Sequences in current databases show that a number of proteins involved in r
espiratory processes are homologous in archaeal and bacterial species. In p
articular, terminal oxidases belonging to oxygen, nitrate, sulfate, and sul
fur respiratory pathways have been sequenced in members of both domains. Th
ey include cytochrome oxidase, nitrate reductase, adenylylsulfate reductase
, sulfite reductase, and polysulfide reductase. These proteins can be assig
ned to the last common ancestor of living organisms assuming that the deepe
st split of the three domains of life occurred between Archaea and Bacteria
and that they were not acquired through lateral gene transfer by one of th
ese domains. These molecular data indicate that several of the most importa
nt respiratory pathways arose early in evolution and that the last common a
ncestor of living organisms was not a simple organism in its energetic meta
bolism. Rather, it may have been able to gain energy by means of at least f
our electron transport chains, and therefore it may have been prepared to f
ace a wide range of environmental conditions.