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Earliest Remains of Genus Homo Volume 50 Number 1, January/February 1997
by Angela M.H. Schuster

[image] A 2.33 million-year-old upper jaw and "chopper tools" from Ethiopia (William H. Kimbel/IHO) [LARGER IMAGE, 40K]

Paleoanthropologists surveying a barren hillside in the Afar region of northern Ethiopia have discovered a 2.33 million-year-old upper jaw belonging to the genus Homo, pushing back the human timeline by more than 400,000 years. Within the same strata, researchers found more than 20 chert flakes and several "chopping tools," river cobbles chipped on two faces--the oldest stone tools ever found in association with hominid remains.

A team led by William H. Kimbel of the Berkeley-based Institute of Human Origins (IHO) discovered the jaw and artifacts at Hadar, the same site where the 3.18-million-year-old Australopithecus afarensis "Lucy" was found more than two decades ago. The new finds shed light on a time period froms 2.0 to 2.5 million years ago that is poorly understood in the paleontological record. Until this discovery, the earliest securely dated Homo fossils were those found at Olduvai Gorge, Tanzania, and Koobi Fora, Kenya, in the 1960s and 1970s. Known as H. habilis and H. rudolfensis, respectively, these fossils date to approximately 1.9 million years ago.

Found by Ali Yesuf and Maumin Alahandu, the jaw had been broken in two. "When the two halves of the maxilla were put together," says Kimbel, "we knew we were not dealing with an apelike Australopithecus." The new jaw is short with a broad parabolic dental arch, similar in form to that found in younger ancestors of modern humans. Because the jaw is sufficiently different in morphology from other known early Homo fossils, however, Kimbel and his team have yet to assign it to a species.

The artifacts resemble the "Oldowan" tool assemblages known from Tanzania, Kenya, and Ethiopia, which date to between 1.80 and 2.35 million years ago. IHO geochronologist Robert C. Walter determined the age of the newly discovered jaw and its associated tools by the single-crystal-laser-fusion method of potassium-argon dating, which is ideal when working with volcanic rocks. There are approximately 20 layers of volcanic ash within the more than 600-foot-thick Hadar Formation, making it possible to obtain highly accurate dates.

"We returned to Hadar in 1990," says Kimbel, "hoping to trace the evolution of A. afarensis and better understand what happened between the demise of that species and the emergence of the Homo line." Abundant antelope fossils found in association with the jaw and tools suggest that the Afar region was an open grassland 2.33 million years ago, a marked contrast to the area's dense woodland environment during Lucy's time. Since the transition from Afarensis to Homo took place between 2.33 and 2.9 million years ago, some researchers have suggested that such climate change may have triggered rapid evolutionary adaptation.

© 1997 by the Archaeological Institute of America