Oct. 12, 2006 — Climate change, naturally induced by tiny shifts in Earth's rotational axis and orbit, periodically wipes out species of mammals, a study published on Thursday says.
Paleontologists have long puzzled over fossil records that, remarkably, suggest mammal species tend to last around two and a half million years before becoming extinct.
Climate experts and biologists led by Jan van Dam at the University of Utrecht in the Netherlands, overlaid a picture of species emergence and extinction with changes that occur in Earth's orbit and axis.
The Earth's orbit is not a perfect circle: it is slightly elliptical, and the ellipticality itself goes through cycles of change that span roughly 100,000 and 400,000 years.
Its axis, likewise, is not perfectly perpendicular but has a slight wobble, rather like a poorly-balanced child's top, which goes through cycles of 21,000 years.
In addition, the axis, as schoolbooks tell us, is also tilted, and this tilt also varies in a cycle of 41,000 years.
These three shifts in Earth's pattern of movement are relatively minor compared with those of other planets.
But they can greatly influence the amount of radiation — heat and light — which Earth receives from the Sun. The effect can be amplified, causing global cooling, affecting precipitation patterns and even creating Ice Ages in higher latitudes, when two or all the cycles peak together.
Van Dam's team looked at 132 species of mammal fossils recovered from three sites in Spain, in layers of soil that dated from 24.5 million years ago to 2.5 million years ago.
What they looked for was the fossilized teeth of rodents, mainly because these could be spotted and identified much quicker than the remains of other animals.
They found that rodent species bit the dust in two regular waves.
One wave of extinction was roughly every 2.4 million years or so and the other was about every million years or so, coinciding with extremes in the cycles of ellipticality, wobble and tilt.
