ne look at the face of a human being and the skull of a Neanderthal is enough to see a striking difference. The Neanderthal face projects out from the cranium.
ne look at the face of a human being and the skull of a Neanderthal is enough to see a striking difference. The Neanderthal face projects out from the cranium.
The human's is aligned more vertically, directly below the cranium. In fact, humans are unique among mammals in their lack of forward projecting faces.
An anthropologist at Rutgers University thinks he has found the most likely anatomical basis for this distinctive facial characteristic. He has determined that the shape and dimensions of the anatomically modern human head and face depend to a large extent on the size and changes over time of one pivotal bone, the sphenoid.
In a close examination of the sphenoid in humans and their archaic hominid relatives, Dr. Daniel E. Lieberman of Rutgers found the bone to be a kind of cornerstone of the skull. Lying at the base of the brain case, behind the palate and in front of the vertebral column and as wide as the skull, the sphenoid joins 17 of the 22 bones in the skull. The face, in a sense, grows forward from the sphenoid.
Comparing modern and archaic skulls, Lieberman noted that the length of the human sphenoid is 20 to 30 percent shorter than the similar bone in other hominids, including Neanderthals.
The longer sphenoid bone had presumably been the major factor dictating the long projecting faces of Neanderthals, which became extinct in Europe 30,000 years ago, and archaic species like Homo heidelbergensis, considered by some paleontologists to be the immediate direct predecessor of modern Homo sapiens.
"So just change the shape of one bone and it has all kinds of effects on how much the face projects in front of the brain case," Lieberman said. "In turn, facial projection affects all sorts of aspects of our overall cranial shape, such as how large our brow ridge is and how steep our forehead is."
The reduction of facial projection, the anthropologist said, contributes to the more globular shape of modern human craniums, compared with the flat, sloping brain cases of Neanderthals.
His investigations were based on X-ray pictures and computed tomography scans of human and early hominid skulls. One difficulty was the scarcity of sphenoid bones, which are fragile and often are not preserved in skull fossils.
From this perspective at least, Lieberman said in an interview, "it doesn't seem to take a lot to turn a Neanderthal into a human."
The shortening of the sphenoid presumably occurred about 125,000 year ago. That was about when anatomically modern humans first appeared in Africa and later drove archaic relatives to extinction.
Lieberman was cautious about possible implications of the findings for the rise of humans or the fall of Neanderthals. Perhaps the changing shape of the face led to adjustments in the placement of the larynx, which might account for the development of more articulate speech in modern humans, compared with that of the Neanderthals. That is only speculation, he emphasized.
Neanderthal speech, like most aspects about these Ice Age hunters, is controversial. Recent evidence by Duke University anthropologists indicated that Neanderthals and other hominids as far back as 400,000 years ago may have been able to speak, though other scientists continue to believe this ability was limited until modern Homo sapiens came along.
In a report of the findings in the current issue of the journal Nature, Lieberman concluded, "Sphenoid reduction, through its effects on facial projection and cranial shape, may account for the apparently rapid evolution of modern human cranial form, and suggests that Neanderthals and other archaic Homo should be excluded from H. sapiens."
This analysis, scientists said, supports recent human evolution research indicating that modern humans arose in Africa and then spread through the world, instead of evolving in different parts of the world from archaic species there.
If the new findings are correct, modern humans would appear to be more closely related to each other than they are to the archaic Homo species that preceded them in various regions of Europe and Asia.
Dr. Fred H. Smith, a paleoanthropologist at Northern Illinois University in DeKalb, who specializes in studies of Neanderthals and other archaic hominids, said the sphenoid hypothesis "makes a lot of sense."
If anything, the faces of modern humans are getting smaller with time. The earliest members of the modern species had huge faces that were especially long from front to back.
Since the end of the most recent Ice Age, about 12,000 years ago, the evolutionary trend has been to smaller faces, presumably because changes in diet reduced the need for robust chewing muscles and jaws.