Geophysics: Flipping the Magnetic Field

Every half-million years or so, for unknown reasons, the earth's magnetic field suddenly flips. Within a period of 10,000 years—a mere instant on the geological time scale—the north and south magnetic poles exchange places. Scientists have long suspected that the mysterious reversals may produce major changes in the earth's topography. Now it seems that there may be even more far-reaching effects. A group of Columbia University scientists has collected evidence suggesting that the flipping field may also play a major role in the evolution of terrestrial life.

Working with 32-to 65-ft.-long sedimentary cores taken from the bottom of the North...