A New Twist on the History of Life

The idea that the wholesale relocation of Earth's continents 520 million years ago, also known as "true polar wander," coincided with a burst of animal speciation in the fossil record dates back almost 20 years to an original hypothesis by Joseph Kirschvink (BS, MS '75), Caltech's Nico and Marilyn Van Wingen Professor of Geobiology, and his colleagues. For more than a century, paleontologists including Charles Darwin have debated whether the so-called Cambrian explosion—a rapid period of species diversification that began around 542 million years ago—was the equivalent of an evolutionary "big bang" of biological innovation, or just an artifact of the incomplete fossil record.

In a new study published in the December issue of the American Journal of Science, a team of researchers including Kirschvink and Ross Mitchell, a postdoctoral scholar in geology at Caltech, describes a new model showing that during the proposed Cambrian true polar wander event, most continents would have moved toward the equator instead of toward the poles.

"It's long been observed that biological diversity is highest in the tropics, where nutrients and energy tend to be abundant," says Kirschvink. "One of the side effects of true polar wander is that sea level rises near the equator but falls near the poles, so the equatorial migration of most Cambrian land masses would have enhanced diversification into previously lower-diversity environments."

Using a model they developed, the team simulated the pattern of continental migration during the Cambrian and found that their results can explain the distribution of Cambrian fossils.

"Our model provides an explanation for why the fossil record looks the way it does, with many Cambrian fossil groups on some continents but few on others," says study coauthor Tim Raub (BS, MS '02), a lecturer at the University of St. Andrews in Scotland.

"The same sea-level rise which flooded those continents that shifted to the tropics and opened new ecological niches for faster speciation also led to more fossil preservation," Mitchell says. "In contrast, the few areas that shifted to the poles became less biologically diverse and also lost rock volume to erosion following sea-level drops due to true polar wander."

The scientists say their new findings could help resolve the debate started so long ago by Darwin. If their theory is correct, the Cambrian explosion is both a true and dramatic pulse of biological innovation and an expression of preferentially preserved shells on selectively submerged continental margins capable of containing fossils.

Funding for the study was provided by the National Science Foundation.