Study finds cooking increases energy from meat, may have driven human evolution

Nov 07, 2011

Next time you're out to dinner, you may want to think twice before ordering your steak rare.

In a first-of-its kind study, Harvard researchers have shown that cooked meat provides more energy than , a finding that suggests humans are biologically adapted to take advantage of the benefits of cooking, and that cooking played a key role in driving the evolution of man from an ape-like creature into one more closely resembling modern humans.

Conducted by Rachel Carmody, a student in Department of Human Evolutionary Biology at Harvard's Graduate School of Arts and Sciences and described in the (PNAS), the research also raises important questions about the way modern humans eat.

"The results of this paper are equally relevant to and to the way we think about today," Carmody said. "It is astonishing that we don't understand the fundamental properties of the food we eat. All the effort we put into cooking food and presenting it – mashing it up, or cutting it, or slicing or pounding it – we don't understand what effect that has on the energy we extract from food, and energy is the primary reason we eat in the first place."

Though earlier studies had examined specific aspects of what happens during the cooking process, surprisingly Carmody said, none had ever fully examined whether cooking affected the in vivo energy value of food.

"There had been no research that looked at the net effects – we had pieces that we could not integrate together, so we didn't know what the overall answer was," Carmody said. "We knew some of the mechanisms that might be at play, but we didn't know how they combined."

To examine those effects, Carmody designed a unique experimental model. Over forty days, she fed two groups of mice a series of diets that consisted of either meat or sweet potatoes prepared in four ways – raw and whole, raw and pounded, cooked and whole, and cooked and pounded.

Over the course of each diet, researchers tracked changes in each mouse's body mass, as well as how much they used an exercise wheel. The results, Carmody said, clearly showed that cooked meat delivered more energy to the mice that raw.

It's a finding, she said, that holds exciting implications for our understanding of how humans evolved.

Though early humans were eating meat as early as 2.5 million years ago, without the ability to control fire, any meat in their diet was raw, and probably pounded using primitive stone tools. Approximately 1.9 million years ago, however, a sudden change occurred. The bodies of early humans grew larger. Their brains increased in size and complexity. Adaptations for long-distance running appeared.

Though earlier theories suggested the changes were the product of increased meat in their diet, Carmody's research points to another possible hypothesis – that cooking provided early humans with more energy, allowing for such energetically-costly evolutionary changes.

Although that theory that had been advanced years earlier by Richard Wrangham, the Ruth Moore Professor of Biological Anthropology and Master of Currier House, it wasn't until Carmody's work that scientists had hard evidence to either support or refute it.

"I'm a biologist by training," Wrangham said. "If you want to understand the anatomical, physiological and behavioral features of a species, its diet is the first thing you ask about. If you want to know what makes a giraffe tick, it's the fact that it eats leaves from the tops of trees. If you want to understand the shape of a flea, it's because it eats blood. But with humans everyone had said what's key about humans is the fact that we are variable, that we are good at solving problems, so human adaptation in general is the result of our brains. But this, right away, strays from the fundamental biological concept of diet.

"That's why Rachel's work is so important," he continued. "For the first time, we have a clear answer to the why cooking is so important cross culturally and biologically – because it gives us increased energy, and life is all about energy."

The impacts of Carmody's work, however, aren't limited to the early days of human evolution. The findings also lay bare the shortcomings in the Atwater system, the calorie-measurement tool used to produce modern food labels.

"That system is based on principles that don't reflect the in vivo energy availability," Carmody said. "Although it measure what has been digested, the human gastrointestinal system includes a whole host of bacteria, and those bacteria metabolize some of our food for their own benefit.

"Atwater doesn't discriminate between food that is digested by the human or the bacteria, and increasing evidence suggests that the bacteria take a pretty good portion of the food we eat," she continued. "In fact, research has shown that one of the ways to increase the value humans get, relative to the bacteria, is by processing food, and cooking is one way to do that."

Carmody's research could also inform how food scientists tackle one of the thorniest of dietary challenges – the prevalence of obesity in Western nations, and malnutrition in developing parts of the world.

"As human evolutionary biologists, we think about energetic gain as being something positive, because it allows for growth and reproduction, and it's a critical component of a species' evolutionary fitness," Carmody said. "But the question in the modern world is: If we now have the problem of excess as opposed to deficit, is that still a positive?

"This research illuminates that that way we've been thinking about food energy value historically, and the way we derive recommendations whether for areas that are experiencing famine or areas where people suffer from energetic excess have been based on assumptions that are not biologically relevant. Instead, they've been based on the treatment of the human body as an efficient digestion machine, when, in fact, it's not, and the degree to which it's not is affected by food processing, including cooking."

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adrianv
3 / 5 (2) Nov 07, 2011
Very interesting! It makes a lot of sense. In addition, I think it is possible that, once cooked, the food would likely last longer, giving early humans access to a lifestyle that is not so dependent on a daily food gathering lifecyle. From an evolutionary perspective, this would be a game-changer as well.
ArtflDgr
not rated yet Nov 07, 2011
It may now explain something else...
canine who came near human habitat but did not attack humans, may over time get meat scraps. given mans proclivity to eat at similar times a day separated from the time of kill, an animal could do their days hunting, and show up for dinner scraps in times of plenty. those who were friendlier and so on, would end up like the humans, bigger and more able than the others.

just a thought...
jmhoward
not rated yet Nov 08, 2011
It is my hypothesis that human evolution resulted from selection for testosterone: "Androgens in Human Evolution," Rivista di Biologia / Biology Forum 2001; 94: 345-362.

I think the underlying change was increased testosterone in, most likely, Homo erectus females. In H. erectus the female body increased in size while the male body did not. This high testosterone female increased exposure of her fetus to increased testosterone which increased androgen receptors within the brain which increased brain size at the expense of the body. Hence, cranial growth occurred at the expense of post-cranial size.

I suggest fire and its effect on meat preservation, taste, and energy therein resulted from the increase in brain size in this group. Furthermore, the subsequent changes in hominids resulted from increases in testosterone within hominids with time. I do not think increased energy from cooked meat was causal in human evolution but was a result of increased brain size as testosterone in
tadchem
not rated yet Nov 08, 2011
Interesting, but do not forget the survival value in sterilizing the food against pathogens. A small family of hunter-gatherers may not be able to eat an entire antelope or deer before the ubiquitous bacteria find it and set up shop, and cooking the meat would make it far safer to eat - later.
Eric_B
not rated yet Nov 08, 2011
"and cooking the meat would make it far safer to eat - later."

to say nothing of yumming salting and seasoning!
alanborky
not rated yet Nov 13, 2011
Cooking may well provide more energy from meat but the eater still has to be sufficiently adapted to be able to assimilate that energy: try feeding cooked or raw meat to, say, a frog or a rabbit.

Then there's the fact domesticated animals, especially pets, which regularly eat raw on the bone meat or garden kills tend to be more lively and aggressive than those that eat factory processed - i.e. cooked - meat.

Cooking can also destroy other nutritional aspects of the meat, e.g. vitamins, enzymes.

Then there's also the issue of what constitutes cooking: I know people who won't eat meat unless it's thoroughly frazzled to a blackened cinder; with others the meat must be so 'rare' the animal's practically still breathing.
wttmartin9
not rated yet Nov 13, 2011
We don't eat meat for its energy in general, protein is more important. If your out for a rare steak with a few potatoes and some vegetables, the energy from the potatoes will be the most important source, then the vegetables, then the meat. So more important in the evolution theory is can we get more protein from cooked meats?