Earth's Tilt Spawns Rise and Fall of Species

Mammal species do not seem to last very long in the grand scheme of things, persisting for an average of 2.5 million years, according to the fossil record. By studying the fossilized teeth of rodents over a span of 22 million years, Jan van Dam of Utrecht University in the Netherlands and his colleagues confirmed this cycle of rodent species rise and fall. But they also found that it closely matched variations in Earth's orbit--and may have definitively linked the two. If you have seven or eight of these in a row all showing the same pattern, it becomes extremely unlikely that it can be reduced to chance, explains paleoecologist Paul Olsen of Columbia University.