Cambrian explosion contradicts evolutionary tree pattern
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Claim
In the "Cambrian explosion," all major animal groups appear together in the fossil record fully formed instead of branching from a common ancestor, thus contradicting the evolutionary tree of life.
Source
- Wells, J., 2000. Icons of Evolution
Responses
- First of all, this is simply not true. Many phyla, including the Cnidarians, Mollusca, Annelida, and Arthropoda appeared in the Precambrian.
- In any case, not all of the phyla present in the Cambrian were anything like their modern representatives. Many, if not all Cambrian organisms appear different, and more primitive than their later relatives. For example, the Cambrian eocrinoid echinoderm Gogia spiralis was attached to the substrate by a short, thick, plate-covered stem and holdfast, while later eocrinoids developed the long, slender stems made up of columnal disks typical of their descendants, the crinoids and blastoids. Equally, neither bryozoans, nor any of the major vertebrate groups most people are familiar with (fish, mammals, reptiles, amphibians, or birds) appear in the Cambrian.
- The basic implication of this critique seems to misunderstand how evolutionary taxonomy works. Classifications like phyla are to some extent arbitrary: a human-created and assigned system of groupings. Furthermore, these groupings are intrinsically hierarchical: all of the descendants of ancient creatures will, by the very nature of the classification system, be classified as sub-variants of previous groups. It is thus not an accident that there was a time when what we distinguish as phyla appear in the fossil record but that there are no new "phyla" appearing today: it is a direct result of the way in which these categories are assigned. All modern life is descended from previous life, and as such, all new categories are "below" the phylum level. Hence, the phyla all emerging at around a given era in the history of life is a direct result of the cladistic evolutionary tree of life and the particular way the classification system is applied to it.
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External Links
- Mark Isaak's page for this claim [1]
