THE discovery of our earliest human-like ancestor has shaken up the roots of humanity. The stunning hominid skull, uncovered in the Djurab desert in Chad, is 6 to 7 million years old and is expected to transform our understanding of human origins.
The skull is around 2 million years older than any accepted hominid fossil skull so far and combines a unique set of both ape and human-like features. The features suggest that hominids did not, as supposed, evolve key traits only the one time en route to becoming modern-day humans.
News of the discovery was leaked to the press late last year, but this is the first time details of the fossil skull have been made public and published in a scientific journal (Nature, vol 418, p 145).
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Named Sahelanthropus, the skull has characteristics which show it was close to the common ancestor of humans and chimpanzees. Although Sahelanthropus’s brain was the size of a modern chimp’s, its face was quite different, with large brow ridges and much smaller canine teeth. From the back, the skull “looks like a chimpanzee”, says Bernard Wood of the George Washington University in Nature (vol 418, p 133). Whereas from the front it could pass for a 1.75-million-year-old australopithecine, a much later group of hominids which include the famous Lucy.
As well as the striking ancestry of Sahelanthropus, it’s that unique blend of features which is causing a commotion among experts. “It’s a great example of how the fossil record keeps showing how wrong our inferences are,” says Susan Anton at the University of Florida in Gainesville. Another newly discovered skull, a relative baby just 1.75 million years old, has been found in Asia which has a different mosaic of features (see “‘dumb’ hominids made it out of africa”). Wood says that Sahelanthropus lends weight to the idea that hominids diversified into many forms as they evolved, each containing a range of different characteristics. “Features were mixed and matched in ways that we are only beginning to comprehend,” she says.
Human and apes supposedly diverged some 5 to 7 million years ago, but the fossil record of this split is sparse. Last year, scientists in Kenya announced they had found the leg bones of an early human-like animal, Orrorin tugenensis, which they dubbed Millennium Ancestor. They believed the creature walked upright around 6 million years ago. But experts have questioned whether Orrorin is a true hominid.
Sahelanthropus is thought to be related to another recently discovered hominid, Ardipithecus, which lived 5.8 to 4.4 million years ago, says Michel Brunet of the University of Poiters in France, who led the international team which discovered the skull.