From Steven Mithen’s Prehistory of the Mind:
The anthropologist Robin Dunbar looked at the size of the brain of H. habilis [2.1 – 1.5 million years ago] from a very different perspective. Recall that we have already referred to his work regarding the relationship between brain size and group size—living within a larger group requires more brain-processing power to keep up with the ever-changing sets of social relationships. When living in groups, primates have to transfer information between each other and the principal way they do this is by grooming each others’ bodies—picking out all the fleas and the lice. Who one chooses to groom, how long one grooms, and who you let watch while you do it, function as much to send social messages as to get rid of parasites. In the Burghers’ zoo chimpanzee group that we looked at in the previous chapter grooming between males reached a peak when their relationships were unstable. Grooming sessions among the males lasted nine times as long in periods when there was an oestrus female in the group; de Waal suggests that the grooming may amount to “sexual bargaining.”
Dunbar found that as group size increases, so too does the amount of time that primates spend grooming. This is not because there are more lice about but because one has to invest more and more time in social communication. But grooming is time consuming, and there are other things to do such as finding food to eat. Dunbar reckons that the longest any primate can afford to groom others is about 30 per cent of its time budget. Once above that limit, the individual may be a mastermind at social relationships, but be very hungry and lack the energy to exploit this knowledge to his or her social advantage.
So what can be done when group size is so large that even spending 30 per cent of one’s time grooming leaves one ignorant of many important social relationships within the group? Well, maybe another means for transferring social information could be used—or in evolutionary terms would be selected for. Dunbar suggests that the other means is language. He argues that language evolved to provide a means for exchanging social information within large and socially complex groups, initially as a supplement to grooming, and then as a replacement for it. Language can do this because it is a much more efficient way of transferring information. An ambidextrous chimp may be able to groom two of his mates at once, but an articulate human can chat away to whoever is listening….
…Aiello and Dunbar concluded that the basis of the language capacity appeared early in the evolution of the genus Homo, at least by 250,000 years ago. A critical feature of their argument is that the subject matter of the earliest language was social interaction; it was in effect a “social language.” There was thus a co-evolution of increasing group size/social intelligence and a capacity for language. Evidence for this may indeed be found in the structure of the brain. The prefrontal cortex is not only the area of the brain responsible for many aspects of language, but also that where the ability to reflect on one’s own an other people’s mental states, which I have argued is a central fact of social intelligence, are found. The general-purpose character of language as we know it today, and its symbolic features evolved, Aiello and Dunbar argued, at a later date—although how much later is left unclear in their work. On a far more intuitive basis, it is indeed difficult to imagine how an Early Human could have had a brain size equivalent to that of ourselves today, but lacked a linguistic capacity.
Further support for a linguistic capacity can be found by looking at the shape of the Early Human brain, as reconstructed from the bumps on the insides of their crania. We saw in Chapter 6 that H. habilis appears to have had a well-developed Broca’s area, which is conventionally associated with speech. Broca’s area also appears well formed on the H. erectus cranium of KNM-WT 15000, a particularly well-preserved 12-year-old boy dating to 1.6 million years ago and found at East Turkana in Kenya. With regard to more recent Early Humans, paleontologists have argued that the brain shape is practically identical to that of Modern Humans. Ralph Holloway, in particular, has argued that both Broca’s and Wernicke’s areas can be identified on Neanderthal brain casts and that they show no difference from their appearance on the brains of Modern Humans.
A third source of evidence for a linguistic capacity is the nature of the vocal tract of Early Humans. There has been a long history of efforts at reconstructing the vocal tract, particularly for Neanderthals. Since it is principally composed of soft tissue—the larynx and pharynx—one must rely on consistent relationships between the organization of soft tissue and those parts of the cranium that can survive in an archeological context. The most recent reconstructions imply that the Neanderthal vocal tract would not have differed significantly from that of Modern Humans: Neanderthals would have had essentially modern powers of vocalization and speech.
This has received support from the discovery of a hyoid bone, surviving in a Neanderthal skeleton buried in the Kebara Cave in Israel and dated to 63,000 years ago. The hyoid is a bone that can provide detailed information about the structure of the vocal tract. Its movement affects the position and movement of the larynx to which it is attached. That found at Kebara, lying in an undisturbed position with the mandible and cervical vertebra, is virtually identical to that of a Modern Human with regard to its shape, muscular attachments and apparent positioning. This implies that the morphology of the vocal tract of the Neanderthal was not significantly different from that of Modern Humans. If the cognitive capacity for language was present, there appears no reason why the full range of human sounds could not have been produced.
Of course the “if” in the last statement is a rather big “if.” On purely logical grounds, however, it would be a little odd if Neanderthals had the vocal structures but not the cognitive capacity for speech.
– Steven Mithen, The Prehistory of the Mind: The Cognitive Origins of Art and Science, 110-111, 140-141