Skull canals spark speech-origins dispute.

Fossil indications of whether Neandertals and other prehistoric populations were capable of talking have proven scarce and subject to conflicting interpretations. The hypoglossal canals, a pair of bony tubes located on the left and right sides of the skull's base, were nominated just last year as skeletal signposts of speech.

These cranial passages carry branches of a nerve that activates all but one of the tongue's muscles. However, they bear no telltale traces of an individual's anatomical readiness to speak, according to a new study.

The findings challenge a proposal that relatively large hypoglossal canals in the skulls of human ancestors who lived about 400,000 years ago reflect their ability to talk much like people do today (SN: 5/2/98, p. 276). In that report, Richard F. Kay of Duke University Medical Center in Durham, N.C., and his coworkers asserted that hypoglossal canal size relative to mouth size averages about twice as large in humans, Neandertals, and some early Homo species as in chimpanzees.

Growth of the hypoglossal canals in the human lineage may have accompanied a thickening of the hypoglossal nerve to coordinate tongue movements needed for speaking, Kay's group theorized.

David DeGusta of the University of California, Berkeley and his colleagues disagree. Many prosimian, monkey, and ape species have hypoglossal-canal-mouth ratios that reach or exceed the modern human range, DeGusta's team reports in the Feb. 16 Proceedings of the National Academy of Sciences.

Hypoglossal-canal-mouth ratios in skulls from two early species in the human evolutionary family, neither of which is thought by anthropologists to have spoken, also fall within the modern human range, the scientists say.

"I think it's pretty clear that hypoglossal canal size has nothing to do with speech," DeGusta says. "The date of origin for human language and the speech capabilities of Neandertals remain open questions."

DeGusta and his team measured the hypoglossal-canal-mouth ratios from skulls of 104 modern humans, 75 nonhuman primates from more than 30 species, and 4 pre-human australopithecines from species dating to 3.2 million years ago. Dissections of five modern human cadavers also yielded no indication that larger hypoglossal canals carry thicker hypoglossal nerves.

The new report provides interesting data on variability in hypoglossal-canal-mouth ratios within species but leaves unexplained the canal's larger average relative size in humans and Neandertals compared with chimps, asserts Kay.

Ranges of hypoglossal canal size vary so much that comparisons of average ratios can offer little insight, DeGusta responds. For instance, he says, some chimps have hypoglossal canals that are proportionately three times as large as those of some modern humans, although only the people speak their minds.