How real is the Cambrian explosion? In a sense, it wasn't an explosion at all in any commonly understood meaning of the term—it was a relatively rapid apparent diversification of animal phyla over the course of at least tens of millions of years, at a rate that is compatible with unexceptional rates of evolution. Even at the most 'explosive' rate that can be inferred from the observations, this is not an event that challenges evolutionary theory, nor should it give comfort to creationists of any stripe.
However, there are controversies here. One camp holds that the rapid divergence of the metazoan phyla in the Cambrian is real: the different phyla all arose sometime around the boundary, 543 million years ago, and then evolved into the various forms we see now. This interpretation is supported by the fossil record, in which the first recognizable representatives of the phyla are found from roughly the same period.
Another interpretation is that the Cambrian explosion is only apparent: that the divergence occurred well before 543 million years ago, and that there was a long period of undetectable evolution. The major groups of animals separated 600 or perhaps even as much as 700 million years ago, flourished as small wormlike forms that would have fossilized poorly, and what the Cambrian represents is an emergence of larger forms with hard body parts that fossilized well. Some of the molecular data supports an early divergence, and there are known pre-Cambrian trace fossils and fossils—the phosphatized embryos of the Doushantuo formation, about 600 million years old, are a good example.
There are also other ambiguities to be resolved. The relationships of many animal phyla are confusing, and who branched from whom remains to be resolved. In the diagram below, the dashed lines in the tree are the problem: do they branch exactly as shown? How deep in time do those branches go?
The fossil record and evolution of 9 of the 35 currently recognized metazoan phyla suggest that most animal phyla diverged/arose at the beginning of the Cambrian (C) period. The thick lines represent the known ranges of fossils from their first appearance in the fossil record. Thin lines represent the inferred metazoan phylogeny based on fossil data. Dashed lines represent an amalgam of three conservative estimates of the inferred metazoan phylogeny.
Continue reading A signature of a radiation in metazoan evolution (on Pharyngula)