Around 55 million years ago, Earth's climate underwent a short but intense bout of global warming, accompanied by dramatic shifts in plant and animal life. Of particular interest to paleontologists is the sudden appearance in the Northern Hemisphere of several groups of mammals--including the primates, the even-toed hoofed mammals and the odd-toed hoofed mammals--at this so-called Paleocene/Eocene boundary. Researchers have long wondered exactly where these creatures, which differed considerably from the animals they replaced, came from. Findings published today in the journal Science indicate that at least one key group, and probably others, originated in Asia.

Working with samples obtained from southern China's Hengyang Basin, an international team of scientists led by Gabriel J. Bowen of the University of California, Santa Cruz, employed a variety of techniques to constrain the ages of important Asian animal groups. Comparisons of these sequences with those from North America and Europe clearly showed that a group of doglike carnivores known as hyaenodontid creodonts (such as the one pictured here) appeared in Asia before showing up in Europe or North America. And members of the primate and hoofed mammal groups, the investigators found, emerged in Asia no later than they did in North America. "Based on the best data we have now for correlating different Asian faunas, it looks quite possible that these groups were present in Asia first," Bowen observes, "but we can't say definitely yet."

The results fit neatly with phylogenetic analyses that have identified Asia as the birthplace of several modern mammal groups, including the primate order to which we belong," paleontologist Chris Beard of the Carnegie Museum of Natural History in Pittsburgh notes in a commentary accompanying the report. "They also point toward global warming as the driving force behind the most profound biotic reorganization of the Age of Mammals."