for National Geographic News
Robert Rohde and Richard Muller are vexed. For the past 542 million years the number of animal species living in the world's oceans has risen and fallen in a repeating pattern, and the scientists haven't the foggiest idea why.
"I wish I knew what it all meant," said Muller, who is a professor of physics at the University of California, Berkeley.
The pattern includes a rise and fall of marine animal diversity every 62 million years and a weaker cycle of rising and falling marine diversity, which repeats every 140 million years. The researchers think that expanding and retreating glaciers may explain the 140-million-year cycle, but they are stumped over what drives the 62-million-year cycle.
The declines in the 62-million-year cycle correspond with some of the best known mass extinctions on Earth.
Among them are the die-off caused by the asteroid or comet widely believed to have doomed the dinosaurs 65 million years ago and the "Great Dying" of 250 million years ago. During the Great Dying, some unknown cause wiped out most life on Earth.
The patterns found in the new study identify five additional declines in marine-animal diversity. Muller and his graduate student Rohde say the pattern is too regular to occur by chance. But they have failed to find a plausible explanation for its existence.
The pair looked for a pattern of asteroid and comet impacts, global climate shifts, volcanic eruptions, fluctuating sea levels, changes in the total amount of plant and animal life, and the reshuffling of the continents. None fit.
Muller went so far as to purchase a lava lamp, plug it in, and studiously gaze at how often the blobs rose to the surface. He thought perhaps magma bubbles from the Earth's core rise in a cyclical pattern. "It's not regular enough," he said.
The researchers' report will appear in tomorrow's issue of the science journal Nature.
Rohde and Muller started with Sepkoski's compendium, a database of every marine-animal fossil ever found. Then they correlated this database with the dates of when the species appeared and disappeared from the fossil record. The dates were gleaned from the International Commission on Stratigraphy's 2004 timescale.
"We took all the best data and put it with the best timescale. It's not what we were looking for, but [Rohde] plotted it up and there it was. We've been wrestling with it ever since," Muller said.
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