95 6854-6859 Woese CR, Fox GE (1977) Phylogenetic structure ofthe prokaryotic domain the primary kingdoms. Proc Natl Acad Sci USA 74 5088-5090... 

The main conclusion from ribosomal RNA phylogeny is that life on Earth is based on three main lines of cellular descent, all derived from a universal ancestor. See Figure 7.2. 

As the tree shows, the true relatives of the archaea are not the bacteria but the eukaryotes. However, because the archaea branch-off closest to the root of the tree, the suggestion is that they are the most primitive of the three kingdoms of organisms whereas the eukaryotes are the least primitive (or the most derived). Placement of the archaea closest to the universal ancestor is supported by the fact that many archaea inhabit extreme environments involving high temperature, low pH, high salinity, and so on. Thus, archaea may well be evolutionary relics of the Earth s earliest forms of life. 

Woese, C. The universal ancestor. Proceedings of the National Academy of Sciences USA 95 6854-6859 1998. 

Woese CR. The universal ancestor. Proc Natl Acad Sci USA. 1998 95 6854-9. 

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