"You seem to conveniently ignore the Cambrian explosion."
Sigh. Taro, there is no reason to believe that there were sudden, big jumps during that time. That was over 500 million years ago, there just isn't much in the way of a fossil record older than that. Much less a fossil record of soft bodied creatures which require extremely rare events to occur. Now, there is evidence that just prior to the Cambrian there was some global geological event based on chemical changes in the rocks. It seems very likely that they Earth had just moved out of one of those iceball stages where the whole Earth was frozen. Given the climatological changes that one would expect, there would have been a massive Darwinnowing...
However, there is a lot of evidence that complex organisms preceded the Cambrian, but we haven't found any of the fossils yet. The best evidence is the massive decline in stromotolites in the fossil recored. Stromotolites exist today, but only in areas where the conditions are so harsh virtually nothing else can live there. Because activity of soft-bodied invertebrates disrupt their structures, if conditions are right for the invertebrates, they clean the stromotolites out. Given the decline of stromotolite populations in the fossil record, it is reasonable to guess that there were complex organisms feeding on them for hundreds of millions of years before the Cambrian. So, they just didn't poof into existence during the Cambrian.
Therefore, complex organisms predated the Cambrian. Likely with a fairly wide variation of body types. What few hard-bodied fossils we have found from the immediate pre-Cambrian, they are all tiny, on the order of a millimeter or so. Like one would expect in very harsh conditions. With the thawing of the Earth and the massive changes in climate, there were strong selection pressures. With the moderating climate, the organisms started to increase in size and predation started to become a larger factor. And that also resulted in selection pressures. One aspect of predation is that it would select for organisms with hard body parts and things like spines for defenses. Which fossilize more readily.
So, that is why the Cambrian looks like there was just an explosion of new creatures. Partly it is a result of conditions be better suited for preservation. Partly it is a result of moderating conditions so that the organisms could grow large enough to find. Partly is it a result of selection pressures at a higher rate than before. And partly it is because organisms started to develop hard body parts to deal with predation. But, there is no reason to believe than the general classes of organisms seen during the Cambrian didn't exist before. The Cambrian was just when those classes were put to the big test.
I am sure the wikipedia entry is far more complete that what I have here. And likely has covered some aspects that I am not aware of. |
