Our family tree does the splits ... (Anthropology).

Anthropologists who study humanity's fossil ancestors fall into two general groups. Splitters tend to interpret skeletons with pronounced shape differences as belonging to separate species; lumpers often regard such disparities as anatomical variations within a single species.

Standing before a packed conference room, Meave G. Leakey of the National Museums of Kenya in Nairobi defended her status as the newly crowned queen of the splitters. She and her coworkers have unearthed a 3.5-million-year-old skull that, in their view, represents not only a new species in the human evolutionary family, but also a new genus (SN: 3/24/01, p. 180). Leakey's group dubbed the ancient skull Kenyanthropus platyops, the first known member of a group of related species that may have spawned our own genus, Homo.

"We didn't propose this new genus lightly or quickly," Leakey said. "This skull just didn't fit into any other known genus."

Kenyanthropus would have existed at the same time as the well-established hominid genus Australopithecus. "If we had assigned this new skull to Australopithecus, it would have made that genus a dumping ground," Leakey contended.

The splitters' naming of ever more early hominid species builds on evidence that East African habitats changed dramatically from 7 million to 5 million years ago and fostered the evolution of many animals, Leakey notes. For example, skeletal remains at Kenya's Lothagam site document widespread extinctions during that period and the rise of ancestors of animals such as giraffes, rhinoceroses, and antelope.

This massive evolutionary turning point coincided with the spread of drier habitats dominated by grasslands interspersed with wooded areas, according to Leakey.

In related work, described by Rene Bobe of the Smithsonian Institution in Washington, D.C., excavations in Ethiopia's lower Omo Valley indicate that another major turnover of animal species occurred around 2.5 million years ago. That's the time when Australopithecus disappeared and two other hominid genera--Homo and Paranthropus--made their debut.

Thereafter, marked transitions in the types of animals found in Omo sediments occurred every 100,000 years, Bobe says. A series of environmental changes probably fostered these species extinctions and emergences, he holds (SN: 7/12/97, p. 26). He also argues that the same changes contributed to the evolution of a handful of Homo species prior to modern Homo sapiens.--B.B.