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No. 59: Sep-Oct 1988

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PERIODIC EXTINCTIONS AND EXPLOSIONS IN TERRESTRIAL LIFE

Claims of a 26-million-year periodicity in biological extinctions have been given wide publicity recently. One theory has it that periodic comet showers have created this regular pulse beat in the history of life. A recent issue of Science presents the results of an analysis of the ups and downs of 20,000 marine genera -- that is, percent extinctions over a period of 600 million years, as revealed by the fossil record. The graph of ten groups of 1000 genera shows at least two things: (1) strong hints of periodicity; and (2) suggestions that extinctions, whatever they really are, "cut across functional, physiological, and ecological lines." The plotters of these graphs, D. Raup and G. Boyajian, claim that whatever the mechanism, "major pulses of extinction result from geographically pervasive environmental disturbances." What besides powerful, external physical forces (read "comets and asteroids") could affect such wide ranges of marine organisms? (Lewin, Roger; "Pattern and Process in Extinctions," Science, 241:26, 1988.) Comment. This all sounds so reasonable that one must wonder why it is given space in Science Frontier. The reason is that we have a suspicion that it is all too easy, too simplistic. Could something more subtle be at work? After all, we really know next to nothing about the real workings of life-as-a-whole, its ups and downs. It is so easy to say that a group of organisms was done in by a temperature change or the fall of acid rain brought on by the impact of an asteroid. We always look for external forces, whereas the real cause of "crises" in the history of life may be intrinsic to life itself.

With a tip of the hat to the Gaia hypothesis, let us think of life-as-a-whole as a most complex, interlinked system. What might be the dynamics of such a megasystem? From the mathematical point of view, many of the processes involved, as life copes with the environment, are doubtless nonlinear, which means that chaotic conditions may sometimes prevail. In fact, the graphs presented below could have been taken right out of a book on chaotic systems. Life's extinctions and explosions might have no connection to asteroids, Ice Ages, or global volcanism. If something as simple as a spherical pendulum can lapse into chaotic motion, life-as-a-whole should show occasional wild behavior, too. Take our planet's weather as another example. Idealists once thought that given enough observations and computer power, weather could be predicted. But, alas, our atmosphere has also turned out to be a chaotic system, and long-term predictions are next to impossible. So, too, with life and its development. Those extinction-explosion plots may be just the paleontological expressions of a chaotic system.

Percentage extinction vs geological time

From Science Frontiers #59, SEP-OCT 1988. 1988-2000 William R. Corliss