ASIA was the mammalian Garden of Eden where animals such as primates and the ancestors of horses and antelopes evolved before spreading east to conquer the world.
The idea that Asia may have been a cradle of mammalian evolution has been popular with palaeontologists. But the fossil record only reveals that the distinctive mammals of the Palaeocene epoch, which began 65 million years ago, were replaced across the northern hemisphere around 55 million years ago by animals more akin to those we see today. How and why the more modern mammals came to dominate has remained a mystery.
Now an international team of researchers has started to resolve this puzzle. They have identified an abrupt change in carbon isotope ratios in fossil-rich rocks from China鈥檚 Hunan province. The same pattern is well known in North America, where it marks the boundary between the Palaeocene and Eocene epochs, some 55 million years ago. About 10,000 years after this transition, a wave of new mammals appeared in the Bighorn Basin of Wyoming.
Advertisement
Crucially, the change in isotope ratios in China reveal that there was a short spell of dramatic global warming at the time. That would have provided a brief opportunity for warmth-loving mammals to travel east.
The evidence is clearest for an extinct group of dog-like predators called hyaenodontid creodonts, reports team member Gabriel Bowen, a graduate student at the University of California at Santa Cruz (Science, vol 295, p 2062). Hyaenodontids first appear in China before the carbon isotope peak, but only appear in North America after the isotope peak. Fossils of the first known primates鈥攁s well as ungulates such as horses with an odd number of toes on each foot, and even-toed ungulates such as deer and cows鈥攁ppear in Asia around this time of global warming, and in the Americas a little later, suggesting they also migrated out of Asia.
Chris Beard of the Carnegie Museum of Natural History in Pittsburgh, who comments on the team鈥檚 findings in an accompanying paper in Science, says it鈥檚 likely that there were several waves of migration. Rodents reached North America about a million years before a larger wave of immigrants, including hyaenodontids, primates and ungulates, which arrived during a time of extreme warmth.
Climate researchers believe the oceans released large amounts of methane, causing a strong but short-lived greenhouse effect that warmed polar regions by as much as 20 掳C. That would have made the climate of land bridges from northern Asia to North America and Europe more hospitable, allowing the small mammals of the time, such as horses the size of cats and early primates the size of small lemurs, to cross.