Bird's-eye view of early primate scene.

Bird's-eye view of early primate scene

Scientists have confidently projected an estimate of what the climate was like around 30 million years ago at the site of the earliest known human and ape ancestors, thanks to a small collection of bird fossils from the same deposit. The analysis of the 25- to 30-million-year-old birds in the Sept. 12 SCIENCE indicates that they inhabited a tropical swampland bordered by rain forests and patches of grassland.

A 1982 study of sediments, plant fossils and primate skeletons from the Egyptian site, known as Fayum, suggested that 30-million-year-old precursors of modern humans and apes spent most of their time in the trees of dense forests with heavy seasonal rainfall. This scenario contrasted with a British scientist's controversial assertion in 1980 that Fayum primates were ground dwellers in a semi-arid, treeless scrubland.

The former description is the one that flies when bird fossils are taken into account, say Storrs L. Olson of the Smithsonian Institution in Washington, D.C., and D. Tab Rasmussen of Duke University in Durham, N.C. Unlike Fayum mammal fossils, which belong to largely extinct species, most of the 30 bird specimens from the site are clearly related to living families of birds, and several may be closely related to living species, Olson and Rasmussen find. Fossils with modern counterparts include those of jacanas, shoebilled storks, herons, cranes, cormorants, ospreys, eagles and other birds that rely on areas of open water. Their habitats today overlap only in Uganda north and west of Lake Victoria, say the researchers, where the climate is tropical, with stable rainfall throughout the year. The area is bordered by forests, open woodlands and grasslands.

"This evidence fits hand-in-glove with other evidence from Fayum that it was once made up of forests and flood-basins," says paleonthropologist Elwyn L. Simons of Duke University, who participated in the 1982 report on the site's ancient climate. There is not enough evidence, he adds, to say for certain whether rainfall came in monsoonlike bursts or was spread throughout the year. "Modern Uganda probably isn't exactly like ancient Fayum," notes Simons.

Nevertheless, bird fossils are an underestimated resource for investigators of early human and ape environments, says Olson. For the period from around 40 million years ago to 20 million years ago, he contends, bird specimens with modern counterparts are better reflections of the climate and habitat than the more abundant remains of now-extinct mammals.