for National Geographic News
A six to seven million-year-old skull from northern Chad that shook the world when its discovery was announced this July may not be what its discoverers' believe it to be: the oldest known member of the human family. "It is an ape and not a human ancestor," said Milford Wolpoff, an anthropologist at the University of Michigan in Ann Arbor.
But the lead scientist who discovered the skull, Michel Brunet of the University of Poitiers in France, stands by his theory that the fossil is a hominid.
Both sides of the debate are published in the October 10 issue of the journal Nature.
The skull and other fossilized remains were first reported in the July 11 issue of Nature by an international team of scientists led by Brunet.
The researchers and the journal touted the fossils then as belonging to the earliest member of the human family so far discovered, proclamations that the scientific community knew would come under scrutiny.
Chris Stringer, head of the Human Origins Program at the National History Museum in London, told National Geographic News in July that discoveries such as this are always complex because evidence is usually incomplete and there is little agreement about what key features characterize a distinct human ancestor.
The analysis by Wolpoff and colleagues centers on the argument that the fossil, formally known as Sahelanthropus tchadensis and nicknamed Toumai, does not have a feature that they believe is essential for it to be considered a hominid: the ability to walk on two feet.
"It does not share the single unifying feature of all humans and hominids, erect posture and obligate bipedal locomotion," said Wolpoff. "It could of course be an ancestor of both humans and chimpanzees, it certainly is early enough, but there is no reason to be sure it is the ancestor of any surviving species."
Wolfpoff and colleagues suggest that another species, Orrorin tugenensis, found in Kenya, well-adapted to walking upright, and announced in 2001 is actually the earliest member of the human family.
Brunet refutes the argument by Wolpoff and colleagues as "not supported by published or unpublished data" in a reply also in the October 10 issue of Nature. He reiterates his belief that Toumai is the earliest known human ancestor.
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