Image: NANCY KLAUD, Courtesy of the Field Museum

The first primates may have scampered among the trees while dinosaurs still ruled the planet, according to the results of a new study. Researchers writing today in the journal Nature conclude that primates--the order of mammals to which humans belong--arose more than 15 million earlier than previously thought. In addition to posing difficulties for widely held ideas about the emergence of this group, the findings could force scholars to rethink the timing of the origin of humans.

Conventional wisdom holds that primates arose no earlier than 65 million years ago (after the demise of the dinosaurs), a date based on the oldest accepted fossil representatives of the group, which hail from roughly 55 million years ago, plus a few million years thrown in for good measure. The problem with that approach to estimating the timing of primate origins, says study co-author Robert D. Martin of Chicago's Field Museum, is that the early primate fossil record is so scrappy. "Our calculations indicate that we have fossil evidence for only about 5 percent of all extinct primates," he reports, "so it's as if paleontologists have been trying to reconstruct a 1,000-piece jigsaw puzzle using just 50 pieces." Indeed, estimates Martin and his colleagues have come up with using a statistical approach place the number of extinct primate species at up to 9,000--many more than the 474 species known from the fossil record.

Taking that into consideration, the last common ancestor of the primates--today represented by lemurs, lorises, tarsiers, New World monkeys, Old World monkeys, apes and humans--probably lived around 81 million years ago, the team proposes. As to why such ancient primates have not turned up in the fossil record, it may be that their remains simply did not have the conditions necessary for preservation. But Martin guesses that the common ancestor was small, nocturnal, and dwelled in tropical forest trees, feeding on fruit and insects (see image).

The new date, although at odds with previous paleontological estimates, actually accords fairly well with conclusions drawn from molecular studies, which have indicated that primates diverged from other mammals some 90 million years ago. It also suggests that humans and chimpanzees parted evolutionary ways earlier than once thought--eight million rather than five million years ago. "We hope our research will help reconcile the discrepancies between the various dates suggested by paleontologists and molecular biologists," Martin remarks, "not just for primates but for other groups of organisms, too."