Walking on Two Feet: The Evolution of Bipedalism

from Steven Mithen’s The Prehistory of the Mind:

The evolution of bipedalism had begun by 3.5 million years ago. Evidence for this is found in the anatomy of A. afarensis, and, more dramatically, by the line of australopithecine footprints preserved at Laetoli in Tanzania. The most likely selective pressure causing the evolution of bipedalism was the thermal stress suffered by the australopithecines when foraging in the wooded savannahs of East Africa. With their tree-climbing and tree-swinging ancestry, the australopithecines had a body already conditioned for an upright posture. The anthropologist Peter Wheeler has shown that by adopting bipedalism australopithecines could achieve a 60 per cent reduction in the amount of solar radiation they experienced when the sun was overhead. Moreover, the energetic costs of locomotion would have been reduced. Bipedalism enabled australopithecines to forage for longer periods without the need for food and water, to forage in environments which had less natural shade, and thus to exploit foraging niches not open to other predators who were more heavily tied to sources of shade and water. The shift to increasingly efficient bipedalism may have been partly related to the environmental change to more arid and open environments that occurred in Africa around 2.8 million years ago, increasing the value of reducing exposure to solar radiation by adopting an upright posture.

      Bipedalism required a larger brain for the muscle control needed for balance and locomotion. But bipedalism and a terrestrial lifestyle had several other consequences for brain enlargement. Some of these have been discussed by the anthropologist Dean Falk. She explains how a new network of veins covering the brain must have been jointly selected for with bipedalism to provide a cooling system for the brain—or a “radiator” as she describes it. Once in place, the constraint of overheating on further expansion of the brain was relaxed as this radiator could easily be modified. Consequently the possibility (not necessity) arose of further brain enlargement.

      Dean Falk also suggests that bipedalism would have led to a reorganization of the neurological connections within the brain: “once feet had become weight bearers (for walking) instead of graspers (a second pair of hands) areas of the cortex previously used for foot control were reduced thus freeing up cortex for other functions.” This of course went with the “freeing” of the hands, providing opportunities for enhanced manual dexterity for carrying and toolmaking. There may also have been significant changes in the perception of the natural environment due to an increase in the distances and directions regularly scanned; and a change in the social environment by an increase in face-to-face contact, enhancing the possibilities for communication by facial expression.

      Perhaps the most significant consequences of bipedalism, however, is that it facilitated the exploitation of a scavenging niche. A “window of opportunity” was opened to exploit carcasses during periods of the day when carnivores needed to find shade. As Leslie Aiello and Peter Wheeler have discussed, with an increasing amount of meat in the diet, the size of the gut could be further reduced, releasing more metabolic energy to the brain while maintaining a constant basal metabolic rate. And in this way a further constraint on the enlargement of the brain was relaxed.

      The main selective pressures for brain enlargement no doubt continued to come from the social environment: the spiralling pressures caused by socially clever individuals creating the selective pressure for even more social intelligence in their companions. And this pressure itself was present due to the need for large social groups that a terrestrial lifestyle in open habitats required, partly as a defense against predators.

      Confirmation of the importance of the social environment for the expansion of brain size was found in Chapter 6. As we saw in that chapter, it is clear that the Oldowan stone tools of early Homo demanded more knowledge to make than those which chimpanzees use today, and therefore those likely to have been used by the australopithecines. But this knowledge probably arose from the enhanced opportunities for social learning in larger groups rather than as a consequence of selection for a domain of technical intelligence. Similarly, the narrow range of environments exploited by early Homo suggests that a discrete domain of natural history intelligence had not yet evolved and that the information requirements for scavenging were also being met as a by-product of living in larger social groups.

      In my reconstruction of the evolution of the mind I only found the first evidence for distinct domains of natural history and technical intelligence at 1.8-2.4 million years ago with the appearance of H. erectus, and the technically demanding handaxes. What were the causes, conditions and consequences for these new domains of intelligence?

      The ultimate cause for these new specialized intelligences was the continuing competition between individuals—the cognitive arms race that had been unleashed when the constraints on brain enlargement had been relaxed. But the evolution of these specific intellectual domains may well reflect the appearance of a constraint on any further enhancement of social intelligence itself. As Nicholas Humphrey noted, “there must surely come a point when the time required to solve a social argument becomes insupportable.” So, just as the possibilities of increasing reproductive success by enhancing general intelligence alone by natural selection had been exhausted by 3.5 million years ago, we might also conclude that the “path of least resistance” for a further evolution of the mind in the conditions existing at 2 million years ago lay not in enhanced social intelligence but in the evolution of new cognitive domains: natural history and technical intelligence.

      In other words, those individuals gaining most reproductive success were the ones who were most efficient at finding carcasses (and other food resources) and most able to butcher them. These individuals gained a better quality of diet, and spent less time exposed to predators on the savannah. As a result, they enjoyed a better state of health, could compete more successfully for mates, and produced stronger offspring. With regard to toolmaking, behavioral advantage was gained by those individuals who were able to have ready access to suitable raw materials for removing meat and breaking open bones of a carcass. The advantages of artifacts such as handaxes may well have been that they could be carried as raw material for flakes, as well as used as a butchering tool themselves. Experimental studies have repeatedly shown that they are very effective general-purpose tools.

      Bipedalism, the scavenging niche, the existence of raw materials, the competition from other carnivores—these were all conditions that enabled the enhanced intellectual abilities at toolmaking and natural history to be selected for. Had one of these conditions been missing, we might still be living on the savannah….

       Just as the tree-living ancestry of the australopithecines enabled bipedalism to evolve, so too did bipedalism itself make possible the evolution of an enhanced vocalization capacity among early Homo, and particularly H. erectus. This has been made clear by Leslie Aiello. She has explained how the upright posture of bipedalism resulted in the descent of the larynx, which lies much lower in the throat than in the apes. A spin off, not a cause, of the new position of the larynx was a greater capacity to form the sounds of vowels and consonants. In addition, changes in the pattern of breathing associated with bipedalism will have improved the quality of sound. Increased meat eating also had an important linguistic spin off, since the size of teeth could be reduced thanks to the greater ease of chewing meat and fat, rather than large quantities of dry plant material. This reduction changed the geometry of the jaw, enabling muscles to develop which could make the fine movements of the tongue within the oral cavity necessary for the diverse and high-quality range of sounds required by language.  

– Steven Mithen, The Prehistory of the Mind: The Cognitive Origins of Art and Science, 204-207, 208

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