Ancestors and urbilateria

As the phylogenetic tree of metazoans has become increasingly informed by molecular sequence comparisons, some early nodes have become increasingly well supported, permitting speculation as to the morphology of the ancestral organisms represented there. Particularly famous among such hypothetical ancestors is Urbilateria (De Robertis and Sasai 1996; Kimmell 1996) defined as the last common ancestor (LCA) of Protostomia and Deuterostomia (here, the P/D LCA). The name implies that this form should be the earliest animal that had the bodyplan of Bilateria, and sometimes Urbilateria has been used in that sense (de Rosa et al. 2005; Carroll et al. 2001). However, the earliest bilaterian was not necessarily (nor probably) an LCA of major taxa. These definitions thus rest on different sorts of ancestors. The characteristics attributed to the urbilaterian may vary according to the sort of ancestor to which Urbilateria is referred. This situation highlights a general problem arising when referring ancestors to nodes in a tree rather than to their first appearance in evolutionary history. Figure 1 offers a terminology for various ancestral stages during the evolution of living clades (see Valentine 2004). Two branches evolve so as to become different living taxa; let us say they become different phyla (although the terminology used here is applicable to taxa of other levels). The LCA of these phyla is not a member of either of the descendant phyla, nor will it generally have either of their bodyplans. The ancestors that founded the clades that include each phylum were produced by speciation of the LCA. The clade ancestors and the LCA thus differ only by species-level features, and may even be morphologically indistinguishable; at any rate, none of them has the bodyplan of either descendant phylum. However, as evolution proceeds, these bodyplans are eventually assembled within each clade, and at those points the stem ancestors of the phyla have originated. To identify stem ancestors within an evolutionary continuum involves the first appearance of some particularly important feature that becomes a synapomorphy for the succeeding clade, the continuum having been broken by a particular change that is designated as defining. (As it happens, this problem has not arisen for phyla, owing to the incompleteness of the fossil record.) Finally, the LCAs of the living taxa of each of the phylaFthe crown groupsFarise; these can be termed crown ancestors. Ordinarily, the stem and crown ancestors would not arise at the same times in both phyla. The LCA and clade ancestors of the phyla may (and probably almost always do) originate when nothing special is going on evolutionarily; the species-level features that arise in the clade ancestors are unlikely to have anything to do with the eventual morphological differences between the descendant bodyplans. The utility of the clade ancestors is that they have disparate living representatives; homologies in these living groups can be used to infer some features in the clade ancestors and the LCA, and differences in their molecular sequences can be used to estimate a molecular clock date for the LCA, which will for practical purposes date the clade ancestors also. The stem ancestor is likely to record important events in evolution, but is only represented (if at all) by fossils, and while the fossils can be dated by radiometric methods (far more accurate than molecular techniques at present), they are highly unlikely to represent the earliest members of the stem group. And while the crown ancestor, like the LCA of the phyla, is datable by molecular techniques, it need not represent any significant evolutionary event. Indeed, a crown ancestor is determined by extinctions among what become stem taxa by definition, and not by the origin of significant novel features, which may or may not be present. The ancestors found at nodes are thus precursory entities to interesting events but may be of little evolutionary importance in themselves. The lineages first showing important synapomorphies are found between the LCA nodes. In the phylogenetic tree of Fig. 2, major metazoan clades are arranged chiefly according to molecular evidence, mostly SSU rRNA, supplemented by data from morphology and other sources. The positions of some LCA, clade, and stem ancestors are indicated. Urbilateria has usually been presented as a reconstruction of either the earliest bilaterian bodyplan, which would be a stem ancestor, or as the P/D LCA (the original definition of Urbilateria; De Robertis and Sasai 1996). The P/D ancestor was once thought to be the crown ancestor of Bilateria (e.g. Erwin and Davidson 2002), but would be relegated to an internal node of that group by molecular evidence that indicates that Acoela is the most basal EVOLUTION & DEVELOPMENT 8:5, 391 –393 (2006)