No. 85: Jan-Feb 1993
Representatives of three body plans (phyla):
jellyfish (coelenterata); aphid (arthropoda);
J.S. Levinton, in a long article in the November 1992 Scientific American, explores the enigma of the Cambrian explosion. Did some unknown evolutionary stimuli prevail 570 million years ago that made the Cambrian different from all periods that followed? Or, has something damped evolutionary creativity since then? Levinton holds that biological innovation has continued unabated at the species level since the Cambrian explosion, but that new body plans; that is, new phyla; have not evolved for hundreds of millions of years. Therefore, something special and very mysterious -- some highly creative "force" -- existed then but is with us no longer. (Levinton, Jeffrey S.; "The Big Bang of Animal Evolution," Scientific American, 267:84, November 1992.)
Comment. If evolution is truly the result of random mutation modulated by natural selection, perhaps mutation was "different-from-random" during the Cambrian! Now that's a heretical thought.