What are the implications of the Cambrian Explosion for evolutionary biology?

Utility of palaeontology: Without fossils, we would never have even known that there was a Cambrian Radiation and we would have kept on believing that animals originated and diversified 700+ million years ago, as molecular clocks falsely tell us. Also included here is importance of exceptional preservation and understanding the various biases in the fossil record.

The power of developmental biology: The Cambrian Radiation was enabled by a single (albeit complex and very large) change in developmental biology, the addition of the third germ layer allowing musculature, skeletons and circulatory systems. That this one change can lead to so much disparity (morphological diversity) is a perfect example of why we should study evo-devo – development really is the key interface between the genotype and the phenotype and has a huge effect on macroevolution.

The speed of macroevolution: There is no doubt that macroevolution can proceed very, very rapidly at the species level, but the Cambrian Radiation allows us to study it at the phylum level, where we see that it can also go pretty fast.

Processes of macroevolution: Canalisation of development has been the most powerful idea that has come out of the study of the Cambrian Radiation. The freak show back then was the result of evolutionary “experiments” in developmental biology, only some of which could work. Those that did got preserved, and we see the traces of that today still, in the body plan of the phyla that still exist: the segmentation and arthrodisation of arthropods or the inversion of the dorsoventral axis in chordates, they were both experimented with during those early phases of triploblast evolution and only specific plans survived. We can now study this idea at the order and family level, but the consistency across all systematic levels is an important contribution that was allowed only by the study of the Cambrian Radiation.

Phylogenetics: Stephen J. Gould did the worst misstep of his career when discussing the Cambrian Radiation. In his book about it, Wonderful Life, and subsequent fights with Simon Conway Morris, he advocated that the Cambrian freaks were isolated phyla, failed experiments of evolution that are now extinct. This is completely wrong. The Cambrian animals can all be placed on the current Tree of Life without having to invoke any extinct phyla. While this has generally been the modus operandi of palaeontology since Hennig formulated and formalised phylogenetic systematics, the uniqueness of the Cambrian animals has made it very hard to actually apply phylogenetics to them. But that it can be done says a lot about the power of the phylogenetic framework and its reflection of reality. I guess it’s more of a philosophical and methodological implication, but it must always be mentioned, if only to stick it to that band of evolutionary biologists who dismiss the fossil record out of hand. The practical implications is that it’s allowed us to properly root the trees of many of our phyla – without the Cambrian critters, we would be even more lost in arthropod phylogeny than we are now (and that’s saying a lot).

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