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
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