Humanity's roots may lie in single, diverse genus.

Six newly discovered fossil teeth from the hominid Ardipithecus, which lived in eastern Africa more than 5 million years ago, have sharpened the scientific debate about our evolutionary origins.

Analyses of the 5.6-to-5.8-million-year-old specimens indicate that they belonged to a previously unidentified species, which anthropologist Yohannes Haile-Selassie of the Cleveland Museum of Natural History and his colleagues are calling Ardipithecus kadabba. Previous fossil finds from the same genus had suggested that the hominids called kadabba were instead a subspecies of the only other known Ardipithecus species, Ardipithecus ramidus (SN: 7/14/01, p. 20).

Even more provocatively, Haile-Selassie's group concludes that 6-to-7-million-year-old fossil teeth that have been attributed by other researchers to two separate hominid genera, Sahelanthropus (SN. 7/13/02, p.19) and Oworin, resemble those of Ardipithecus and probably belonged to members of that genus. That would put all of the Homo sapiens ancestors of 5 to 7 million years ago in one genus, which evolved gradually.

"It appears that the evolution of dentition in these early hominids occurred through a slow evolutionary process resulting in [anatomical] changes through time," Haile-Selassie says.

The new fossil teeth show one facet of gradual evolution, the investigators report in the March 5 Science. The upper canines curved to the outside of the lower canines and were sharpened by premolars adjacent to the lower canines. This arrangement was intermediate between that of fossil apes, as well as living chimpanzees, and later hominids.

Some other features of A. kadabba's teeth resemble those of fossil and modern apes, and still others look like those of later hominids, the researchers add.

Moreover, a foot bone found during earlier excavations of A. kadabba ends in a humanlike joint that represents a key evolutionary step toward achieving a two-legged stride, Haile-Selassie adds.

His team recovered the ancient teeth during its 2002 field season at a site in Ethiopia's fossil-rich Middle Awash region. Measurements of argon gas trapped in volcanic ash above and below the teeth yielded an estimated age for the teeth.

Early hominid finds probably represent separate lineages that migrated to eastern Africa from western Asia or from other parts of Africa, argues David R. Begun of the University in Toronto in a comment published with Haile-Selassie's report.

However, Daniel E. Lieberman of Harvard University says that if Sahelanthropus, Orrorin, and Ardipithecus belonged to a common genus, "I would not be surprised."

Michel Brunet of the University of Poitiers in France led the team that discovered remains of Sahelanthropus. He notes that comparisons among the specimens are difficult. Complete canines have been found only for A. kadabba and Orrorin, whereas a skull has been unearthed only for Sahelanthropus.

For now, "it's clearly not possible to say that Ardipithecus and Sahelanthropus are the same genus," he says.