Evidence Unearthed in Debate Over Man’s Earliest Ancestor

Is a fossil creature that grabbed headlines three years ago really the earliest known ancestor of modern humans, or does it belong elsewhere on the evolutionary tree?

The answer has been hotly debated, but now two studies argue that it does indeed belong on the human branch.

In 2002, scientists announced finding jaw fragments, some isolated teeth and a skull of a creature nicknamed “Toumai” in Chad. At some 6 million to 7 million years old, the fossils came from about the time of a major split in the evolutionary tree, with one branch leading to humans and the other branch leading to chimpanzees.

The researchers argued that the creature, which they dubbed Sahelanthropus tchadensis, belonged on the human branch, making it the oldest known hominid. Others disagreed. In any case, the skull provided a puzzling combination of human and chimp traits and raised what one expert called a “wheelbarrow full of questions” about evolution.

Michel Brunet of the University of Poitiers in France, who led the team that made the original discovery, presented more evidence supporting the contention that the creature was probably a hominid in Thursday’s issue of the journal Nature.

A big question is whether the creature walked upright.

Brunet and colleagues reported discovering two new jaw fragments and the crown of a tooth in the same area as the earlier findings. Analysis showed similarities to hominid fossils and differences from ape traits.

They also presented a computerized reconstruction of the skull, because the fossil had been distorted in the ground. The reconstruction confirmed that S. tchadensis shared several features with later hominids. In addition, the position of the hole where the spinal cord entered is like what’s seen in humans but not apes, which suggested upright walking, they wrote.

David Begun of the University of Toronto said the chances were “pretty good” the creature walked upright, although “I’ll be convinced when they find a knee joint.”