One of the amazing things about the animal kingdom right from the Cambrian on is that there's only about 35 body plans, basic designs, yet there are millions of species representing everything from insects to whales. -- Rudolf A. Raff

Video: Raff examines models of Cambrian-era creatures

In his lab at Indiana University, Biologist Rudy Raff studies body plans. He knows that vertebrates like us and echinoderms like the sea star have a common ancestor, but he wonders why we evolved so differently. For clues, he looks back at the Cambrian Explosion, 10 to 20 million years of evolutionary development that produced all the body plans known to us today.

Raff can't say for sure why so many animals appeared so quickly 520 million years ago, but he does offer several hypotheses:

First, he turns to genetics. Before the Cambrian, genes were only able to control the production of simple animals, such as sponges and jellies. During the Explosion, assemblages of animal genes may have evolved sufficiently to direct the production of more complex groups of creatures -- animals with heads, brains, arms and legs.

It's possible as well that during the Cambrian, the waters' oxygen levels increased. And finally, the Cambrian Explosion might have happened because of the development of a biological arms race that continues to this day. As more animals appeared and the food web diversified, the pressure to adapt to increasingly competitive predator prey interactions may have dramatically amplified the evolutionary response.

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