A literal reading of the fossil record suggests that the animal phyla diverged in an "explosion" near the beginning of the Cambrian period. Calibrated rates of molecular sequence divergence were used to test this hypothesis. Seven independent data sets suggest that invertebrates diverged from chordates about a billion years ago, about twice as long ago as the Cambrian. Protostomes apparently diverged from chordates well before echinoderms, which suggests a prolonged radiation of animal phyla. These conclusions apply specifically to divergence times among phyla; the morphological features that characterize modern animal body plans, such as skeletons and coeloms, may have evolved later.Darwin (1) recognized that the sudden appearance of animal fossils in the Cambrian posed a problem for his theory of natural selection. He suggested that fossils might eventually be found documenting a protracted unfolding of Precambrian metazoan evolution. Many paleontologists today interpret the absence of Precambrian animal fossils that can be assigned to extant clades not as a preservational artifact, but as evidence of a Cambrian or late Vendian origin and divergence of metazoan phyla (2-6). This would make the Cambrian the greatest evolutionary cornucopia in the history of the Earth. Definitive representatives of all readily fossilizable animal phyla (with the exception of bryozoans) have been found in Cambrian rocks, as have representatives of several soft-bodied phyla (6). Recent geochronological studies have reinforced the impression of a "big bang of animal evolution" by narrowing the temporal window of apparent divergences to just a few million years (4).The evidence for a Cambrian explosion of animal phyla is based on the absence of fossils of triploblastic metazoans from rocks predating the Cambrian. This negative evidence is not entirely convincing. Tiny unskeletonized animals with no possibility of preservation in the fossil record may have existed before the Cambrian (7, 8). Even if larger, soft-bodied animals were present, conditions appropriate for their preservation may not have existed for much of the The authors are in the . In particular, the famous Lagerstatten of the Cambrian (8, 10) resulted from taphonomic conditions that are exceptionally rare at other times in the rock record (9). Nevertheless, some Vendian trace fossils and body fossils suggest that animals with coeloms existed before the Cambrian (6, 8, 11, 12). Calibrated rates of gene sequence divergence provide another avenue for dating divergence times between animal phyla (13). An early study by Runnegar, based on hemoglobin, suggested Precambrian divergences (14) but was criticized for not testing assumptions of rate constancy (15). A more recent study based on 18S ribosomal RNA (rRNA) sought evidence of rapid divergences in the inability of sequence data to resolve phylogenetic relationships (16). The burgeoning database of gene sequences provides an opportunity to examine the divergence times of metazoan phyla from large data sets based on severa...

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