THE diversity of animals living in the oceans rises and falls in 62-million-year cycles. But whether the fluctuations represent something intrinsic to biological evolution or are an aspect of fossilisation remains a mystery.
Robert Rohde and Richard Muller of the University of California at Berkeley used a pre-existing fossil database to count how many genera of marine animals were alive during the past 540 million years. They found a strong 62-million-year cycle of fluctuating diversity (Nature, vol 434, p 208).
The cycles do not coincide with geophysical cycles such as those affecting climate, volcanism or sea level. Nor do…
