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Life Cycles

Are there periodic booms and busts in the diversity of life on Earth? Hear a tale of fossils and Fourier transforms

Brian Hayes

Life has its ups and downs. Half a billion years ago, the sudden proliferation of hard-bodied creatures in the Cambrian period was an uptick; the mass extinction that wiped out the last of the dinosaurs at the end of the Cretaceous was a major downer.

Diversity of living organisms...Click to Enlarge Image

Several students of the history of life have suggested that these peaks and valleys have a regularity to them—that they are not just random fluctuations but periodic oscillations, possibly synchronized to some external pacemaker. In the 1970s Keith Stewart Thomson, then of Yale University (and now a columnist for this magazine), noted surges in the diversity of various animal groups at intervals of 62 million years. Then Alfred G. Fischer and Michael A. Arthur of Princeton University suggested that extinctions come in waves every 32 million years. Later, David M. Raup and J. John Sepkoski, Jr., of the University of Chicago offered a revised mass-extinction timetable with a period of 26 million years. Now there's yet another sighting of cyclic tides in biodiversity, this time with super-imposed wavelengths of 62 million years (again!) and 140 million years. The new report comes from Richard A. Muller, a physicist at Lawrence Berkeley National Laboratory, and Robert A. Rohde, a graduate student in physics at the University of California, Berkeley. Their analysis was published in Nature this past March.

To a naive observer, the sheer variety of these proposals invites a certain skepticism. If there's a loud and steady drumbeat in the history of life, shouldn't everyone hear the same rhythm? On the other hand, if the signal is faint and has to be teased out of a noisy background, could we be perceiving patterns in what is really random noise? Just how do you go about detecting such an oscillation, and how do you know whether or not it's real?

A reading of the various claims and counterclaims on periodicity in the fossil record did not answer these questions for me. I felt an urge to explore the data for myself, to see just how much teasing it needs. Some years ago, such an undertaking would have been unthinkable for anyone but insiders and experts—and I am neither. But computational science is a great equalizer. The tools and data are now widely available. The mathematics required is not too daunting. Muller and Rohde have posted a detailed and very helpful technical supplement—almost a how-to manual—on the Nature Web site. If you're willing to write a few programs, you too can create mass extinctions on your home computer.

Of course access to tools and data does not guarantee the skill to use them well—as I shall demonstrate forthwith.

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