Scientific American Sep 2018

(Jeff_L) #1
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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The Cultural


Drive Hypothesis


3Çx`žxäÇß ̧‰`žx³îin teaching and innovating
generation after generation should have larger
brains—or so postulates the cultural drive hypothesis.
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ä ̧`žD§UxšDþž ̧ßäD³lx³xîž`䞳ÿšž`šD``øßDîx
copying of others’ behaviors selects for better
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leads to enhanced social behaviors and technical
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UßDž³äD³lø§îž­Dîx§āßxDîxßx‡`žx³`āž³îxD`šž³
and copying. Humans have mastered this virtuous
circle better than any other species.
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