Earlier Human Speech?
DURHAM, N.C. -- Duke University Medical Center anthropologists have offered anatomical evidence from skulls suggesting that human vocal abilities may have appeared much earlier in time than is suggested by the first archaeological evidence for speech.
The scientists measured the pencil-sized "hypoglossal canal" - which carries the motor nerve controlling the tongue -- in the skulls of humans, apes and fossil hominids. They found that the canal in Neanderthals and early humans more closely matched that of modern humans than did the smaller canals of apes and proto-humans such as Australopithecus.
The scientists, Professors of Biological Anthropology and Anatomy Richard Kay and Matt Cartmill, and student Michelle Balow, published their findings in the April 28 Proceedings of the National Academy of Sciences.
In their studies, the scientists made rubber casts of the hypoglossal canals in skulls of chimpanzees, gorillas and humans, as well as those of three specimens of the early "man-ape" Australopithecus, two archaic members of the genus Homo, two Neanderthals and one early Homo sapiens.
"Our conjecture is that the size of this canal reflects the fineness of the motor control over the tongue in people," Cartmill said. "People don't need a big nerve to the tongue so they can eat; people don't process their food any better than apes do. And that's what the tongue is mainly for in most mammals: for the stereotyped behavior of manipulating food to position it for chewing."
The researchers found that the canals in humans measured about twice as large as those in chimpanzees. The Australopithecus canals proved to be ape-sized, but by contrast, the Neanderthal and early human canals fell within the human range.
However, to correct for possible differences in the size of the tongues controlled by the nerves, the scientists plotted canal size versus the size of the oral cavity. Still, the apes and Australopithecus samples measured about half the relative size of those of humans and Neanderthals, although there was some overlap. The scientists believe the slight overlap may occur because the size correction is imperfect, or that variations exist in the sizes of the structures that run through the canal.
Because Neanderthals first evolved about 300,000 years ago, the Duke scientists' evidence disagrees with the theory that human speech may not have arisen until about 40,000 years ago, based on the unambiguous appearance of symbols in the archaeological record. Some researchers infer the presence of language abilities from such symbols as body ornamentation and deliberate burial practices, as well as evidence of such collective action as hunting or the design of habitation sites.
The Duke findings also disagree with those of scientists who measured the base of the Neanderthal skull in an attempt to reconstruct the vocal tract. Those scientists concluded that Neanderthals and earlier hominids may not have had the ability to produce the full range of sounds that humans produce.
Kay and his colleagues emphasized that further measurements must be made to refine their data. For example, besides the hypoglossal nerve, the canal carries two tiny arteries and a vein, and the Duke scientists are assuming that these structures are about the same size in apes, humans and fossil hominids. Thus, the scientists plan to compare these blood vessels in apes and humans to confirm that they are similar.
The researchers will also conduct measurements of more fossil hominids to fill in the roughly two-million-year gap between the fossils of Australopithecus and the archaic humans they studied. Such further studies might reveal a steady increase in the size of the canal, Cartmill said.