University of Bristol

Intrinsic causes

Intrinsic causes involve some change or mechanism within the Precambrian fauna itself which enabled rapid evolution and diversification to take place leading to the Cambrian 'explosion' and the origin of the metazoan phyla.

One theory has stemmed from the growing field of developmental genetics. As animal embryos develop, their body plans are coded for by their genes, it would be expected that the genes controlling the growth of a mammal embryo would be very different from those controlling the growth of an insect embryo. A study published in 1994 (see references Quiring et al. 1994) caused a stir by discovering that there are identical master control or hox genes which code for the development of eyes in mice and fruit flies, even though such structures are very different.

Since then other hox genes have been isolated which code for most of the structures seen in the basic bilaterian body plan. There seems to be a fundamental genetic link between all metazoans no matter how distantly related thay are. The significance of this for the Cambrian explosion is obvious. Development or mutation of just one hox gene in the ancestral metazoan could potentially initiate a large morphological change in the animal. Here we have a mechanism for the kind of rapid evolution and development of a large range of body plans in a very short space of time required for the Cambrian explosion event.


The distribution of the Hox genes in the fruit fly Drosophila embryo. Each gene codes for a specific part of the body.

From Nature, Carroll S.B. 1995. 376, pg 479-85.

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timing triggers fossils controversies

This report was written by Abby Lane and was last updated on 20th January 1999