The quandary arose two years ago, when a Nobel Prize-winning physicist and his geologist son startled the scientific community. They produced compelling evidence that the mighty dinosaurs had become extinct—
not from gradual, natural processes, but because a huge asteroid had crashed into the Earth. And now a new study indicates that similar mass extinctions, whether sudden or more gradual, happened regularly during the Earth’s history, possibly every 26 million years. “Until now, people have studied one extinction event,” said John Sepkoski, a University of Chicago paleontologist and coauthor of the new study, to be published next month in the U.S. Proceedings of the National Academy of Sciences. But his studies included fossils “covering the past 250 million years” and revealed evidence of cataclysmic events occurring in cycles.
Since Sepkoski discussed his preliminary data in August, some eager astronomers have searched the heavens for evidence of a major event—such as the solar system moving through galactic debris or passing near another star—that recurs every 26 million years. If the exploration bears fruit, or if scientists can substantiate the Chicago study in other ways, the way in which mankind views the evolution of life on Earth may change irrevocably.
In searching for that explanation, scientists cannot ignore the asteroid theory about the demise of the dinosaurs. In 1980, when a team of scientists from the University of California at Berkeley—including geologist Walter Al-
When Sepkoski began his study eight years ago, he was interested only in fluctuations in the diversity of animal types over the ages. He hoped that his findings would explain apparent changes in the rate of evolution. But eight months ago Sepkoski’s colleague at the University of Chicago, paleontologist David Raup, noted what appeared to be a “periodic pulse” in the fossilized evidence of animal deaths. By applying various methods of computerized statistical analysis to established fossil counts, they concluded that large-scale extinctions took place every 26 million
years. According to that calculation, the next rash of extinctions should occur in 15 million years. Said Sepkoski: “We were very surprised. We had assumed that each extinction period would be unique and that the periods would occur randomly.”
Scientists have long known of such
events. The death of the dinosaurs at the end of the Cretaceous period 65 million years ago is the most dramatic, because the victims were among the most interesting animals that ever roamed the Earth. But other extinctions have been more severe. Fossil evidence shows that roughly 250 million years ago more than 90 per cent of the species in the ocean became extinct. In all, Sepkoski and Raup claim to have found 10 almost equally spaced extinction periods. Concluded Sepkoski: “Our results suggest that there is some underlying principle,
some simple explanation for the pattern.”
varez and physicist Luis Alvarez-presented their catastrophic extinction theory, colleagues greeted it with skepticism. But since then many other scientists have studied the Berkeley data and conducted their own analyses, and the idea has gained credence. The Alvarez theory suggests that 65 million years ago, when dinosaurs were abundant, successful and diverse, many forms of life ended abruptly when an object at least 10 km wide collided with the Earth. The impact, according to the theory, tossed enough debris into the atmosphere to block out sunlight for many months, preventing plant growth and therefore depriving animals of food. The disappearance of dinosaur fossil evidence in rock sediments coincides with the appearance of an unusual layer of rock bearing exceptionally high concentrations of rare elements usually associated with comets and certain kinds of asteroids and meteorites. Sophisticated analyses of the so-called “Cretaceous-Tertiary boundary” by the Berkeley scientists and, since then, by other groups show as much as 30 times the normal amounts of the trace elements iridium ánd osmium.
Such evidence has convinced Arizonabased geologist Eugene Shoemaker, for one, that there is a connection between large meteoritic impacts and catastrophes on Earth. Added Shoemaker, a well-known authority on asteroids and comets: “I don’t think the one at the end of the Cretaceous period is the only one;
I think there were lots of them.” In fact, the Alvarez group has found similar iridium abundances at another extinction period, at the end of the Eocene epoch, 35 million years ago. Said Walter Alvarez: “The Sepkoski-Raup data, if it is
true, does not conflict with our findings at all.”
But, even if both the asteroid hypothesis and the periodic extinction pattern are true, scientists still must find a common cause. In 1980, when the Alvarez group first presented its results, Victor Clube and William Napier, astronomers at the Royal Observatory in Edinburgh, pointed out that every 100 million years the solar system passes through giant molecular clouds, possibly picking up a new supply of comets each time. That encouraged paleontologists to look for extinctions 100 million years apart. But now, with the apparent pattern of 26 million years to work with, scientists are looking for other extraterrestrial cycles as well. Richard Schwartz, an astronomer at the University of Missouri in St. Louis, is working on a report arguing that the extinction periods might coincide with movements of the sun—and the solar system—up and down in the plane of the galaxy. His theory involves a time frame of approximately 32 million years, instead of 26 million years. Schwartz argues that the asteroid impact at the time of the demise of the dinosaurs is probably a coincidence.
The Berkeley and Chicago studies are prompting scientists to ask questions that might have seemed absurd a decade ago, and to view the history of life on Earth from a new vantage point. Said Sepkoski: “Our study is another indication that we are not alone on this Earth, that we are influenced by processes beyond its bounds.” Increasingly, scientists are considering the traditional Darwinian idea, involving a leisurely evolution as species succeed by adapting to the environment, as only part of the total picture. “Most of the time the Darwinian mechanism of evolution operates,” said Walter Alvarez. “But during mass extinctions, very successful species often do not survive. Then, those lucky enough to make it evolve and diversify rapidly, filling in the empty niches.” In Ottawa, Dale Russell, curator of fossil vertebrates at the National Museum of Natural Sciences, has taken a unique approach. He believes that evolution would have produced intelligent creatures from the dinosaurs if a catastrophe had not occurred. With the help of a taxidermist, Russell built a five-foot Fiberglas model of such a creature, looking remarkably like a human being. Concluded Russell: “It looks as if the actors in the drama might change after an extinction event, but the story would remain the same.” If that is true, human beings can ultimately thank the jolt of an errant asteroid, or some cyclical extraterrestrial event, for allowing them to step into their role as Earth’s most intelligent creatures.
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