Hi Rusty,
By amazing coincidence I've just finished
In the Blink of an Eye: the Cause of the Most Dramatic Event in the History of Life, by Andrew Parker. This book lays out the foundations, the rationale, & a potential solution to the "problem" of the Cambrian explosion. Personally I couldn't help shaking the feeling that he was overstating the case on a few points, but hey ho.
It goes something like this. Evolution in the Precambrian proceeded at a relatively slow rate. The reason for this is that one major sensory adaptation had yet to take place: vision. Predation did occur, but relied on vibration/chemical sensory detection. All you had to do to avoid predation was to not smell, & be quiet. The only way the predators could then get you was to "happen" upon you, or you drift into them (in the case of ambushers/filter feeders). Clearly, there was only so far you could go with limited sensory apparatus developmental-wise, so selective pressure in this regard was limited, resulting in limited evolutionary impetus.
At the base of the Cambrian the first fossil evidence of eyes appears, specifically the first belonged to trilobites. This changed the entire arena with regard to predator prey relationships. No longer was predation necessarily a passive affair. A predator could now target its prey from afar, & since most of those predators were themselves prey, prey could now take preventative measures against predation. This resulted in an instantaneous arms race with enormous selective pressures in operation that previously did not exist. And large selective pressures equals rapid evolution. With the advent of eyes, so the selective pressures were there for new modes of locomotion, physical protection, escape, & camouflage. And remember, all of this "exploded" onto a scene where nobody occupied any of the new ecological niches. There was no pressure from "incumbent" niche holders (at least initially) to prevent evolutionary innovation.
The corrolory question is, why did eyes take 3 bn years to form if they were so useful? A fair question, & there are answers that are all highly speculative (in my opinion), but that's a different issue to the one you raise. The fact is, there is no evidence of eyes in the Precambrian, & they appear at the start of the Cambrian explosion. Clearly such a biologically efficient sensory apparatus would revolutionise the selective & adaptive landscapes of most organisms in the upper sunlit ocean, resulting in rapid adaptation, followed by a slowing of evolutionary tempo as selective pressures eased as adaptation was achieved.
Parker makes use of a modern study to support his low light = slow evolution hypothesis. Isopods are probably most familiar to us as the woodlouse, but most species are marine. There was a study off of Australia that sought to discern the biodiversity at different depths of various crustacea (to which Isopods belong). What was noticed that the largest species occurred at the greatest depth, & that biodiversity was lowest there. A similar study was performed the other side of the Indian Ocean, & some interesting facts revealed themselves. The shallow species were many, varied, & small, the Aussie shallow isopods were diffent species to the Indian ones. The deeper you went, however, the larger the isopods got, & the lower the biodiversity became. There came a point where the largest & deepest isopod,
Bathynomus propinquus was actually the same species as the Australian large-deep-isopod (barring a few minor differences such as spine number, etc). Both the Aussie shallowest & deepest isopods had been separated from each other for the same length of time as their Indian equivalents, but in the
darkest environment, evolution had proceeded much, much slower than in the shallower sunlit environments, resulting in negligable morphological change. ie. No light (or lack of vision)
that is uniform in all oganisms in any given environment results in slower rates of evolution.
Interesting, non?
Mark
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