Mark I. Vuletic

Last updated 21 March 2008

Analysis

(i) First, let us put to rest the claim made by some creationists that there are no fossils before the Cambrian explosion. There are plenty of fossils even of multicellular Precambrian organisms such as jellyfish, coelenterates, and annelids (McGowan 1984:103). There are far fewer fossils before than during and after the Cambrian explosion, but they are there, so the existence of Precambrian organisms should not be in doubt.

(ii) The smaller number of fossils before the Cambrian explosion is expected because the majority of Precambrian organisms were soft-bodied, making them less likely to become fossilized. This is not to say that Precambrian fossilization was impossible, just that it was less likely, so we would expect even fewer fossils than normal.

(iii) It is currently debated whether the Cambrian explosion represents a sudden increase in diversity or merely a sudden increase in fossilization probability. Some paleontologists believe that most of the diversification of body plans occurred before the Cambrian explosion, but happened in soft-bodied organisms less amenable to fossilization. During the Cambrian era, however, global environmental changes created selection pressure for hard shells, which increased the probability of fossilization. According to this account, which receives some confirmation both from Precambrian fossils and from molecular clocks, the apparent "explosion" of body forms in the Cambrian era is an illusion.

(iv) Even if the full diversification of body plans did happen in the Cambrian explosion, the diversification rate is consistent with normal rates of evolution. Given how many niches were still open to exploitation at the beginning of the Cambrian era, there is nothing inherently improbable about so many new forms having appeared when they did.

References

McGowan C. 1984. In the Beginning...: A Scientist Shows Why the Creationists Are Wrong. Buffalo: Prometheus.