A publication of the Archaeological Institute of America
Newly discovered fossils found in Chad and the Republic of Georgia are raising questions about the chimpanzee-human evolutionary split and why early humans spread from Africa into Eurasia.
Michel Brunet of the University of Poitiers and his colleagues have announced the discovery of a cranium, jaw fragments, and teeth at a desert site in northern Chad. The fossils have mixed characteristics, more human in the facial region and teeth, but more ape-like in the braincase. Brunet, who named them Sahelanthropus tchadensis, after the Sahel region where they were found, says they are from a creature in the hominid (human) line rather than that of the apes.
The discovery demolishes the accepted date of the last human-chimpanzee common ancestor, thought to be between 5 and 7 million years ago. "If Sahelanthropus really is 6-7 million years old and does, indeed, show distinctive hominid features," says Robert Martin of The Field Museum, "the actual time of divergence...must have been even earlier, probably between 8 and 10 million years ago."
Sahelanthropus also forces a rethinking of our family tree, which most scholars now view more as a "bush" with many branches. "If the new find has been properly interpreted," says Martin, "it indicates that certain advanced hominid features had been developed in at least one lineage well before the first known australopithecine [the group to which Lucy belongs]. This increases the likelihood that the australopithecines are, in fact, on a peripheral side-branch of the hominid bush." But Martin cautions that, "Even with this latest find of Sahelanthropus, we are far from having a complete picture."
Another team of scholars has turned up a Homo erectus cranium at Dmanisi, 50 miles from the Georgian capital of Tbilisi. The earliest human to depart Africa, H. erectus spread into Eurasia, reaching as far east as Indonesia. The new fossil, along with two crania found in 1999 and several jaw fragments, dates around 1.75 million years ago.
The newest cranium held a brain estimated at only about 600 cubic centimeters (37 cubic inches), one-quarter smaller than the other two. Team member David Lordkipanidze of the Georgian Academy of Sciences says this may undermine the notion that large brains were a driving force in early human dispersal from Africa. He believes other factors--anatomical and environmental--may have been involved. That may be true, but Martin notes that there is a natural range of brain size in every species and that "for primates generally, including ourselves, the smallest brains are about half the size of the largest."