Jaw Offers Clue to Hidden Chapter of Human Development
In the arid badlands of Ethiopia, researchers have uncovered evidence that humanity’s direct ancestors used tools 2.3 million years ago, offering a new clue to a crucial hidden chapter of human development, the Berkeley-based archeologists announced today.
Two local Afar tribesmen working for the Institute of Human Origins in the Hadar region of Ethiopia discovered pieces of a primitive human jaw that belong to the genus Homo--the family that includes all of modern humanity--amid a collection of crude stone tools and fossil mammal bones.
The find offers a rare look into one of the most mysterious periods of human development--the time when dramatic shifts in the climate of Africa may have spurred the evolution of modern humanity from its more primitive ancestors. Almost nothing is known about this critical period, when the human family of species first appeared.
“This is very important,” said F. Clark Howell, an authority at UC Berkeley on early human origins. “This opens a whole new window into a time when we think Homo is emerging, and that is very exciting.”
Experts said the discovery appears to be one of the oldest known specimens of the root stock of humankind. Only one other Homo fossil of such antiquity is known to exist.
An international team of 16 scientists describes the new find in the December issue of the Journal of Human Evolution.
The fossil jaw is at least 400,000 years older than other, more complete collections of fossils belonging to the Homo lineage, institute science director William H. Kimble said. At the same time, it is about 700,000 years younger than a more primitive pre-human species that also lived in the area.
“The critical thing here is that the earliest good record of our own lineage begins only about 2 million years ago,” Kimble said. “Before that, the record of Homo is pitiable. We are trying to fill in some voids.”
Especially provocative, the researchers said, is the fact that the fossil was discovered near ancient tools. Scattered near the jaw fragments were 20 stone flakes and several “chopping tools” fashioned from chipped river cobbles, the researchers said. When they excavated, they found 14 more stone artifacts.
There is no direct way to prove that the individual whose bones were found among the tools made or used them, but together they represent the oldest known firmly dated association of stone tools with a fossil human ancestor, the scientists said.
“It was one of the most wonderful surprises,” said Donald C. Johanson, president of the private institute that made the find public. “It is the oldest known association of our own genus with primitive stone tools.”
Other researchers, however, reserved judgment on the significance of the discovery.
“It has been known for a long time that stone tools existed at this time,” said Andrew Hill, a Yale University anthropologist involved in discovering the other known Homo fossil from this period. “It depends on what conclusions you draw from the association with tools. It is interesting, but it is very difficult to say what it tells you.”
The chipped stones all bear many of the characteristics of early human manufacture that are typical of tools found in Tanzania, Kenya and Ethiopia, which date from 1.8 million years and 2.3 million years ago. No one knows what species made them.
The researchers are not sure to which primitive human species the fossil belongs, leaving open the possibility that the jaw could be evidence of a previously unknown branch of the human family tree.
In many ways, Kimble said, the teeth and jawbones resemble those belonging to more modern human species, such as Homo habilis, but there is not enough fossil material to assign it to a known species.
“The basis for the caution is that there are a few similarities [to known species], but they are by no means overwhelming,” Kimble said. “Because of the time between the next-oldest fossil specimens--at least 400,000 years--we are awaiting additional evidence before we make a judgment.”
To date the find, researchers at the institute and the University of Toronto used a new technique of potassium argon dating, which allows them to determine the age of volcanic rocks with unusual precision. The researchers analyzed grains of volcanic ash extracted from deposits just above the layer in which the fossils and the tools were found.
Several paleontologists speculate that the evolution of the Homo lineage was triggered by global climate changes about 2.8 million years ago.
Columbia University experts who analyzed ocean sediments off the coast of Africa determined recently that the cradle of humankind in East Africa was rocked three times by abrupt cycles of colder, drier climate that coincide with major events in human evolution.
“The time period between 2 million years ago and 3 million years ago has been fingered as a time when global change may have influenced the evolution of a variety of lineages, including our own,” Kimble said. “We do indeed find at the time horizons that yielded the jaw a definite change in conditions.”
