The supposed great antiquity of the archaebacteria remains an unproved prejudice, but it is a plausible one. The methanogenic phenotype seems to cover a phylogenetic span as great as or greater than the span covered by any other comparable bacterial phenotype. This implies that the methanogens are as old as or older than any other bacterial group. Moreover, methanogenic metabolism (the reduction of carbon dioxide to methane) is ideally suited to the kind of atmosphere thought to have existed on the primitive earth: one that was rich in carbon dioxide and included some hydrogen but virtually no oxygen. The name archaebacteria implies that these organisms were the dominant ones in the primeval biosphere. When conditions changed, the methanogens' need for an anaerobic environment confined them to a limited range of relatively in accessible niches.
The measurements that revealed the existence of the archaebacteria (differences in R N A sequences) were genetic ones and were purely quantitative. They revealed nothing about the quality of the differences—the phenotypic differences—between the archaebacteria and the true bacteria. If our interpretation of the archaebacteria as a primary kingdom separate from that of the true bacteria is correct, then on detailed inspection the archaebacteria should prove to be as different from true bacteria in their molecular phenotype as either group is from eukaryotic cells.