ABSTRACT
The evolutionary history of microorganisms can be reconstructed using evidence from the fossil and geological records (Schopf, 1993; Schopf and Cornelius, 1992). However, the microfossil record is far from complete and unique morphological characters do not exist for most microbial groups. In lieu of these data, two types of molecular data found in extant organisms have proved useful in the study of early evolution: comparative analysis of biochemical pathways (Wiichtershiiuser, 1992; Morowitz, 1992) and analysis of macromolecular sequence data. The best-studied molecule for sequence analysis is undoubtedly the small subunit ribosomal RNA (rRNA), but many other macromolecules, especially proteins, are elucidating the relationships among prokaryotic kingdoms and domains.
