Phylogenetic meaning of the kingdom concept: an unusual ribosomal RNA from Giardia lamblia

An analysis of the small subunit ribosomal RNA (16S-like rRNA) from the protozoan Giardia lamblia provided a new perspective on the evolution of nucleated cells. Evolutionary distances estimated from sequence comparisons between the 16S-like rRNAs of Giardia lamblia and other eukaryotes exceed similar estimates of evolutionary diversity between archaebacteria and eubacteria and challenge the phylogenetic significance of multiple eukaryotic kingdoms. The Giardia lamblia 16S-like rRNA has retained many of the features that may have been present in the common ancestor of eukaryotes and prokaryotes.