Wrangham starts off with some critical observations: No known human tribe lives on a predominantly raw food diet, and those modern people who attempt to live on a largely raw food diet have demonstrated difficulties maintaining body mass, energy levels, and fertility. This points to the hypothesis that modern humans are actually "adapted to eating cooked food in the same essential way as cows are adapted to eating grass, or fleas to sucking blood, or any other animal to its signature diet. We are tied to our adapted diet of cooked food, and the results pervade our lives, from our bodies to our minds. We humans are the cooking apes, the creatures of the flame."
Traditional Chinese medicine has for milennia maintained that humans need to eat cooked food to get adequate food energy (Pinyin: gu qi). In The Tao of Healthy Eating, traditional Chinese physician Bob Flaws writes:
"Traditional Chinese medicine suggests that most people, most of the time, should eat mostly cooked food. Cooking is predigestion on the outside of the body to make food more easily digestible on the inside. By cooking foods in a pot on the outside of the body, one can initiate and facilitate the stomach's rottening and ripening in its pot on the inside of the body. cold and raw foods require that much more energy to transform them into warm soup within the pot of the stomach. Since it takes energy or qi to create this warmth and transformation, the net profit from this transformation is less. Whereas, if one eats cooked foods, less qi is spent in the process of digestion. This means that the net profit of digestion, i.e. qi or energy, is greater."
This perspective contradicts the common belief that raw food is better than cooked because cooking can destroy nutrients. But as Flaws points out, net nutrient delivery matters more than gross amount of nutrient in the raw food. Let's assume that a carrot has 10 units of X nutrient, but only 10% of it is available to humans because it is locked in an largely indigestible cellulose envelope. Let's say that cooking destroys 50% of that nutrient (a gross overestimation for proper cooking), but increases the availability to 50%. The net delivery of X from the raw carrot is 1 unit, but the net from the cooked carrot is 2.5 units.
Wrangham presents multiple lines of evidence that humans and non-humans have a greater net macronutrient absorption from cooked than from raw foods, resulting in cooked foods delivering more energy than raw foods.
Wrangham includes some of the research I discussed in my series on raw vegan diet, which found that a high proportion of people eating diets high in raw foods (70% or more raw) are underweight and have low fertility. Belgian researchers showed that humans can digest only about 65% of the protein in raw eggs, but 91-94% of the protein from cooked eggs.  Another team showed that enzymatic digestion of heated beef protein increased by four times over raw beef protein.  This occurs because cooking denatures protein more effectively than stomach acid, making it more vulnerable to enzymatic digestion.
Wrangham's hypothesis competes with the Man-The-Hunter hypothesis which maintains that humans evolved big brains and small guts by route of increased meat-eating. However, Wrangham points out that the hunting hypothesis can't account for some of the facts. Increased meat-eating might explain the transition from Australopithecines to Homo habilis (habilines), but not the transition from the habilines to Homo erectus:
"Meat-eating accounts smoothly for the first transition, jump-starting evolution toward humans by shifting chimpanzeelike australopithecines into knife-wielding, bigger-brained habilines, while still leaving them with apelike bodies capable of collecting and digesting [raw] vegetable foods as efficiently as did australopithecines. But if meat eating explains the origin of the habilines, it leaves the second transition unexplained, from habilines to Homo erectus. Did habilines and Homo erectus obtain their meat in such different ways that they evolved different kinds of anatomy? Some people think the habilines might have been primarily scavengers while Homo erectus were more proficient hunters. The idea is plausible, though archaeological data do not directly test it. But it does not solve a key problem concerning the anatomy of Homo erectus, which had small jaws and small teeth that were poorly adapted for eating the tough raw meat of game animals. These weaker mouths cannot be explained by Home erectus's becoming better at hunting. Something else must have been going on."Increased meat eating can't explain whey we have such small mouths and jaws.
"Given that the mouth is the entry to the gut, humans have an astonishingly tiny opening for such a large species....To find a primate with as relatively small an aperture as that of humans, you have to go to a diminutive species, such as a squirrel monkey weighing less than 1.4 kilograms (3 pounds). In addition to having a small gape, our mouths have a relatively small volume––about the same size as chimpanzee mouths, even though we weigh some 50 percent more than they do. Zoologists often try to capture the essence of our species with such phrases as the naked, bipedal, or big-brained ape. They could equally well call us the small-mouthed ape."Compare the jaws of any raw food eating animal to human jaws. The largely vegetarian chimp has a gape much larger than that of a human:
The carnivorous cat has a gape nearly half the size of its head, and the jaws are very powerful for cutting through raw meat.
You can see some other big yawns here. Compare to the modern human gape:
By the way, although Wrangham does not mention it, the shrinkage and reorganization of the mouth laid the foundation for speech. Thus, we may owe our linguistic abilities to the mastery of fire and cooking. I seem to recall reading that another anthropologist had proposed this hypothesis more than 20 years ago, but I no longer have the book that had the reference.
If evolution from Homo habilis to Homo erectus had been driven by increased consumption of raw meat, with technology and cooking as an afterthought, we would expect to have seen it maintain the large powerful ape mouth and jaws, retained the large, sturdy teeth, and increased the shearing action for adaptation to meat eating. Instead, from the habilines to the erectines the mouth and teeth shrank.
