Scientific American - September 2018

September 2018, ScientificAmerican.com 37

poor at most of them and have a lower metric. The
strong correlations between primate intelligence
and both brain size measures and performance in
laboratory tests of learning and cognition validate
the use of the metric as a measure of intelligence.
The interpretation also fits with neuroscientific anal-
yses showing that the size of individual brain compo-
nents can be accurately predicted with knowledge of
overall brain size. Associated with the evolution of
large primate brains are bigger and better-connected
regions—neocortices and cerebellums—that allow ex-
ecutive control of actions and increased cortical pro-
jections to the motor neurons of the limbs, facilitating
controlled and precise movements. This helps us to
understand why big-brained animals show complex
cognition and tool use. [For more on primate brains,
see “Are We Wired Differently?” on page 60.]
Plotting the intelligence measure on a primate
family tree reveals evolution for higher intelligence
taking place independently in four distinct primate
groups: the capuchins, macaques, baboons and great
apes—precisely those species renowned for their so-

cial learning and traditions. This finding is exactly the

pattern expected if cultural processes really were driv-

ing the evolution of brain and cognition. Further anal-

yses, using better data and cutting-edge statistical

methods, reinforce these conclusions, as do models

that make quantitative predictions for brain and body

size based on estimates of the brain’s metabolic costs.

Cultural drive is not the only cause of primate

brain evolution: diet and sociality are also important

because fruit-eating primates and those living in large,

complex groups possess large brains. It is difficult,

however, to escape the conclusion that high intelli-

gence and longer lives co-evolved in some primates

because their cultural capabilities allowed them to ex-

ploit high-quality but difficult-to-access food resourc-

es, with the nutrients gleaned “paying” for brain

growth. Brains are energetically costly organs, and so-

cial learning is paramount to animals gathering the

resources necessary to grow and maintain a large

brain efficiently.

Graphic by Federica Fragapane

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brains—or so postulates the cultural drive hypothesis.

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and copying. Humans have mastered this virtuous

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