The skull of an ape discovered on the sandblasted floor of Chad's Djurab Desert has provided a stunning view into the evolution of hominids, the lineage that includes humans but not other apes. “It is unquestionably one of the great paleontological discoveries of the past 100 years,” says paleoanthropologist Daniel Lieberman of Harvard University.
The six new fossils, which include a nearly complete cranium, two lower jaw fragments, and three teeth, show a unique combination of features, prompting paleontologist Michel Brunet of University of Poitiers in France and his international team to assign it to a new genus and species, Sahelanthropus tchadensis. It was also found in an unexpected place: the ancient shore of Lake Chad in western Africa. Most other early hominid fossils have been uncovered in eastern Africa. Nicknamed Toumaï, which means “hope of life” in the Goran language of Chad, it was found in beds where shifting sand dunes have exposed thousands of fossils of crocodiles, elephants, and other creatures. Brunet's team compared the mix of extinct species from the site to fauna at other reliably dated sites in Africa and concluded that the cranium is 6 million to 7 million years old. That age pushes the limits of many molecular studies dating the split between the hominid and chimpanzee lineages to 5 million to 7 million years ago (Science, 15 February, p. 1217).
In keeping with its age, the skull looks most like that of an ancient ape, with a brain the size of a chimpanzee's, large incisors, and widely spaced eyes like a gorilla's. But the shape and size of its canines and lower face resemble those of human ancestors that came later. The partial skull, featured on the cover of the 11 July issue of Nature, fills a crucial gap at the dawn of human evolution, when next to nothing is known. The next oldest published hominid skull is almost 3 million years younger.
Paleoanthropologists are stunned by the new skull's antiquity and surprising mix of apelike traits and more advanced hominid features. But they do offer a caveat. If the new skull is from a female rather than a male, the canines are “less striking” and more in line with those of living and extinct apes, says anthropologist Carol Ward of the University of Missouri, Columbia. This debate could be settled if Brunet's team finds skeletal bones that show that Toumaï was bipedal--and hence a hominid.