Here's a habiline skull:
|Source: www.anhb.uwa.edu.au/.../ skulls/s10_homo_habilis|
And here's an erectine skull:
The erectine jaw and teeth are much smaller relative to body size. Erectines had a smaller gape and must have had a softer diet than the habilines. The skeletal remains provide the best available evidence that some tribe of Homo habilis discovered something that made for a much softer and energy-rich diet, giving rise to Homo erectus.
One might think that the use of knives and hammers alone selected for smaller mouths. Perhaps habilines simply cut the meat into small pieces or pounded it tender. Although initially plausible, on further examination, this loses credibility, because it can't explain how an animal adapted to a diet consisting predominantly of raw vegetation can continue eating that vegetation while adapting to the raw meat portion of the diet.
Wrangham notes that "Peter Lucas has calculated that the size of a tooth needed to make a crack in a cooked potato is 56 percent to 82 percent smaller than needed for a raw potato." Thus, so long as human ancestors continued to eat raw plants, they needed large teeth and jaws. And they definitely needed to eat plants.
"The problem is that tropical hunter-gatherers have to eat at least half of their diet in the form of plants, and the kinds of plant foods our hunter-gatherer ancestors would have relied on are not easily digested raw.Tropical wild game simply does not provide adequate amounts of fat or carbohydrate to prevent excessive intake of protein resulting in ammonia and urea accumulation, especially in the annual dry seasons, when the whole carcass fat levels of game will drop as low as 1 percent to 2 percent.
By the way, Wrangham notes:
"Starchy foods make up more than half of the diets of tropical hunter-gatherers today and may well have been eaten in similar quantity by our human and pre-human ancestors in the African savannas."Moreover, if raw meat was a staple of our ancestors, we would expect modern humans to have some significant resistance to toxins produced by bacteria that infect raw meat. But we are still vulnerable to those bacterial toxins.
In addition, there is a major economic problem with the meat-eating hypothesis. Wrangham has studied chimps directly, watching them hunt and eat. The typical chimp has to spend about 6 hours daily chewing its bulky, chewy raw foods. They hunt opportunistically, but will only spend 15 to 20 minutes on a hunt. If not successful in that time frame, they give up and return to eating plants. Why?
Wrangham explains that because digestion of raw food takes more time than digestion of cooked food and costs a lot of energy, a chimp has to devote eight or nine hours daily to feeding in order to get adequate energy. Australopithecines and habilines probably had similar constraints. This would have prevented them from investing much time in hunting:
"Males who did not cook would not have been able to rely on hunting to feed themselves. Like chimps, they could hunt in opportunistic spurts. But if they devoted many hours to hunting, the risk of failure to obtain prey could not be compensated rapidly enough. Eating their daily required calories in the form of their staple plant foods would have taken too long."As Wrangham explains, a division of labor into hunting and gathering would not solve this problem, so long as the food was consumed raw.
"Suppose that a hunter living on raw food has a mate who is willing to feed him, that his mate could collect enough raw foods for him (while satisfying her own needs) and would bring them back to a central place, to be met by her grateful mate. Then suppose the male has had an unsuccessful day of hunting....The hungry hunter needs to consume, say, two thousand calories, but he cannot eat after dark. To do so would be too dangerous, scrabbling in the predator-filled night to feel for the nuts, leaves, or roots his gatherer friend brought him. If the hunter slept on the ground, he would be exposed to predators and large ungulates as he fumbled for his food. If he were in a tree, he would find it hard to have his raw foods with him because they do not come in tidy packages.
"So to eat his fill he would have to do most of his eating before dusk, which falls between about 6 and 7 P.M. in equatorial regions. If he had eaten nothing while on the hunt, he would need to be back in camp before midday, and there he would find his mate's gathered foods 9assuming she had been able to complete her food gathering so early in the day). He would then have to spend the rest of the day eating, resting, eating, resting, and eating. In short, the long hours of chewing necessitated by a raw diet would have sharply reduced hunting time. It is questionable whether the sexual division of labor would have been possible at all.In short, cooking (and other culinary technologies that make food softer and easier to digest) made it possible for humans to pursue increased meat-eating. It freed men from the need to continuously feed on plant foods, giving them time to devote to hunting meat.
"The use of fire solved the problem. It freed hunters from previous time constraints by reducing the time spent chewing. It also allowed eating after dark. The first of our ancestral line to cook their food would have gained several hours of daytime. Instead of being an opportunistic activity, hunting could have become a more dedicated pursuit with a higher potential for success. Nowadays men can hunt until nightfall and still eat a large meal in camp. After cooking began, therefore, hunting could contribute to the full development of the family household, reliant as it is on a predictable economic exchange between women and men."
Simply put, cooked food delivers more energy and nutrition in a smaller, more easily digested package than raw food. Wrangham argues that since Homo erectus had a larger brain and a much smaller face, mouth and teeth than Homo habilis, probably some tribe of Homo habilis first controlled fire and used it for cooking. The resulting increase in energy and nutrient availability led to rapid selection for smaller guts and larger brains and bodies. By providing protection from nocturnal predators, control of fire also enabled human ancestors to give up tree-dwelling. It also supported the sexual division of labor (hunting and gathering/cooking) present in human cultures.
Wrangham's Catching Fire will provide plenty of food for thought for anyone interested in ancestral nutrition.