Researchers working in Ethiopia have unearthed the remains of a creature thought to occupy a spot on the family tree quite close the evolutionary branching point between humans and chimpanzees. According to a report published today in the journal Nature, these remains belong to a new subspecies of the hominid genus Ardipithecus, previously known fossils of which date to 4.4 million years ago. The new fossils, however, come from sediments dated to between 5.2 and 5.8 million years ago, and thus may represent the earliest known human ancestor.

Yohannes Haile-Selassie, a doctoral candidate at the University of California at Berkeley, discovered the fossils in Ethiopia's Middle Awash study area, a locale famous for yielding other key fossils, such as the 4.4 million-year-old Ardipithecus ramidus and the 2.5 million-year-old Australopithecus garhi. Subsequent analyses of the fragmentary remainswhich include a jawbone with teeth; hand, foot and arm bones; and a piece of collar bonerevealed a bipedal creature with teeth similar to those of later hominids. These teeth and certain skeletal features are more primitive than those belonging to Ardipithecus ramidus, however, thus leading Haile-Selassie to designate the fossils as a new subspecies, Ardipithecus ramidus kadabba.

A.r.kadabba is not without competition for the distinction of being the earliest human ancestor. Earlier this year a French team unveiled the 6 million-year-old remains of Orrorin tugenensis, a new species that the discoverers regard as hominid. (These researchers consider Ardipithecus a chimpanzee ancestor.) For his part, Haile-Selassie asserts that more information is required to resolve Orrorin's place in the family tree. And with excavations resuming in the Middle Awash this fall, more clues to our elusive evolutionary past may yet come from Ardipithecus.