WHEN LIFE EXPLODED

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Why no new phyla? Some scientists suggest that the evolutionary barrel still contained plenty of organisms that could quickly diversify and fill all available ecological niches. Others, however, believe that in the surviving organisms, the genetic software that controls early development had become too inflexible to create new life-forms after the Permian extinction. The intricate networks of developmental genes were not so rigid as to forbid elaborate tinkering with details; otherwise, marvels like winged flight and the human brain could never have arisen. But very early on, some developmental biologists believe, the linkages between multiple genes made it difficult to change important features without lethal effect. "There must be limits to change," says Indiana University developmental biologist Rudolf Raff. "After all, we've had these same old body plans for half a billion years."

The more scientists struggle to explain the Cambrian explosion, the more singular it seems. And just as the peculiar behavior of light forced physicists to conclude that Newton's laws were incomplete, so the Cambrian explosion has caused experts to wonder if the twin Darwinian imperatives of genetic variation and natural selection provide an adequate framework for understanding evolution. "What Darwin described in the Origin of Species," observes Queen's University paleontologist Narbonne, "was the steady background kind of evolution. But there also seems to be a non-Darwinian kind of evolution that functions over extremely short time periods--and that's where all the action is."

In a new book, At Home in the Universe (Oxford University Press; $25), theoretical biologist Stuart Kauffman of the Santa Fe Institute argues that underlying the creative commotion during the Cambrian are laws that we have only dimly glimpsed--laws that govern not just biological evolution but also the evolution of physical, chemical and technological systems. The fanciful animals that first appeared on nature's sketchpad remind Kauffman of early bicycles, with their odd-size wheels and strangely angled handlebars. "Soon after a major innovation," he writes, "discovery of profoundly different variations is easy. Later innovation is limited to modest improvements on increasingly optimized designs."

Biological evolution, says Kauffman, is just one example of a self-organizing system that teeter-totters on the knife edge between order and chaos, "a grand compromise between structure and surprise." Too much order makes change impossible; too much chaos and there can be no continuity. But since balancing acts are necessarily precarious, even the most adroit tightrope walkers sometimes make one move too many. Mass extinctions, chaos theory suggests, do not require comets or volcanoes to trigger them. They arise naturally from the intrinsic instability of the evolving system, and superior fitness provides no safety net.