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No. 85: Jan-Feb 1993

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Biology's big bang

Representatives of three body plans
Representatives of three body plans (phyla):
jellyfish (coelenterata); aphid (arthropoda);
eohippis (chordata);
The title refers to the so-called "Cambrian explosion," that period that began some 570 million years ago, during which all known animal phyla that readily fossilize seem to have originated. The biological phyla are defined by characteristic body plans. Humans, for example, are among the Chordata. Some other phyla are the Arthropoda (insects, crustaceans), the Mollusca (clams, squids), the Nemotada (roundworms), etc. All of these phyla trace their ancestries back to that biologically innovative period termed the Cambrian explosion. Even at the taxonomic level just below the phylum, the class (i.e., the vertebrates), most biological invention seems to stem from the Cambrian.

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.

From Science Frontiers #85, JAN-FEB 1993. 1993-2000 William R. Corliss