Evolutionary psychologists have suggested that the large human brain, like the peacock's tail, is the result of sexual selection. Ancestral peahens were more attracted to mates with bigger and more ornate tails. Therefore, size and ornateness of tail were selected for. Therefore, we now have peacocks with ludicrously exuberant tails. By analogy, hominid females were more attracted to mates with bigger brains. Therefore, bigger brains were selected for. Therefore, we now have humans with ludicrously big and complicated brains.
On the face of it, this seems plausible. Sexual selection has been held to account for evolutionary "super-growth" in various animal features. However, sexual selection only favours exaggerated development of features in the male. It is the peacock, not the peahen, that has the dazzling tail. In humans, the ratio of brain size to body size is essentially the same in both sexes. Moreover, even if brainier males did attract more mates in the evolutionary past, this would not explain why their ancestors started to become brainier in the first place. The sexual selection hypothesis of human brain evolution might contain a grain of truth, but it cannot be the whole truth.
Let us consider an alternative "just so story". It is generally agreed that bipedalism predated the increase in hominid cranial capacity. What were the immediate advantages of bipedalism? First and foremost, it freed the hands for tasks other than locomotion, such as using tools. Some elementary use of objets trouvés as tools probably began very early in human evolution, or even before; other ape species, such as chimpanzees, can also use tools. The individuals who were best at using tools were likeliest to survive and leave offspring, so the capacity for tool use was selected for. Greater capacity for tool use implies better hand control, which could only have been achieved by increased development of brain areas concerned with hand movements and sensations. Therefore, natural selection favoured the growth of those parts of the cerebral cortex that process sensory information from the hands and control the fine movements of digits47.
The better the control of hand movements became, the greater the possibilities for tool use. As a result, the range of tool uses increased. In time, hominid populations became more dependent on tools. This made it increasingly advantageous for the young to learn about tools: how to find them, how to use them and - later - how to make them. A brain that was good at learning from adults in the community, acquiring and developing skills and passing on those skills to others, became useful and then indispensable. Ultimately, teaching tool-using skills to the young became necessary for the survival of the next generation. Therefore, the winners of this part of the evolutionary race were hominids whose brains were best at learning and communicating. They were also best at distinguishing quickly and accurately among individuals within the community. It was clearly advantageous to be able to recognise adults who could teach essential skills, and adults who were best avoided.
By the time tool use had advanced from the incidental and occasional to the essential-for-survival, the hominid brain had (according to our scenario) become good at hand control, learning, communicating, and recognising individuals: in other words, at manual skills, language and facial recognition. The learning was predominantly early learning, so the period of childhood -i.e. dependence on parents and other adults - must have gradually grown longer as learning became more elaborate and the need for it greater.
Early learning forges new synaptic connections, increasing brain size. One of the most remarkable features of the modern human brain is its increase in volume between birth and adulthood. Both human and chimpanzee babies have roughly 350 cc brains at birth. An adult chimpanzee has a 450 cc brain, an increase of about 30% during maturation. But an adult human has quadrupled in size to 1200-1400 cc. This difference is almost entirely due to the formation of new axon branches and new synapses, the physical concomitants of early learning and environmental influence.
The brains of the earliest known Australopithecines were about the same size as those of chimpanzees. Even their hominid successors had cranial capacities of only 600-700 cc. At this stage in evolution, the part of the
47 In the brains of modern humans, far more of the sensory part of the cerebral cortex is devoted to the hands than to any other part of the body except the genitals. The hands are similarly over-represented in the motor cortex. We propose that this "takeover" began in the very earliest hominids.
cerebral cortex that had grown most was presumably concerned with hand control. Not until H. habilis appeared was the 1000 cc barrier broken. By this stage, culture was probably elaborate enough for significant communication and facial recognition skills to have developed.
Of course, this is a much over-simplified picture. Many other factors must have been at work. Horrobin drew attention to the fat content of human brains (and human milk), which is distinguished by extremely high levels of certain unsaturated fatty acids. The richest sources of these fatty acids are bone marrow and small aquatic organisms. Significantly, early hominid communities lived near lakes or rivers and their diet seems to have included bone marrow, which is not a part of the diet of other apes. This idea, that some early hominids were semi-aquatic, is consistent with -among other features - the development of elaborate vocal communication, which demands considerable breath control and is found also in whales and dolphins. If there was a semi-aquatic phase during human evolution, many aspects of the development of the large human brain - including language capacity - can be explained.
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