Universal phylogenetic tree based on small-subunit ribosomal RNA sequences. Sixty-four rRNA sequences representative of all known phylogenetic domains were aligned and a tree was produced using a likelihood method. That tree was modified to the composite one shown by trimming lineages and adjusting branchpoints to incorporate results of other analyses. The scale-bar corresponds to 0.1 changes per nucleotide (reprinted with permission from Pace, Science 276:734-740, 1997).