Human biology reorganized itself to cope with
the punishing burden of an oversize brain.
That shift completely reshaped who we are.
IN ;;;; THE SWEDISH TAXONOMIST CAROLUS LINNAEUS DUBBED OUR SPECIES Homo sapiens, Latin for “wise man.” It’s a matter of open debate
whether we actually live up to that moniker. If Linnaeus had wanted
to stand on more solid ground, he could have instead called us Homo
megalencephalus: “man with a giant brain.”
Regardless of how wisely we may use our brains, there’s no disputing
that they are extraordinarily big. The average human brain weighs in
at about three pounds, or ;,;;; grams. Our closest living relatives, the
chimpanzees, have less than one-third as much brain—just ;;; grams.
And if you compare the relative size of brains to bodies, our brains are
even more impressive.
As a general rule, mammal species with big bodies tend to have big
brains. If you know the weight of a mammal’s body, you can make a
fairly good guess about how large
its brain will be. As far as scientists
can tell, this rule derives from the
fact that the more body there is, the
more neurons needed to control
it. But this body-to-brain rule isn’t
perfect. Some species deviate a
little from it. A few deviate a lot. We
humans are particularly spectacular
rule breakers. If we were an ordinary
mammal species, our brains would
be about one-sixth their actual size.
Competing theories seek to explain
the value of a big brain. One idea,
championed by psychologist Robin
Dunbar of the University of Oxford, is
that complicated social lives require
big brains. A relatively large-brained
baboon can make a dozen alliances
while holding grudges against several
rivals. Humans maintain far more,
and more complicated, relationships.
Managing a social network can
yield signi;cant bene;ts: When a ;ght
breaks out, it pays to have friends at
your back. But keeping tabs on one’s
social life requires e;ort. Dunbar and
his colleagues have found that it takes
longer for people to answer questions
when they have to think about what’s
going on in other people’s minds. And
the more “mind reading” a question
requires, the more it activates the brain.
A di;erent perspective comes
from Daniel Sol of the Center for
Terrestrial Ecology and Applied
Forestries in Barcelona, Spain, who
studied the introduction of animals
to new habitats. Comparing successful and unsuccessful invaders, he
found that in both birds and mammals, big-brained species are more
likely to be successful than are small-brained ones. ;is research suggests
that larger brains are more adept at
problem solving, which translates
into a better chance of survival.
It is possible that both Dunbar and
Sol are at least partially correct. But
no matter how the debate resolves,
a puzzling question remains: If big
brains are so useful, then why are they
relatively rare? ;e answer is that
nothing in nature comes for free—and
where the brain is concerned, the cost
can be enormous. In fact, scientists are
discovering that a lot of human biology has reorganized itself to cope with
the burden of an oversize brain.
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University College London, and Peter
Wheeler of Liverpool John Moores
University o;ered the ;rst possible
reason for the rarity of big brains.
Neurons, they pointed out, have a
voracious appetite. ;ey require lots of
energy to produce their voltage spikes
and to release neurotransmitters. ;ey
get that energy from oxygen and food,
mostly glucose. A three-pound human
brain burns up to ;; times as many
calories as three pounds of muscle.
We cannot ignore this demand, even
for a moment. A few minutes without
oxygen may not do too much damage
to our muscles but can irreparably
harm the brain. ;e brain also requires
a constant supply of food. Twenty-;ve percent of all the calories you eat