ABSTRACT
In recent times, two sets of convergent arguments have been used to favour the hypothesis of a hot autotrophic origin of life over the classic heterotrophic origin proposed to occur in a colder prebiotic soup (Baross and Hoffman, 1985; Woese, 1987; Wiichtersbiiuser, 1988; Woese et al., 1990; Pace, 1991). Geological and astronomical data suggested that conditions governing the Earth at the time when life arose (3.8-4.2 Ga) consisted of more extensive volcanism and higher temperatures. These conditions resembled those of hyperthermophilic biotopes today (Baross and Hoffman, 1985; Nisbet, 1985; Shock, 1996). 16S rRNA sequence-based phylogenetic analyses, on the other hand, revealed that presentday hyperthermophiles are the deepest-branching organisms (Woese eta/., 1990; Stetter, 1996). The proposed bacterial rooting of the tree of life has further supported this view (Woese et al., 1990; Brown and Doolittle, 1995; Baldauf et al., 1996). However, the position of the root and the hot origin of life are themselves controversial (Doolittle and Brown, 1994; Forterre, 1996; Lazcano and Miller, 1996). In this regard, even the possibility that the last common ancestor (or cenancestor) was the only survivor of the last large meteorite bombardment has been formulated in order to make its putative hyperthermopbilic nature compatible with a cold origin of life (Miller and Lazcano, 1995).
