The XVIIIth (New) International Congress of Zoology

Le XVIIIème (Nouveau) Congrès International de Zoologie

Taken literally, the fossil record tells us that animals first appeared on Earth less than 600 million years ago, in the prelude to the Cambrian explosion. Estimates based on gene sequence comparisons suggest that the protostome and deuterostome lineages diverged at least several hundred million years earlier. The apparent incongruence between molecular and palaeontological data is to some extent to be expected. The former relate to genetic distances, whereas the latter trace the appearances of phenotypic characters. Characters are acquired sequentially during the evolution of a lineage, and the first members of a lineage are likely to lack the features characteristic of the crown group. Nonetheless, the last common ancestor of protostomes and deuterostomes, existing at the time of divergence of the two groups, should have possessed the characters that are shared by the groups and expressed in the phenotype. What these characters were, and what the possibilities were of their being preserved in the fossil record, are questions for combined biological and palaeontological attack. The recent discoveries of fossilized metazoan embryos in early Cambrian and late Neoproterozoic rocks give us an additional tool to trace metazoan roots in the fossil record, and to bring together evolutionary and developmental biology in the study of extinct early animals. Evolution is further constrained by geological events, anda proposed runaway global glaciation, 'Snowball Earth', 600 million years ago may represent a bottleneck after which metazoan evolution could finally gather momentum.