"Whatever the truth about Sahelanthropus, it is still a find of great importance," said Stringer in a comment sent to National Geographic News.
The skull was found in a location over 1,000 miles (1,600 kilometers) from the sites in East Africa where the search for human origins has been concentrated. It is also the only relatively complete fossil discovered in a fossil gap of five million years between the ancestral apes of nine million years ago and the australopithecines, or southern apes of Africa, which are generally regarded as close human relatives from four million years onward, said Stringer.
Teeth, Face and Muscle Scars
Brunet and colleagues identified Toumai as an early hominid based on characteristics such as the tooth type, thickness of the enamel, the shape and position of the head, and facial features.
Wolpoff and colleagues looked at these same features and report in Nature that these features have other explanations and interpretations, leading them to conclude that Toumai is not a hominid.
For example, when Wolpoff and colleagues look at Toumai's teeth they see evidence linking the fossil to apes where Brunet and colleagues find evidence linking the fossil to early hominids.
According to the analysis by Brunet and colleagues, the lower canine teeth show no evidence of honingthe sharpening of tooth edges against each other. Non-honing is considered as one of the first hominid features.
However, according to the interpretation of Brunet and colleagues' description of the upper canine teeth, Wolpoff and colleagues see evidence for honing and thus a link to apes.
Brunet, in his reply, says that to interpret the wear on the upper canine as honing is "roughly equivalent to describing an African millet pestle as a Samurai sword." Brunet argues that the tooth in question resembles those of later hominids.
The back and forth between the two groups of researchers extends to other features, such as tooth enamel and facial features.
One of the things that struck the scientific community when Toumai was revealed to the world was the resemblance of some of Toumai's facial features to hominids of the genus Homo not known in the fossil record until 1.8 million years ago.
Wolpoff and colleagues write that the brow of Toumai is hominid-like and not like living apes due to a consequence of mechanics to counter forces during chewing. Brunet and colleagues say the brow is the result of where the face lies relative to the base of the head.
The crux of the argument for Wolpoff and colleagues is that the size and position of the scars on the Toumai skull left by the neck muscles indicate that Toumai did not habitually hold its head in an upright position over the spine and was thus not a biped.
"I expect it could hold its head upright; any ape can," said Wolpoff. "What the bone reflects, through the position and size of the scars left by the neck muscles, is that it did not habitually keep its head in that position."
Brunet says that this analysis is based on measurements taken from published photographs of the skull that are distorted. Undistorted, he writes, the evidence left by the neck muscles "is within the range of fossil hominids" and is "nothing like that of any quadrupedal ape."
Given the current patchy state of scientific knowledge about human origins, it is too early for scientists to say where fossils such as Sahelanthrpous and Orrorin lie in relation to the human evolutionary line, according to Stringer.
The Toumai discovery, he says, demonstrates how much evidence has been missing up to now:
"The earliest stages in the evolution of humans and our closest relatives, the chimpanzees and gorilla, were probably as complex as we see 9 million years ago in the diversity of apes in Europe and Asia, and in the human-like forms 2.5 million years ago in Africa, when nature was seemingly experimenting in how to evolve the first real humans."
Related Stories from National Geographic:
Skull Fossil Opens Window Into Early Period of Human Origins
Skull Fossil Challenges Out-of-Africa Theory
Human Fossil Adds Fuel to Evolution Debate
Fossils From Ethiopia May Be Earliest Human Ancestor
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