Is a fossil (search) 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 around the time of a major split in the evolutionary tree, with one branch leading eventually to humans and the other branch leading to chimps.
The researchers argued that the creature, which they dubbed Sahelanthropus tchadensis (search), belongs on the human branch and so is the oldest known hominid. Some 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 at that time.
Many scientists now think S. tchadensis was probably a hominid, and more evidence appears in Thursday's issue of the journal Nature. It comes from Michel Brunet (search) of the University of Poitiers in France, who led the team that made the original discovery, and colleagues.
Other experts said the new work strengthens the case for hominid status but doesn't clinch it.
"This isn't a smoking gun," said David Begun of the University of Toronto.
A big question is whether S. tchadensis walked upright, because that's a key characteristic of hominids. Brunet, in an e-mail, said given the available evidence it would be a "great surprise" if it didn't walk upright. But he agreed with other scientists that to be sure, scientists would have to find and analyze skeletal bones that carry signatures of upright walking, like a knee, hip or foot.
In Nature, Brunet and colleagues report discovering two new jaw fragments and the crown of a tooth in the same geographical area as the earlier findings. Analysis shows similarities to hominid fossils and differences from ape traits, they said.
They also present a computerized reconstruction of the skull, because the fossil had been distorted in the ground. The reconstruction confirms that S. tchadensis shared several features with later hominids, the researchers wrote. In addition, the position of the hole where the spinal cord enters is like what's seen in humans but not apes, which suggests upright walking, they wrote.
Rick Potts, director of the Human Origins Program at the Smithsonian Institution's National Museum of Natural History, said the position of that hole doesn't necessarily prove S. tchadensis walked upright. Still, Potts said he thinks the creature was probably a hominid.
Begun agreed, and said the chances are "pretty good" the creature walked upright, although "I'll be convinced when they find a knee joint."
Bernard Wood of George Washington University said he finds too little evidence to declare S. tchadensis a hominid with certainty, although it might well be true. If it isn't, the creature might have belonged to a branch of the evolutionary tree that has no living representatives, he said.