Chapter 24. Human Origins and Evolution

Humans have large brains relative to body size.

In mammals, brain size is typically correlated with body size. Humans are relatively large-bodied mammals, but our brains are large even for our body size (Fig. 24.13). It is our large brains that have allowed our species’ success, extraordinary technological achievements, and at times destructive dominion over the planet.

FIG. 24.13 Brain size plotted against body size for different species. Humans have large brains for their body size. Data from Fig. 2.4, p. 44, in H. J. Jerison, 1973, Evolution of the Brain and Intelligence, New York: Academic Press.

Data from Fig. 2.4, p. 44, in H. J. Jerison, 1973, Evolution of the Brain and Intelligence, New York: Academic Press.

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What factors acted as selective pressure for the evolution of the large human brain? Again, speculation is common, but because a large brain is metabolically expensive to produce and to maintain, we can conclude that the brain has adaptive features that natural selection acted in favor of. What are the selective factors? Here are some possibilities:

Tool use. Bipedalism permitted the evolution of manual dexterity, which in turn requires a complex nervous organization if hands are to be useful.
Social living. Groups require coordination, and coordination requires some form of communication and the means of integrating and acting upon the information conveyed. One scenario, for example, sees group hunting as critical in the evolution of the brain: Natural selection favored those individuals who cooperated best as they pursued large prey.
Language. Did the evolution of language drive the evolution of large brains? Or did language arise as a result of having large brains? Again, we will probably never know, but it is tempting to speculate that the brain and our extraordinary powers of communication evolved in concert.

Probably, as with bipedalism, there was no single factor but a mix of elements that worked together to result in the evolution of large brain size. We tend to focus on brain size because it is a convenient stand-in for mental power and because we can measure it in the fossil record by making the reasonable assumption that the volume of a fossil’s cranium reflects the size of its brain. However, it should be emphasized that our minds are not the products solely of larger brains. Rather, the reorganization of existing structures and pathways is the more important product of the evolution of the brain. Not only was the brain expanding over those 7 million years of human evolution, it was also being rewired.

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The human brain evolved through natural selection. What we have today is a learning machine capable of generating many more skills and abilities than just those that directly enhance evolutionary fitness. For example, let’s assume that the brain evolved to make group hunting more efficient. The result is that the brain does indeed enhance our ability to hunt together, but there are numerous by-products of this brain that take its abilities beyond just hunting. Making art, for instance, has nothing to do with group hunting, and yet the brain allows us to do it. A brain evolved for an essential but mundane task like group hunting lies at the heart of much that is wonderful about humanity.