Raw food takes too long to digest and
offers too few calories to grow a human brain. Cooking it is the key.
Gathered
around a blazing fire, our ancient ancestors probably huddled to pass the
archaic kebab, munching cooked meat and figuring out how they might share it
and plan to get more of it. Eating cooked food allowed these early hominids to
spend less time gnawing on raw material and digesting it, providing time--and
energy--to do other things instead, like socialize. The strenuous cognitive
demands of communicating and socializing forced human ancestors to develop more
powerful brains, which required more calories--calories that cooked food
provided. Cooking, in other words, allowed us to become human.
A new paper
examines the metabolic restrictions of a raw diet, and suggests that our
primate cousins are limited by their inability to heat their dinners. It
bolsters the cooking hypothesis of Richard Wrangham, a primatologist and
professor of biological anthropology at Harvard who believes cooking is our
legacy.
Brazilian
biomedical scientists Karina Fonseca-Azevedo and Suzana Herculano-Houzel note
that the largest primates do not have the largest brains, a perplexing
question. Encephalization (a larger brain size per body size
than you'd expect) has long been thought to be a key feature setting humans
apart from other primates, and mammals as a whole, but there is no consensus on
how or why this happened.
"We
consider this disparity to be a clue that, in primate evolution, developing a
very large body and a very large brain have been mutually excluding strategies,
probably because of metabolic reasons," the authors write. They're the
first to try and quantify these limits.
"You
would think, 'Surely people have thought about this stuff before,'"
Wrangham said in an interview. "But nobody has ever thought about the fact
that cooking gives you more energy."
This is a
central thesis of Wrangham's 2009 book, "Catching Fire." He argues that the control of fire
allowed early hominids to not only cook their food, but obtain warmth, allowing
them to shed body hair and in turn run faster without overheating; to develop
calmer personalities, enabling social structures around the hearth; and even to
form relationships among men and women--in short, to become human.
"My day
job is studying chimpanzees in the wild, and I have often studied feeding
behavior. I have tried to survive on what chimps eat," he said.
Really?
"If I
don't have any food with me, I just eat what they eat. And that told me that
what they eat is totally unsatisfying," he continued. "I thought
about what would happen if humans had to live like chimps. And that took me
very rapidly to the conclusion, within a few minutes, that as long as we've
been human, it's hard to imagine how we could live on raw food."
Wrangham's
ideas follow the expensive-tissue hypothesis. That concept predicts an inverse
relationship between brain size and gut size--to accommodate a large,
human-sized brain, our guts shrank relative to our primate cousins. Imagine the
pot belly of a gorilla, Wrangham notes. This paper doesn't even address gut
size, just the requirements of our hungry brains.
"In
order to be able to apply a sufficient number of calories to the brain, you
have to be able to cook your food," Wrangham said. "You can only
afford to have a brain if you can supply a lot of energy to it."
The idea is
that raw food just doesn't provide enough calories. You have to get out more
than you put in, and raw food takes a lot more work (meaning calories) for your
muscles and organs to chew and digest, resulting in a net decrease in the
amount of calories available for the rest of your cells. But you can only spend
so many hours of the day eating--there must be time to sleep, forage and
procreate, too. This limits the amount of calories you can get per day, and it
turns out this is directly related to how many neurons you can grow, according
to Fonseca-Azevedo and Herculano-Houzel.
The duo crunched
numbers to figure out the metabolic costs of a human-sized brain, which is the
third most energy-expensive organ in the human body, ranking below only
skeletal muscle and the liver in terms of metabolic needs. The more neurons the
brain has, the more energy it needs. If we ate an only-raw diet, to maintain
the body size we humans possess, as well as the number of neurons our brains
possess, people would have to eat for more than 9 hours per day, they found.
Cooked food
allowed Homo erectus to overcome these limitations, they conclude.
Like
Wrangham, the authors also see a feedback loop in the history of feeding. Along
with increasing the efficiency of our food intake and eliminating limits on
growth, eating cooked food would have increased the time humans could spend
around the fire, time spent together. Socializing, along with other
"cognitively demanding" activities--like developing speech, social
structure and civilization--would have required more brain power. And humans
could afford to develop these more powerful brains, thanks to their improved,
cooked diet. This positive feedback drove the rapid increase in neurons that
took place in human evolution, the authors say.
It makes
perfect sense, if you think about it. Cooking does some of the work of digestion
for us, as Wrangham puts it.
"Molecules
are moving faster under the influence of heat; they are breaking up or shaking
apart from each other, and that's essentially what happens in digestion, the
denaturating of proteins," he said. "They lose their structure, and
become more accessible."
As an
example, he and others have investigated the effects of cooking on starch
molecules and humans' ability to digest cooked versus raw grains. Simply
cooking starchy foods increases the net energy gain by 30 percent, he said.
"The
grains themselves represent long chains of glucose, which are very difficult to
digest until they have been gelatinized; you are opening up these chains,"
he said. Take, for example, a simple white sauce of flour and butter. You have
to stir constantly over even heat, letting the water in the butter invade the
starch molecules in the grain. "Then you get this change in consistency,
where the whole thing becomes a continuous colloid, and the starch grains have
become gelatinized. The result is that it will be easier to digest,"
Wrangham said. "Our body pays fewer calories for the digestion."
Although
this paper doesn't address the raw foodist culture directly, Wrangham does,
arguing that humans are not adapted to eating raw food. Subsisting on raw foods
is a hugely effective way to lose weight, which in and of itself suggests that
it's not enough for healthy development. Your body needs more calories than it
is consuming, so it starts consuming your fat reserves as a supplement.
"If you
want to lose weight, a raw food diet is the best thing you can do--it's very
well-balanced in vitamins, and well mixed in terms of macro nutrients. But you
have to be careful ... the evidence is pretty strong that it would be really
dangerous to bring a child up on a raw diet," he said. "Partly for
that reason, I'm delighted to see this paper that emphasizes the importance of
cooking for the evolution of the brain. I hope it will deter people who are
overly enthusiastic about their raw food diets."
The paper is published today in the
Proceedings of the National Academy of Sciences.
Posters comments: There are plenty of food safety related
reasons we cook and refrigerate, freeze, dehydrate, etc. so much of our food,
too. That includes our cooking instruments and serving items. One interested
can research the idea of shelf life, also. Here is one link on food safety: http://en.wikipedia.org/wiki/Food_safety
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