from Richard Klein’s The Dawn of Human Culture:
More research is required to demonstrate that the brain enlarged abruptly in steps as we have suggested, but no one questions that brain size increased roughly threefold over the 5- to 7-million-year span of human evolution. Body size also increased over the same interval, but to a much smaller degree, and the result is not only that living people have large brains, but also that they are highly encephalized, that is, they have brains that are exceptionally large for their body mass. Mammals are generally more encephalized than other kinds of animals, and even the earliest mammals had brains that were about four times larger than those of like-sized reptiles. Much of the difference in size came from the development of the cerebral cortex, the folded mantle of gray matter that we think of first when we visualize a human brain.
The original mammals were probably mainly nocturnal, and their enlarged brains may have functioned to process information from multiple senses—smell, touch, and hearing, as well as sight—as they sought food and safety. Mammalian brains continued to evolve, but in most groups, encephalization—the ratio of brain size to body mass—plateaued early on. The most conspicuous exception to this generalization concerns the Primates, which have routinely spawned more encephalized forms during their entire history, spanning the last 65 million years or so. People are of course Primates, and in this light, their extraordinary encephalization can be seen as the culmination of a long-standing evolutionary trend.
UCLA neuroscientist Harry Jerison notes that the human brain is roughly six times larger than we would predict from the relationship between brain size and body size in other mammals. Even if we restrict the survey to monkeys and apes and scale them to human body size, human brains are about three times larger than we would expect. The fossil record suggests that whenever encephalization has occurred, it occurred rapidly, and the human brain illustrates the point especially well. It may actually have been the most rapidly evolving organ in the history of the vertebrates.
The benefits of a larger brain are obvious, but there are also costs. In modern humans, the brain accounts for only about two percent of body weight, but it consumes roughly twenty percent of the body’s metabolic resources. In addition, large brains and the constraints imposed on the birth canal by bipedalism vastly complicate birthing. A survey of other mammals suggests that human brains should be even larger at birth, or more precisely, that the human gestation period should be perhaps three months longer. Restricting it to nine months increases the likelihood that the fetus will make it out, but it also means that newborn human infants are more helpless than those of apes and other mammal species, and this imposes a further cost, mostly on mothers. Obviously the brain got bigger anyway, so the pros must have outweighed the cons, and Jerison proposes that the most general benefit was the ability to accumulate novel behaviors, such as those we detect through time in the archeological record. Jerison also notes that a major function of the brain, and more particularly of the cerebral cortex, is to build a mental image or model of the “real world,” which in his words is “the brain’s way of handling an otherwise impossible load of information and is the biological basis for mind.” Brain expansion after 600,000 years ago presumably increased the amount of data that the human brain could process, and this in turn allowed the development of more sophisticated mental models. “Brains are, after all, information-processing organs,” notes Jerison, “and [natural] selection for brain size must have been selection for increased or improved information-processing capacity.”
Humans before 600,000 years ago surely had sophisticated mental models of their world, but rapid brain expansion about this time may have enhanced their ability to communicate these models to others, that is, it may mark a major step in the development of human language. No topic is more intriguing and more difficult to address concretely than the evolution of language, but as Jerison points out, language is almost a kind of sixth sense, since it allows people to supplement their five primary senses with information draw from the primary senses of others. Seen in this light, language becomes a kind of “knowledge sense” that promotes the construction of extraordinarily complex mental models, and language alone may have provided sufficient benefit to override the costs of brain expansion.