Older ancestors: primate origins age in new analysis. (This Week).

The evolutionary roots of primates, the group of mammals that gave rise to humans, are murky. Paleontologists generally think that the first primates appeared about 65 million years ago, whereas genetic analyses of the DNA from living primates yield an estimate of 90 million years.

That DNA-derived number comes closer to the mark, according to a research team led by biologist Simon Tavare of the University of Southern California in Los Angeles. Using a new statistical model of primate evolution, the scientists conclude that the oldest common ancestor of today's primates lived approximately 81.5 million years ago.

"We hope our method reconciles discrepancies between the dates suggested by paleontologists and molecular biologists," says study coauthor Robert D. Martin, a paleontologist at the Field Museum in Chicago.

In their model, the entire primate evolutionary tree is reconstructed from several inputs. First, they considered the minimum number of primate species alive today, 235. Then, they entered quantitative descriptions of anatomical diversity in these animals and known fossil primate species from different time periods, starting with the oldest undisputed finds, dated at 55 million years old. Finally, they assumed an average duration of 2.5 million years for each species.

From these data, the model yields estimates of the length of time between the oldest known fossil and the earliest common ancestor of a given group of species. The researchers describe the technique in the April 18 Nature.

The common ancestor of primates, the model suggests, arose before dinosaurs disappeared. This ancestor resembled a modern dwarf lemur, weighing 1 to 2 pounds, and led a nocturnal life in tropical forests, the scientists theorize. Early primates therefore would have expanded northward from tropical areas, rather than originating in northern regions, as many researchers now assume.

Philip D. Gingerich, a paleontologist at the University of Michigan in Ann Arbor, says that the absence of primate fossils older than 55 million years places the scenario devised by Tavare's group into the category of sheer speculation.

Richard F. Kay, a paleontologist at Duke University Medical Center in Durham, N.C., is less dismissive, but he's still skeptical. "It's a surprisingly old age estimate," Kay says. "I wouldn't reject this new model out of hand, but I'd be very cautious about it."

Tavare's group asserts that the dearth of old fossils reflects unfavorable conditions for bone preservation in tropical habitats. In fact, they say, their model indicates that extinct primate species number between 8,000 and 9,000, far more than the 474 identified so far.