Shift in Earth’s Crust, Diversity of Life Coincide

Five hundred million years ago, the Earth moved with such force that it cracked apart the existing supercontinent, possibly setting off the most prolific explosion of life in the planet’s history, according to Caltech researchers.

Reading clues frozen into rocks by ancient magnetic fields, the geologists were able to piece together the history of this catastrophic event. The findings are reported in today’s issue of the journal Science.

The dramatic diversity of life that suddenly appears in the fossil record about 530 million years ago--the so-called Cambrian explosion--produced the ancestors of virtually all living things on Earth today. However, researchers have not been able to explain what sparked this evolutionary burst.

Now it appears it might have come from an equally dramatic rearrangement of the Earth.

Most exciting to David Evans, a Caltech graduate student and one of the co-authors of the paper, it proves that enormous hunks of Earth can shift suddenly and dramatically in very short periods of geological time. Previously, the continental plates were thought to be governed by what Evans calls a “plate tectonic speed limit” that kept their motions at a slow crawl--no more than several inches per year.

During this Precambrian period, however, the researchers say, the Earth’s crust may have been moving many feet per year--enough to shift regions of land from the poles to the equator and vice versa.

“It’s the first time this has been documented,” he said. “The geodynamics community has really embraced this.”

For now, the relationship between the shifting continents and the flowering of life is based mostly on conjecture, says Caltech geologist Joseph Kirschvink, the lead author of the paper. However, the pieces fit together nicely, he says.

“Life diversified like crazy about a half-billion years ago, and nobody really knows why,” said Kirschvink. “When we started assembling a real picture, it suddenly became apparent that this burst of motion was synchronous: The continents were moving at the same time. . . . That was the key insight,” he said

He speculates that relatively sudden transportation of species living in warm regions to cold ones, and vice versa, would have forced the kind of rapid adaptation to changing environments that drives evolution. What’s more, he notes, emerging traits are more likely to survive in small inbreeding populations. “That is a great script for increasing diversity,” he said.

As for why the Earth’s crust took such a sudden turn, it probably resulted from an imbalance in distribution of the continents that ultimately proved unstable. The Earth normally spins around its axis in a way that lines up almost perfectly with the magnetic fields produced deep inside its molten core. As long as the continental mass is evenly distributed, the configuration is stable.

However, if a large chunk of crust gathered in one place, it could throw the spinning crust off balance--like a ball with a weight glued to it. In order to equalize the distribution of mass, the weight would have to shift. And because the entire solid part of the Earth would move in one piece, it wouldn’t be slowed down rubbing against other continents.

Geologists know that 550 million years ago, all the land mass on Earth was collected in a single supercontinent, called Gondwanaland. Twenty million years later, the planet suddenly blossomed with life in a way not seen before or since.

For now, the connection between the two is speculative. “There’s lots of work to do,” said Kirschvink.