One very obvious difference between the vocalizations of all our ape cousins and humans is the presence of air sacs in all the apes. Air sacs are essentially large bags that attach to the throat and lie atop the upper chest. Apes use them to make great big sounds, noises that make them sound larger than they are, rather like putting a truck horn on a Volkswagen to command respect. Humans do not have air sacs. Dutch linguist Bart de Boer has looked into the question of whether the absence of air sacs in humans has something to do with the rise of language. If it does, we can date language to at least 800 thousand years ago he told the Evolang conference in Barcelona this evening (Friday, March 14, 2008).
The fossil evidence comes from the hyoid bone, the only bone in the vocal system, and therefore the only source of fossil evidence for what was going on in the vocal tract’s evolution. (An earlier presentation said we cannot describe the vocal tract on the basis of the hyoid bone; however, de Boer presented evidence that all the apes have a hyoid bone with a cup shape in the middle while human hyoids lack the cup. So perhaps we can at least tell from the hyoid whether or not there was an air sac. See: Stop Your Yacking.) Humans and Neanderthals have almost identical hyoids and just published data shows that Homo heidelbergensis of 800 thousand years ago had a human-like hyoid. No erectus hyoid fossil has ever been found, so we cannot push the loss of air sacs back to them. An Australopithecus afarensis hyoid with the characteristic cup has been found, but as I recall that was a fossil from an infant and so is not quite definitive. The young of a species often show traits that disappear in the adult. (See: The Selam Fossil)
But what evidence is there that the loss of air sacs might be related to the rise of speech? De Boer has modeled the sounds that follow the addition of an air sac to the vocal system. Air sac result in lower frequency sounds and a smaller acoustic range than humans enjoy. Air sacs shorten the articulatory range and the mouth is less able to shape the sound that comes out from an air sac vocalization. De Boer hypothesized that when what you say becomes more important than how you sound, air sacs give way. So the disappearance of air sacs is likely a good bit of evidence that speech of some sort has appeared.
During the question session following the presentation, one woman asked if it was true even today for humans that what you say is more important than how you say it. De Boer said he liked to believe it would, but the young woman seemed unconvinced. On the other hand, if what you say doesn’t matter so much, the conference as a whole is wrongly focused.
Another questioner pointed out that there is a sexual appeal in having a big air sac and that mating strength would be a counterbalance to the pressures from language to reduce the sac. De Boer agreed but thought that the descent of the larynx may well have been a counterbalance to the loss of air sacs. He said this explanation is not perfect, however, because the lowered larynx would likely only offer an alternative way of deepening the voice after the sac had already disappeared.