Tracing origins with molecular sequences: metazoan and eukaryotic beginnings.

Milestones in the evolution of the eukaryotic cell are being discovered through the analysis of molecular sequences. As sequence data become increasingly plentiful, our ability to reconstruct the most distant evolutionary branchings of evolutionary trees is limited only by the mathematics of phylogenetic reconstruction. Analysis of ribosomal RNAs agrees with traditional analyses of morphological and developmental characters that all multicellular animals probably arose from a common ancestor, but highlights one of the major limitations of the various mathematical algorithms used. Refined methods of sequence analysis also suggest a previously unsuspected sister relationship between the eukaryotic nucleus and eocytes, a group of extremely thermophilic, sulfur-metabolizing bacteria, that questions the classical eukaryote/prokaryote division.