Life Goes for a Spin

Fossils locked away in rocks from the Cambrian Period record one of the most astounding episodes in the history of evolution. Beginning 540 million years ago, life on the earth took on radically new forms. Nearly all of the major groups (phyla) of animals that we see today arose at that time. The change was as rapid as it was complete, with much of the evolutionary innovation taking place over as little as five million years; scientists often refer to this period as the "Cambrian explosion."

Scientists have advanced many theories to explain that big bang in the diversification of life, none of them fully convincing. Now Joseph L. Kirschvink of the California Institute of Technology and his colleagues believe they may have solved what he calls "one of the outstanding mysteries of the biosphere." Life on the earth turned upside down, Kirschvink proposes, because the earth itself turned upside down: an abnormally rapid reorganization of the earth's crust, tens of times faster than normal continental drift, touched off sharp climatic shifts that in turn unleashed a torrent of evolutionary change.

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Writing in the July 25, 1997, issue of Science, Kirschvink and his co-authors (Robert L. Ripperdan of the University of Puerto Rico and David A. Evans, also at Caltech) point to several lines of evidence showing that the early Cambrian was a time of extraordinary geologic upheaval. One giant continental landmass, called Rodinia, was torn asunder but the pieces almost immediately regathered into another supercontinent, Gondwana. Sedimentary rocks record abrupt shifts in the chemistry of the oceans. And new uranium-lead dating techniques prove that these changes all occurred rapidly and simultaneously with the Cambrian explosion.

One way to learn more about this tumultuous era is to retrace the detailed motions of the continents during the early Cambrian. Such reconstructions are possible (though difficult to perform) by examining the magnetism of rocks from that time. When volcanic rocks cool, they preserve an imprint of the earth's magnetic field; once locked in, the magnetic record in the rock does not shift. The direction and inclination of the field in the rock indicates both the latitude of the rock and its orientation relative to the North Pole at the time it solidified.

When Kirschvink and his collaborators studied rocks from Australia, they found that the continent rotated by 90 degrees between 534 million and 505 million years ago. Paleomagnetic data collected around North America appear to show a similarly abrupt dislocation of the continents relative to the earth's axis. The tectonic regions associated with Australia and North America cover two thirds of the earth's continental crust, leading the researchers to a startling conclusion: that the entire surface of the earth rotated 90 degrees in a geologically brief 15 million years. The earth's crust must have been shifting at least 30 centimeters per year, or about 10 times the usual rate of continental drift.

Such motions violate the "plate tectonic speed limit" that has operated for at least the past 200 million years, Evans notes. The researchers therefore suspect a different mechanism, called true polar wander, was responsible. In essence, they believe the crust of the earth became so unbalanced that it slid across the mantle until parts of the surface that started out at the poles reached the equator, and vice versa. Because the entire solid lithosphere of the earth moves at once, true polar wander can produce much faster motions than normal continental drift, in which the various tectonic plates are bumping and grinding against one another.

The rapid geologic wanderings of the early Cambrian would have given a major kick to life on the earth. "The repeated reorganizations in global wander during [a true polar wander] event should have fragmented any large-scale ecosystems that were established, generating smaller, more isolated populations and leading to a higher evolutionary branching rate among existing groups," the authors write. A succession of major changes in climate and ocean circulation could thus explain why the rate of evolutionary innovation jumped by about a factor of 20 compared with the pre-Cambrian.

Kirschvink's analysis does not prove that continental reorganization alone was responsible for the Cambrian explosion. Other possible explanations (including the rising levels of oxygen in the atmosphere and the appearance of the first true carnivores) may also have played a role. But Kirschvink is confident that true polar wander offers an important new tool for understanding the geology and biology of the Cambrian--a time that gave rise to the famous Burgess Shale, and a dramatic turning point in the march of life.