Figure 5. Estimates of species divergence times vary greatly.

Metazoan species tree annotated with estimates of ancestral divergence times, obtained from the TimeTree database (timetree.org) using the species tree obtained from the NCBI taxonomy [54]. Nodes are plotted at the mean age estimate across all surveyed literature that contained that divergence [55]. The relative timing of these mean estimates violates the branching order of the commonly accepted tree (inset box), as can be seen from the distorted layout in which several branches appear to be traveling backwards in time. In the accepted tree, the Opisthokonta, Metazoa, and Bilateria nodes all pre-date Ecdysozoa and its sister node Chordata. This uncertainty is further reflected in the fact that minimum and maximum age estimates for each node (red bars) differ by hundreds of millions of years. To attempt to resolve these issues, an “expert result” (starred) was selected for each node from a single article that was deemed to have the “best” estimate for that divergence [45]. Across the tree, this expert result is consistently much older than the mean age estimate for the same node, indicating that there may be systematic underestimation of node ages in the literature. The expert results are more consistent with the branching order of the accepted tree, although they do not correctly place Opisthokonta earlier than Metazoa.