Cambrian explosion

Calvin, Melvin 84 Cambium layer 29 Cambrian explosion 9 cAMP 536,544,544s, 556,557 second messenger 557 Cancer 573-575... 

Morris SC. The Cambrian explosion Slow-fuse or megatonnage Proc Natl Acad Sci USA 2000 97 4426. 

Beardsley, T. Weird Wonders Was the Cambrian Explosion a Big Bang or a Whimper Scientific American, June 1992, pp. 30-31. 

Figure 7.1 The three Cambrian explosions of animal life, and the probable temporal extensions (5-10 million years) of their geological strata.

The Cambrian explosion was traditionally defined as the appearance of the first skeleton-bearing Metazoa, but now we can... 

No new body plan has been invented after the Cambrian explosion because all ecological niches were already occupied. 

This seems a more reasonable hypothesis, but surely it cannot apply to Cambrian animals. They too had embryonic developments and body plans, and the Cambrian explosion means precisely that those ancestral plans were modified. If the constraints on embryonic development did not prevent the modification of body plans before the explosion, why should have prevented it after the explosion ... 

As we can see, none of the hypotheses that have been proposed so far is satisfactory, and this is probably due to the fact that the conservation of body plans and the origin of body plans are treated as if they were two disjoined problems. In reality, what we need to explain is not the conservation of body plans per se, but from a certain point onwards. More precisely, the problem consists in understanding why the body plans were modifiable before but not after the Cambrian explosion. 

The temptation to declassify the Cambrian explosion, and to assimilate it to processes that can easily be accounted for, is strong, but leads to an unbridgeable contradiction between experimental data and theoretical previsions. The only reasonable conclusion, therefore, is that the Cambrian explosion was not an adaptive radiation, i.e. it was not a simple process of adaptation to the environment, but something very different. 

Such a conclusion is directly suggested by the very characteristics of the explosion. All adaptive radiations that came after the Cambrian have never modified the body plans, while the Cambrian explosion was characterised precisely by modifications of those plans. And, in a similar way, no adaptive radiations have ever changed the phylotypic stage of developing embryos, while the Cambrian explosion did precisely that. 

The message that nature herself appears to be sending us is that the Cambrian explosion was a rare event in the history of life, comparable perhaps only to the origin of life or to the origin of the mind. And what is so special about these rare episodes of macroevolution is the appearance of biological characteristics which have never been changed ever since. The mechanism that we are looking for, in conclusion, must explain precisely why the Cambrian explosion was so different from a normal adaptive radiation, even if this means that we cannot explain it with classical mechanisms. 

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 chance of understanding the rest of metazoan evolution, because animals are, first of all, what their embryos make of them. 

The mathematical model, in conclusion, allows us to add a new biological property to the Bauplan, an idea that can be expressed in this way a body plan is a supracellular memory, or the body plan is the body s memory. The proof that such an addition is not only new, but also useful, can come of course only from its power to solve real biological problems. One of which is precisely the problem of the Cambrian explosion. 

It is likely, in other words, that the first animals had embryonic developments totally programmed by genes, and that only later did developmental strategies evolve that could also exploit the supracellular information of the body plan. This shift from a continuous (one-phased) development to a discontinuous (two-phased) one would have been a tranformation of enormous importance, and could well correspond to the Cambrian explosion. This is a new hypothesis, and it may be worthwhile to consider its predictions for what happened before, during and after the Cambrian explosion. 

We have therefore a new model which can be summarised in this way the Cambrian explosion was the transition from a primitive type of development that was totally controlled by genes to a discontinuous type of embryonic development that could also use, from a certain point onwards, the supracellular information of the body plan. 

It may be useful to underline that the model has a number of positive features (1) it explains why body plans were modifiable before the Cambrian explosion and had to be conserved afterwards (2) it explains why animals were small before the explosion and could grow to much bigger sizes afterwards and finally (3) it explains why the explosion took place in a geologically brief period of time. 

The first kind of development (being continuous or single-phased) is an evolutionary precondition for the second one (which is two-phased or discontinuous), and this suggested that there might have been a transition from the first to the second developmental strategy in the history of life. Such a transition, incidentally, could well correspond to the Cambrian explosion, i.e. to the appearance of all known animal phyla in a geologically brief period of time. 

---