From: bivalve (bivalve@mail.davidson.alumlink.com)
Date: Fri Jul 18 2003 - 21:31:24 EDT
Although the evolution of improved sense organs, including better eyes, probably played a role in the Cambrian radiation by providing novel options in the predator-prey arms/defense race, no one factor probably explains all of it.
A few specific details:
>the image-forming eye.<
I am not sure that arthropod compound eyes exactly form images. However, they do provide visual acuity.
>The only light sensitivity might well have been light sensitive patchs which didn't form images.<
Light sensitivity is quite likely. The relevant genes are very widespread. However, there is no definite evidence of very well-developed eyes before the Cambrian.
> But these animals were probably too primitive to have good hearing so life mostly consisted of touch, taste, smell and the detection of light and dark only. They would get their food via accidentally bumping into it.<
Smell can be quite effective at a distance.
>And the evolutionary outcome? There are considerably fewer species active at night compared with the day.<
I am rather doubtful about this claim. Most mammals are nocturnal, as are quite a lot of insects. Many marine species are also more active at night.
>But as one went deeper into the seas, thus into the dark, the diversity of species declined significantly.<
This depends very strongly on the choice of taxon and the sampling. Deep sea diversity appears to be quite high for many groups.
>Evolution was going forward at a snails pace in the dark.<
Ignoring examples of rapid evolution in snails, the deep sea seems to provide a refuge for some archaic-looking groups, but there are also a variety of relatively young species there.
However, this does not necessarily refute the underlying idea about the Cambrian. Many of the deep sea forms of modern origin may initially evolve in the light and then go down. It may also be of interest that the Cambrian radiation seem sto have come later to the deep seas than to the shallows.
>What he found was that animals after 544 million years ago had eyes. Before that, they didn't.<
There are a fair variety of Phanerozoic forms that managed without eyes, e.g. agnostid trilobites.
>the Proterozoic and Phanerozoic--the world of dark and the world of light.<
Given the etymology of Phaneozoic, this borders on a pun.
Dr. David Campbell
Old Seashells
University of Alabama
Biodiversity & Systematics
Dept. Biological Sciences
Box 870345
Tuscaloosa, AL 35487-0345 USA
bivalve@mail.davidson.alumlink.com
That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa
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