ABSTRACT
There is now evidence that life arose at high temperature (W oese, 1987) and therefore that all the metabolic pathways we now observe are evolutionary successors of pathways that operated at high temperatures-possibly above 100 ·c. 'Modem', mesophilic, pathways will have adapted to overcome the difficulties presented by low-temperature metabolism; but those successors of the ancestral organism which still operate at high temperature are presumably closest in nature to it, and so an examination of thermophilic (and probably archaeal) metabolic pathways is the best place to seek clues to the nature of early metabolism. In particular, we may be helped by a study of the constraints which may have been imposed on these early metabolic pathways by high temperature. The first step must be a judgement as to the temperature at which the ancestral organism(s) lived. Overall, the evidence in favour of any particular temperature is not strong, but the best indications probably come from those taxonomic trees which show that, in general, thermophiles are more deeply rooted than mesophiles (Figure 21.1 ), and the more deeply rooted a taxonomic branch, the higher the growth temperature of the organism. Thus, among the prokaryotes Aquifex (top~ - 85 ·q is more deeply rooted than Thermotoga (top~ - 80-85 ·c), which is more deeply rooted than Thermus (topt-65-75 ·q (Woese, 1987; Achenbach Richter et al., 1987; Olsen et al., 1994). The highest known temperature optimum for growth is 105 ·c for Pyrodictium (Stetter, i 982) and we must therefore consider the possibility that the ancestral organism grew at a higher temperature; but a growth temperature in the range 90-125 ·c seems likely.
