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Already Carroll suspects that the Cambrian explosion was powered by more than a simple expansion in the number of Hox genes. Far more important, he believes, were changes in the vast regulatory networks that link each Hox gene to hundreds of other genes. Think of these genes, suggests Carroll, as the chips that run a computer. The Cambrian explosion, then, may mark not the invention of new hardware, but rather the elaboration of new software that allowed existing genes to perform new tricks. Unusual-looking arthropods, for example, might be cobbled together through variations of the genetic software that codes for legs. "Arthropods," observes paleoentomologist Jarmila Kukalov-Peck of Canada's Carleton University, "are all legs" — including the "legs" that evolved into jaws, claws and even sex organs.
Beyond Darwinism
Of course, understanding what made the Cambrian explosion possible doesn't address the larger question of what made it happen so fast. Here scientists delicately slide across data-thin ice, suggesting scenarios that are based on intuition rather than solid evidence. One favorite is the so-called empty barrel, or open spaces, hypothesis, which compares the Cambrian organisms to homesteaders on the prairies. The biosphere in which the Cambrian explosion occurred, in other words, was like the American West, a huge tract of vacant property that suddenly opened up for settlement. After the initial land rush subsided, it became more and more difficult for naive newcomers to establish footholds.
Predation is another popular explanation. Once multicelled grazers appeared, say paleontologists, it was only a matter of time before multicelled predators evolved to eat them. And, right on cue, the first signs of predation appear in the fossil record exactly at the transition between the Vendian and the Cambrian, in the form of bore holes drilled through shelly organisms that resemble stacks of miniature ice-cream cones. Seilacher, among others, speculates that the appearance of protective shells and hard, sharp parts in the late Precambrian signaled the start of a biological arms race that did in the poor, defenseless vendobionts.
Even more speculative are scientists' attempts to address the flip side of the Cambrian mystery: why this evolutionary burst, so stunning in speed and scope, has never been equaled. With just one possible exception — the Bryozoa, whose first traces turn up shortly after the Cambrian — there is no record of new phyla emerging later on, not even in the wake of the mass extinction that occurred 250 million years ago, at the end of the Permian period.