Cambrian animals

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 ... 

We can find burrow-holes in the fossil mats, but we can t find evidence of scars or notches on the thick shells and hides of the first Cambrian animals. It seems that for a time, everyone was a vegetarian and fed off the mats, and it was a world without scars. That would end soon enough. 

Paleozoic Cambrian 600 Myr Climate warms, O2 levels approach current level most animal phyla present, including some that failed to survive algae and cyanobacteria diversify... 

The first well-documented episode of extinction came at the time of transition from the Precambrian to Cambrian era, about 600 milhon years ago. Many species for which we have fossil evidence, the Edicarian animals, simply did not survive this... 

Animals appeared in the Ediacaran, slightly ahead of the Cambrian but most of them were soft-bodied and did not survive except the jelly fish. The first 10 million years of the Cambrian, now known as the Tommotian, brought about a bewildering array of micro-fossils, none larger than a few millimeters. A quote from Stanleys text is illuminating in that context.1... 

Secondary ova, those that were produced by a finished prehistoric animal, have been found frequently, and when dinosaur eggs revealed the fully formed embryo there was no doubt about the saurian way of reproduction.15 Precambrian eggs have been found16 but never one of the primary eggs, which must have been there en mass for each species a few thousand years before the entry of a species into the fossil record. Until now, there was no good reason to look for them and, of course, there is no handbook for identification purposes. Certainly the avalanche of exquisite fossils from China, which is pushing the age of vertebrates deeper into the Cambrian, will provide an excellent chance to find some of the pro-forms postulated by the Genomic Potential Hypothesis.17... 

The Cambrian arthropods start the series and because, as mentioned above, there were no macroscopic ancestors that could have been converted slowly by mutation and adaptation to the first animals in the shale, they set the tone for the evolutionary calendar of events. There are no recognizable intermediates leading to fishes, would it not follow that they too should have evolved by the same route that brought about arthropods The step from fishes to amphibians is informative. Coelacanth and its purported successor Ichthyostega are contemporaries. Would that not suggest quite... 

Fig. 13.5. This figure is a summary of the various postulates of the genomic potential hypothesis. Coding sequences evolve from straight chemistry in innumerable pools on the earth s surface to provide a nearly unlimited reservoir of nucleic acid polymers which form themes and variations that are taken up by the various foci of cell formation. Once a cell has formed, it never loses its basic character, remaining either a micro-organism or developing a potential for a large animal during this 3.5 million years of the Archean period which would show up at varying times after the Cambrian period. Each new species eventually appears from its own pro- form and persists until the present or extinction.

The genomist will look at the first animals and predict that all of them will remain just what they were in the Cambrian until the present or extinction.8, 9 New forms will come from species-specific precursors at later times. The fossil record gives a nod to the new world. 

In the last twenty years, in conclusion, paleontology has discovered that in Cambrian times there have been not one but three different explosions ofanimallife one documented by trace fossils, a second which left behind small shelly fossils, and finally the classical explosion that was dominated by trilobites. It must also be added that Cambrian life was preceded by the so-called Ediacara fauna, a vast assembly of soft-bodied animals (almost all with radial symmetry). Many scholars now regard them as a failed evolutionary experiment, while others believe that they may have left modified descendants (Figure 7.1). 

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

This is a very weak suggestion, because a high number of new ecological niches did become available after the Cambrian, and not just once but many times over. When animals invaded the land, for example, they had at their disposal absolutely virgin territories for hundreds of millions of years. And as for the sea, species have been literally decimated various times by great mass extinctions, which certainly created plenty of opportunies for new experiments in body plans. 

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. 

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. 

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. 

The second model illustrates the point that more than one kind of memory can be responsible for the reconstruction from incomplete information that takes place during the (epigenetic) formation of an organism. Barbieri proposes that two kinds of memory are in fact responsible for the development of multicellular animals - one for the earlier stages, the other for the later ones. He shows how the existence of these two kinds of memory might account for the pattern of macroevolution, notably the Cambrian explosion. 

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