1. Department of Physics and Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720, USA

Correspondence to: Richard A. Muller¹ Correspondence and requests for materials should be addressed to R.A.M. (Email: ramuller@lbl.gov).

It is well known that the diversity of life appears to fluctuate during the course of the Phanerozoic, the eon during which hard shells and skeletons left abundant fossils (0–542 million years ago). Here we show, using Sepkoski's compendium¹ of the first and last stratigraphic appearances of 36,380 marine genera, a strong 62 3-million-year cycle, which is particularly evident in the shorter-lived genera. The five great extinctions enumerated by Raup and Sepkoski² may be an aspect of this cycle. Because of the high statistical significance we also consider the contributions of environmental factors, and possible causes.

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