"Wobbles in the Earth's orbit may explain the puzzling regularity with which
new mammalian and other species appear and vanish in the fossil record,
a study has found.
Mammalian species tend to survive for an average of 2.5 million years before
being snuffed out, said Jan van Dam of Utrecht University, Netherlands
and colleagues. The team of paleontologists and geologists surveyed
about 22 million years' worth of fossil data.
They found that peaks of species turnover—periods in which many
species go extinct, to be replaced by new ones—seem to
correspond to changes in Earth's orbit, which cool the planet.
The group studied the fossil record of rodents in Spain, which they said
provides a detailed account of when these species rose and fell.
Writing in the Oct. 12 issue of Nature, they argued that turnover rates
showed a complex pattern consisting of two different cycles.
One, longer, has peaks roughly every 2.5 million years; the second peaks
every million years.
Their timing mirrors oscillations in the Earth's behavior,
they added. The 2.5-million-year peaks occur when the Earth's orbit
is closer to being a perfect circle, and the million-year peaks come when
the Earth is shifting its degree of tilt on its axis.
Both processes result in ice-sheet expansion, global cooling
and changes to precipitation patterns, the researchers argued.
"The astronomical hypothesis for turnover offers a plausible explanation
for the characteristic duration of the mean species life span in mammals,
and may explain similar durations in other biological groups" they wrote.
Although the researchers did not venture a prediction of when the next
peak in turnover rates would be, extrapolation from the historical figures
they provided suggests it would come in slightly under 600,000 years."