Astrobiology Workshop, Macquarie University July 12-13 2001
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Rebuilding Ancient Genes to Study the Last Common Ancestor
David Saul (University of Auckland)
"...it will in the future be possible to synthesize these presumed components
of extinct organisms. Thus one will be able to study the physico-chemical properties
of these molecules and make inferences about their functions"
Pauling and Zuckerkandl (1963)
The conditions in which life emerged remains one of the greatest unknowns of biology and there are vociferous advocates for both hot and cold starts. Excitement was kindled in this topic by phylogenetic analyses based on 16S rRNA genes and the discovery of 'hyperthermophilic' microorganisms (organisms that grow at or above 95°C). The Tree-of-Life constructed from these analyses placed the hyperthermophiles on short branches close to the origin. The obvious implication was that life had arisen in a high temperature environment. In addition to this observation, there is a substantial body of circumstantial evidence of a hot start from other disciplines. However, for every supporter of the hot start, there are as many opponents - particularly adamant are the supporters of the hypothesis that life started as an RNA world.
Reliable information about the evolutionary ascent from the Last Universal Common Ancestor (LUCA) would provide valuable clues for resolving some of these issues and would establish a more stable platform from which to extrapolate to earlier ancestors. Modern computational methods for phylogenetic inference can provide this information. In addition to estimating the evolutionary relationships of extant organisms, they can also infer the most probable genotype of ancestors. The programs can be used to predict the nucleotides or amino acid sequences of ancestral genes and gene products, and molecular genetics techniques can then be used to synthesise the ancestral genes in vitro. The encoded protein is then expressed and its thermal properties provide empirical evidence of the growth habits of extinct organisms.