Mass extinction comes every 62 million years, UC physicists discover

Photo of David Perlman
Cycles of Extinction and Biodiversity. Chronicle Graphic
Cycles of Extinction and Biodiversity. Chronicle Graphic


Their findings are certain to generate a renewed burst of speculation among scientists who study the history and evolution of life. Each period of abundant life and each mass extinction has itself covered at least a few million years -- and the trend of biodiversity has been rising steadily ever since the last mass extinction, when dinosaurs and millions of other life forms went extinct about 65 million years ago.

The Berkeley researchers are physicists, not biologists or geologists or paleontologists, but they have analyzed the most exhaustive compendium of fossil records that exists -- data that cover the first and last known appearances of no fewer than 36,380 separate marine genera, including millions of species that once thrived in the world's seas, later virtually disappeared, and in many cases returned.


"We've tried everything we can think of to find an explanation for these weird cycles of biodiversity and extinction," Muller said, "and so far, we've failed."

But the cycles are so clear that the evidence "simply jumps out of the data," said James Kirchner, a professor of earth and planetary sciences on the Berkeley campus who was not involved in the research but who has written a commentary on the report that is also appearing in Nature today.

"Their discovery is exciting, it's unexpected and it's unexplained," Kirchner said. And it is certain, he added, to send other scientists in many disciplines seeking explanations for the strange cycles. "Everyone and his brother will be proposing an explanation -- and eventually, at least one or two will turn out to be right while all the others will be wrong."

Muller and Rohde conceded that they have puzzled through every conceivable phenomenon in nature in search of an explanation: "We've had to think about solar system dynamics, about the causes of comet showers, about how the galaxy works, and how volcanoes work, but nothing explains what we've discovered," Muller said.



Sepkoski's fossil record of marine life extends back for 540 million years to the time of the great "Cambrian Explosion," when almost all the ancestral forms of multicellular life emerged, and Muller and Rohde built on it for their computer version.

Muller has long been known as an unconventional and imaginative physicist on the Berkeley campus and at the Lawrence Berkeley Laboratory. It was he, for example, who suggested more than 20 years ago that an undiscovered faraway dwarf star -- which he named "Nemesis" -- was orbiting the sun and might have steered a huge asteroid into the collision with Earth that drove the dinosaurs to extinction.

"I've given up on Nemesis," Muller said this week, "but then I thought there might be two stars somewhere out there, but I've given them both up now."


Perhaps, they suggested, there's an unknown "Planet X" somewhere far out beyond the solar system that's disturbing the comets in the distant region called the Oort Cloud -- where they exist by the millions -- to the point that they shower the Earth and cause extinctions in regular cycles. Daniel Whitmire and John Matese of the University of Louisiana at Lafayette proposed that idea as a cause of major comet showers in 1985, but no one except UFO believers has ever discovered a sign of it.




Rohde, however, prefers periodic surges of volcanism on Earth as the least implausible explanation for the cycles, he said -- although it's only a tentative one, he conceded.

Said Muller: "We're getting frustrated and we need help. All I can say is that we're confident the cycles exist, and I cannot come up with any possible explanation that won't turn out to be fascinating. There's something going on in the fossil record, and we just don't know what it is."