The Origins of Human Intelligence

What leads to intelligence of an advanced, human kind? First one needs a certain kind of brain; ours is built to the same basic plan as the Old World monkeys and apes. Like us the world of color vision has led to reduced dependence on tactile hairs, olfaction, and auditory signals. To produce human intelligence it would certainly seem to help if the sensory inputs to the brain can be combined with the motor skills available when one has an intricate organ like the hand (or, in the case of elephants, the trunk) in front of the eyes. For selection to produce an ape brain the size of ours would appear to be trivial; simply one more division of brain cells in a developing chimp would produce human-sized brains. Considering the enormous value of large brains to humans, why haven’t the chimps opted for that one extra division of cells? Such a brain would not be organized like the human brain, of course, but in any case the chimps’ “metabolic budget” simply could not sustain such a brain. The brain is an enormously energy-devouring organ and the chimpanzee lifestyle can’t afford it. Even asleep a human infant can require up to 80 percent of its metabolic energy just to maintain its brain. Developing and supporting such an expensive organ requires many conditions. For example, while marsupial reproduction has its own advantages, it does not have placentation to supply the rich resources to support a greedy infant’s brain in the womb.

Once one begins to consider the features of human intelligence, one quickly realizes that this involves very special physiological adaptations, adventitious traits inherited from ancestors, the consequences of the dietary adaptation right on through to a very special, supportive, social organization. Human adaptation had to allow the mother to not only be “bipedally effective,” but also to give birth to a large-brained infant, whose brain then grows even larger after birth. During the period of late pregnancy and lactation both mother and infant are dependent and vulnerable. To sustain them requires a social group that is willing to provision them.

Humans and chimps are very closely related. We shared a common ancestor only 6 - 7 million years ago. Chimps and humans share 99 percent of their genes. By this measure we are more closely related to chimps than are horses to zebras or foxes to dogs or sheep to goats. If humans manage to wipe themselves out with some rampaging disease (leaving near relatives largely unharmed), could one or more of the great apes fill the human niche? Sadly, I think the “Planet of the Apes” scenario will remain just an entertaining fantasy.

Many of these issues were brought home to me vividly when I briefly babysat a young orphan chimp, Koby, at Jane Goodall’s Gombe research station. Despite his small size and tender age, Koby’s upper body strength was embarrassingly close to my own. This brought home to me the fact that the lifestyle of the great apes means that their young must consume energy in early muscular development, and there is not enough energy left over to also support a greedy brain.

Contributed by: Dr. Irven DeVore