Fossil primates emit elusive species clues.

Fossil primates emit elusive species clues

Paleoanthropologists who attempt to decipher the evolutionary history of humans and other primates, express increasing skepticism concerning their ability to identify long-extinct species from fossil evidence alone. New glimpses of the pitfalls of trying to squeeze primate species out of bone emerged last week at the American Association of Physical Anthropologists' annual meeting in Milwaukee.

Even the concept of "species" provokes dispute among investigators. Many assume a species consists of organisms that look alike and can mate to produce fertile offspring. But skeletal anatomy often changes rapidly in response to environmental influences, making a simple list of skeletal traits unreliable as a guidepost to species recognition, asserts William H. Kimbel of the Institute of Human Origins in Berkeley, Calif.

Moreover, the primate fossil record -- largely made up of partial skulls and teeth -- often yields underestimates of the number of related species represented in a collection of bones, says Ian Tattersall of the American Museum of Natural History in New York City.

Tattersall and Jeffrey Schwartz of the University of Pittsburgh examines 77 skulls from the seven modern lemur species making up the genus Lemur. Each species has distinctive external features, Tattersall notes. But identifying the species on the basis of skulls and teeth alone proved extremely difficult, he reports. In fact, Tattersall contends that most investigators would classify no more than three species in this skeletal sample. The major problem: Different lemur species share numerous anatomical features of the head and teeth that apparently evolved independently, thus shrouding the boundaries between species.

"The genus is the [meaningful] category with regards to teeth and crania, Tattersall says.

He argues that cranial and dental analyses may have led to an inappropriate lumping together of separate species of hominids, the evolutionary family that includes modern humans. For instance, he holds that fossils classed as Homo erectus -- a hominid species that lived in Africa and Asia from about 1.6 million to 300,000 years ago -- actually encompass several species, only one of which represents a direct human ancestor (SN: 4/25/87, p.264).

Terry Harrison of New York University concurs with Tattersall's cautions about deriving species from bones, but he sees no reason to split up H. erectus. The widespread reliance on primitive and advanced skeletal features to establish evolutionary connections between groups -- an approach known as cladistics -- best identifies broad evolutionary levels, such as the family of hominids, Harrison says. But cladistics often cannot resolve controversies about genera or species within an evolutionary family, he maintains. The limited fossil record and the parallel evolution of similar features in related species -- as documented among lemurs by Tattersall and Schwartz -- often confuse evolutionary reconstructions, Harrison says.

To date, the far-flung fossil remains of H. erectus have shown no more anatomical variation than modern human populations, supporting the traditional view of H. erectus as a single species, he adds.

Body-size differences between the sexes also impede the identification of fossils from ancestral primate species, points out Jay Kelley of Brown University in Providence, R.I. Proposed sex differences in size have long stoked controversy over the earlier known hominid species, Australophithecus afarensis. Many anthropologists classify A. afarensis as one species, with males considerably larger than females. Others claim the size differences reflect separate species (SN: 3/23/91, p.182).

Similar arguments plague assessments of fossil apes dated at between 5.5 million and 23 million years old, Kelley notes. In his analysis of the only good population sample of a fossil ape from that time period, Kelly concludes that marked size differences between the sexes "probably characterized many early primate species to a much greater extent than modern primates."

He compared several hundred teeth belonging to Lufengpithecus, which lived about 7 million years ago in China, with the teeth of modern orangutans, which display large sex differences in body and tooth size. The fossil ape's cheek teeth fell into two groups with more striking size differences than those observed among orangutans, Kelley reports. Noting that sex contrasts in tooth size closely correspond to sex differences in body size among all living primates, he argues that Lufengpithecus represented a single species in which these gender disparities exceeted the modern limit.