Weebles wobble but never fall over. Apparently so does the Earth. From the New York Times today, one more possible piece in the puzzle:
October 12, 2006
Study Links Extinction Cycles to Changes in Earth's Orbit and Tilt
By JOHN NOBLE WILFORD
If rodents in Spain are any guide, periodic changes in Earth's orbit may account for the apparent regularity with which new species of mammals emerge and then go extinct, scientists are reporting today.
It so happens, the paleontologists say, that variations in the course Earth travels around the Sun and in the tilt of its axis are associated with episodes of global cooling. Their new research on the fossil record shows that the cyclical pattern of these phenomena corresponds to species turnover in rodents and probably other mammal groups as well.
In a report appearing today in the journal Nature, Dutch and Spanish scientists led by Jan A. van Dam of Utrecht University in the Netherlands say the "astronomical hypothesis for species turnover provides a crucial missing piece in the puzzle of mammal species- and genus-level evolution."
In addition, the researchers write, the hypothesis "offers a plausible explanation for the characteristic duration of more or less 2.5 million years of the mean species life span in mammals."
Dr. van Dam and his colleagues studied the fossil record of rats, mice and other rodents over the last 22 million years in central Spain. The fossils are numerous and show a largely uninterrupted record of the rise and fall of individual species. Other scientists say rodents, thanks to their large numbers, are commonly used in studies of such evolutionary transitions.
As the scientists pored over some 80,000 isolated molars, the most distinct markers of different species, the patterns of turnovers emerged. They seemed often to occur in clusters, which seemed unrelated to biology. And they occurred in cycles of about 2.5 million and 1 million years.
The longer-term cycle, the scientists determined, peaks when Earth's orbit is closer to being a perfect circle. The short cycle corresponds to shifts in the tilt of Earth's axis. The "pulses of turnover," the scientists determined, occurred mainly at times when the different cycles left Earth a colder world.
Previous studies have invoked climate change to explain mammalian species turnover, but they have been challenged or only partly supported by other research.
Paleontologists and mammal experts not involved in the research said the findings and interpretations were provocative and likely to inspire other investigations. One objective, they said, was to extend the study to small mammals beyond Spain, preferably to other continents.
"It's very intriguing," said John J. Flynn, a paleontologist at the American Museum of Natural History in Manhattan. "But this will be controversial. Any time you invoke periodic and external forces to explain patterns in biology and climate, it stirs up controversy."
Dr. Flynn said some recent research had led other scientists to conclude that there was no strong correlation between climate changes and species turnover.
While scientists go off looking for fossil rodents outside Spain, there is no apparent cause for concern that another species turnover is nigh.
Dr. van Dam said the 2.5-million-year cycle "has entered the critical stage corresponding to a relatively circular orbit." But any period of high turnover may be tens of thousands of years away, he said. And it may be good news for both mice and men that the climate system has changed significantly in the last three million years.
Ever since the establishment of the northern ice cap, Dr. van Dam said, the climate system has been reacting differently, as reflected in the succession of ice ages. "So it is not easy to predict what the 2.5-million-year cycle will do," he said.
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