Anatomical and embryological observations by early biologists laid the groundwork for understanding animal phylogeny, but, as was true for phylogenetic relationships in Bacteria, Archaea, and Eukarya as a whole, a modern understanding of evolutionary relationships among animals had to wait for the revolution in molecular sequencing. Over the past two decades, comparisons among DNA, RNA, and amino acid sequences have greatly improved our understanding of animal phylogeny. Molecular comparisons support many of the conclusions reached earlier on the basis of comparative anatomy and embryonic development, including the early divergence of sponges and the separation of radially and bilaterally symmetrical animals. Moreover, molecular sequence comparisons confirm the view that choanoflagellates are the closest protistan relatives of animals.

Other hypotheses, however, have been rejected. For example, the traditional phylogenetic division of bilaterians into acoelomate, coelomate, and pseudocoelomate groups gets little support from molecular studies. Similarly, molecular sequence comparisons do not support the once widespread view that segmented bodies indicate a close relationship between earthworms and lobsters.

Quick Check 1 Which animals are more closely related to sponges, Cnidaria or Bilateria?