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WHY are people brighter than chimps? Many think it’s because the part of the
brain linked to intelligence, the frontal cortex, is much bigger in people than
in apes when compared to overall brain size. But now it turns out this isn’t
true at all: that part of our brains is actually about the same relative
size.

Two key reports published in 1912 and 1968 suggested that people have larger
frontal cortices relative to their overall brain size than monkeys or other
mammals. Because the frontal cortex turned out to be a key centre for
problem-solving, planning and general intelligence, researchers suggested that
it might explain why humans have such unique brainpower.

But these reports were based on limited studies. They looked at only one or
two brains from a few species, and didn’t include most of the great apes. What’s
more, the animals were dead, and it’s possible that some brain regions had
shrunk more than others after death. Other small studies since then have come up
with conflicting results.

Now Katerina Semendeferi from the University of California at San Diego has
looked at the brains of a far larger sample of living animals using magnetic
resonance imaging. Her team analysed brain scans of 15 great apes—chimps,
bonobos, gorillas and orang-utans—along with four gibbons and five
monkeys, and compared the results to brain scans from 10 people.

Monkeys and gibbons did indeed have a relatively small frontal cortex as a
percentage of their overall cerebral cortex size—gibbons, for example,
averaged 29 per cent. But the figure for people and all the great apes was at
least 35 per cent. Both humans and orang-utans, for example, averaged 38 per
cent.

Richard Passingham, an Oxford University neuroscientist, points out that
there’s another way of explaining why humans are brighter than chimps. It could
be that the size of the entire brain in comparison to body size is the crucial
point. This ratio is roughly constant for most animals. Elephants, for instance,
have huge brains because they have to control so much muscle.

But people have far larger brains for our body size than other primates, so
our frontal cortices are larger too. “They must be over three times larger than
would be expected for a hypothetical great ape of the same weight,” says
Passingham. “Such a difference must be of immense consequence for our capacity
to plan and reason.”

Semendeferi says that the difference in brainpower between people and chimps
may not depend on size alone. Instead, it could be down to better neural
circuits connecting parts of the frontal cortex. “Behaviours are supported by
neural systems, not localised chunks of tissue,” she says.

Or it may be due to the relative size of more specialised parts within the
cortex itself. Both Passingham and Semendeferi agree that researchers need to
look more closely at the sizes of these smaller regions. It may be that a
section like the prefrontal cortex—central to high-level human thinking,
creativity and emotion—is larger in people than in apes by any measure.