Higher intelligence would be an advantage everywhere if there were no costs accompanying it. But what matters are not merely the positive benefits of a trait, but also the costs involved. The changes in human anatomy and physiology that lead to higher intelligence do not come cost free.
For example, one way for a subgroup of human beings to develop higher intelligence is for that group to evolve larger brains. However, larger brains impose at least three important costs:
1) Larger brains require larger amounts of energy. Since its neurons must be repeatedly recharged, the brain requires an amount of energy that is far out of proportion to its size. (A typical human brain contains only about 2% of the body's mass, but uses about 20% of a resting person's energy.) Larger brains, of course, require more energy than smaller ones.
2) Larger brains require larger heads, which creates strains on the muscular and skeletal structure. Standing on two legs is a complex problem in balancing, and weight situated near the top of our structure is particularly hard to balance. Larger brains and heads make this problem even greater.
3) Larger brains (and heads) require wider female pelvises, and the wider pelvises result in less efficiency in walking and running. The large size of the human head creates serious difficulties in childbirth (which is why human females typically have far more difficulty in delivering their young than do females of most other species). As brains and heads became larger, wider female pelvises were required to accommodate them. Since it was difficult for such a change to be confined to one sex only, the male members of those groups also wound up with wider pelvises. But wider pelvises and hips result in lower running speeds (see section 14-7) and less efficiency in both walking and running.
The combination of costs that are involved in having larger, more powerful brains explains why many human groups did not evolve brains as large as those typically found in north Asians and north Europeans. However, in cold climates, the advantages of higher intelligence outweighed the costs just described.
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