The mechanism that we are looking for, therefore, must explain precisely why was the Cambrian explosion so different from a normal adaptive radiation, even if this means that we cannot explain it with classical mechanisms.

The stumbling-block

The fact that we have no explanation for the Cambrian explosion does not seem to surprise anybody, and is indeed understandable, because biologists know only too well where the stumblig-block comes from. The body plan is built during embryonic development by a sequence of genetic and epigenetic processes, but so far we have a reasonable knowledge only of the genetic side of the story. About the epigenetic contribution we are still in the dark, and it is this which prevents us from understanding the “logic” of the whole process.
The Cambrian explosion, on the other hand, had its roots precisely in the developing strategies of Cambrian embryos, and as long as the logic of development remains a mystery, we have no chance of understanding what happened. And we have also little change of understanding the rest of metazoan evolution, because animals are, first of all, what their embryos make of them.
What is probably less known, is that it is not embryologists that are to be blamed for our present ignorance of epigenesis. It is our culture as a whole that carries that responsability. The real culprit is the fact that we can build a “genetic” machine (a computer), but not an “epigenetic” system, a machine that is capable of doing what any embryo does. This is the great challenge of the future, and not for embryology alone, but for all of science: how does a system manage to increase its own complexity?.
While physics has built its fortunes on mathematics, biology is still an essentially empirical science, and mathematical models are mainly used for descriptive purposes, not as guiding principles. In this case, however, the problem is not that biologists are sceptical about mathematical models of epigenesis, but the fact that such models do not exist. As a matter of fact, one does exist, but nobody has taken notice, which amounts to almost the same thing.