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"Mmm yummy, tasty, juicy...puh puh puh!" |
(Wired.com, 23, Oct 2012) - Eating a raw food diet is a
recipe for disaster if you’re trying to boost your species’ brainpower. That’s
because humans would have to spend more than 9 hours a day eating to get enough
energy from unprocessed raw food alone to support our large brains, according
to a new study that calculates the energetic costs of growing a bigger brain or
body in primates. But our ancestors managed to get enough energy to grow brains
that have three times as many neurons as those in apes such as gorillas,
chimpanzees, and orangutans. How did they do it? They got cooking, according to
a study published online today in the Proceedings of the National Academy of
Sciences.
“If you eat only raw food, there are not enough hours in the
day to get enough calories to build such a large brain,” says Suzana
Herculano-Houzel, a neuroscientist at the Federal University of Rio de Janeiro
in Brazil who is co-author of the report. “We can afford more neurons, thanks
to cooking.”
Humans have more brain neurons than any other primate — about
86 billion, on average, compared with about 33 billion neurons in gorillas and
28 billion in chimpanzees. While these extra neurons endow us with many
benefits, they come at a price — our brains consume 20 percent of our body’s
energy when resting, compared with 9 percent in other primates. So a
long-standing riddle has been where did our ancestors get that extra energy to
expand their minds as they evolved from animals with brains and bodies the size
of chimpanzees?
One answer came in the late 1990s when Harvard University
primatologist Richard Wrangham proposed that the brain began to expand rapidly
1.6 million to 1.8 million years ago in our ancestor, Homo erectus, because
this early human learned how to roast meat and tuberous root vegetables over a
fire. Cooking, Wrangham argued, effectively predigested the food, making it
easier and more efficient for our guts to absorb calories more rapidly. Since
then, he and his colleagues have shown in lab studies of rodents and pythons
that these animals grow up bigger and faster when they eat cooked meat instead
of raw meat — and that it takes less energy to digest cooked meat than raw
meat.
In a new test of this cooking hypothesis, Herculano-Houzel
and her graduate student, Karina Fonseca-Azevedo, now a neuroscientist at the
National Institute of Translational Neuroscience in São Paulo, Brazil, decided
to see if a diet of raw food inherently put limits on how large a primate’s
brain or body could grow. First, they counted the number of neurons in the
brains of 13 species of primates (and more than 30 species of mammals). The
researchers found two things: one, that brain size is directly linked to the
number of neurons in a brain; and two, that that the number of neurons is
directly correlated to the amount of energy (or calories) needed to feed a
brain.
After adjusting for body mass, they calculated how many
hours per day it would take for various primates to eat enough calories of raw
food to fuel their brains. They found that it would take 8.8 hours for
gorillas; 7.8 hours for orangutans; 7.3 hours for chimps; and 9.3 hours for our
species, H. sapiens.
These numbers show that there is an upper limit on how much
energy primates can get from an unprocessed raw diet, Herculano-Houzel says. An
ape’s diet in the wild differs from a modern “raw food diet,” in which humans
get sufficient calories from processing raw food in blenders and adding protein
and other nutrients. In the wild, other apes can’t evolve bigger brains unless
they reduce their body sizes because they can’t get past the limit of how many
calories they can consume in 7 hours to 8 hours of feeding per day. But humans,
she says, got around that limit by cooking. “The reason we have more neurons
than any other animal alive is that cooking allowed this qualitative change —
this step increase in brain size,” she says. “By cooking, we managed to
circumvent the limitation of how much we can eat in a day.”
This study shows “that an ape could not achieve a brain as
big as in recent humans while maintaining a typical ape diet,” Wrangham says.
Paleoanthropologist Robert Martin of The Field Museum in
Chicago, Illinois, agrees that the new paper does “provide the first evidence
that metabolic limitations” from a raw food diet impose a limit on how big a
primate’s brain — or body — can grow. “This could account for small brain sizes
of great apes despite their large body sizes.” But “the jury is still out” on
whether cooking was responsible for the first dramatic burst of brain growth in
our lineage, in H. erectus, Martin says, or whether our ancestors began cooking
over a fire later, when the brain went through a second major growth spurt
about 600,000 years ago. Hearths show up in the archaeological record 800,000
years ago and the regular use of fire for cooking doesn’t become widespread
until more recently.
But to Herculano-Houzel’s mind, our brains would still be
the size of an ape’s if H. erectus hadn’t played with fire: “Gorillas are stuck
with this limitation of how much they can eat in a day; orangutans are stuck
there; H. erectus would be stuck there if they had not invented cooking,” she
says. “The more I think about it, the more I bow to my kitchen. It’s the reason
we are here.”
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