ABSTRACT
The rooted global phylogenetic model of Woese and colleagues (Woese et al., 1990; Woese, 1994), based on 16S rRNA sequences, predicts that the oldest of extant organisms are thermophilic and that the common ancestor to all extant organisms (cenancestor) was also thermophilic. The model further indicates that the earliest organisms were phenotypically similar to present-day hyperthermophiles isolated from volcanic and geothermal environments. Testing these predictions is difficult since experimental approaches are limited to extant organisms. The most common alternative approaches are to develop models of the early earth that incorporate thermal, tectonic and geochemical earth history and to look for organic chemical and specific isotopic signatures of distinct physiological groups of microorganisms along with isotopic signatures that reflect historical temperature. Trying to infer the evolutionary history of organisms on the basis of molecular phylogenetic trees from the biogeochemical signatures in the geological record is also problematic. For example, even though there is evidence for oxygenic photosynthesis, methanogenesis and microbial sulfate reduction 3 billion years ago, it is not possible to conclude that these ancient genetic lineages were the source of present-day cyanobacteria (sic), methanogens and sulfate reducers. In fact, the 16S rRNA-based phylogenetic tree does not support an ancient lineage for cyanobacteria or, for that matter, oxygenic photosynthesis, but it does indicate that archaeal sulfur reduction and methanogenesis are ancient physiologies. These findings, however, do not negate the possibilities for multiple origins of specific physiologies, for present-day organisms having evolved from one of these lineages, or for similar physiologies having evolved more than once.
