Book Summary: “Catching Fire: How Cooking Made Us Human” by Richard Wrangham

Title: Catching Fire: How Cooking Made Us Human
Author: Richard Wrangham
Scope: 3 stars
Readability: 4 stars
My personal rating: 5 stars
See more on my book rating system.

If you enjoy this summary, please support the author by buying the book.

Topic of Book

Wrangham argues that cooking food was a key milestone in the evolution of the human species.

If you would like to learn more about key innovations in human history, read my book From Poverty to Progress: How Humans Invented Progress, and How We Can Keep It Going.

Key Take-aways

  • One of mankind’s most important innovation was cooking food over a fire.
  • Cooking effectively pre-digests meat and starches, making it possible for humans to fully digest all their energy and nutrients.
  • Primates spend 5-6 hours per day digesting their food, while humans need little more than one hour to digested cooked food. This gives an enormous time bonus to the human species.
  • Less need for digestion, led humans to evolve smaller guts. This left more energy to grow the brain, which consumes about 25% of our energy usage, far higher than for any other animal.
  • Compared to other primates, humans have smaller mouths, teeth, stomach, colons and weaker jaws.
  • Eating cooked food is nearly universal among humans. Raw food usually only consumed by Hunter Gatherers as snacks while on the hunt. This is because raw food does not provide enough energy to survive.
  • Cooking also enabled the sexual division of labor with men hunting and women gathering and cooking. Cooking enabled men to hunt all day and know that they still would have food to eat if they failed to kill.
  • Cooking also promoted the male-female pair bond because women cooking alone at camp were vulnerable to hungry males. If she is married, this greatly reduces the danger of having her food stolen (as other primates regularly do).

Important Quotes from Book

The question is old: Where do we come from?.. What made us human?

This book proposes a new answer. I believe the transformative moment that gave rise to the genus Homo, one of the great transitions in the history of life, stemmed from the control of fire and the advent of cooked meals. Cooking increased the value of our food. It changed our bodies, our brains, our use of time, and our social lives. It made us into consumers of external energy and thereby created an organism with a new relationship to nature, dependent on fuel.

Among primates we are the only dedicated carnivores, and the only ones to take meat from large carcasses… So it is easy to imagine that the rise of meat eating fostered various human characteristics such as long-distance travel, big bodies, rising intelligence, and increased cooperation. For such reasons the meat-eating hypothesis, often called “Man-the-Hunter,” has long been popular with anthropologists to explain the change from australopithecine to human.

But the Man-the-Hunter hypothesis is incomplete because it does not explain how hunting was possible without the economic support gathered foods provided.

The habilines show that there were two changes in the path from ape to human, not just the one implied by Man-the-Hunter. The two steps involved different kinds of transformation and occurred hundreds of thousands of years apart—one probably around . million years ago, and the second between . million and . million years ago. It makes no sense that the two kinds of change should have been prompted by the same cause.

Meat eating accounts smoothly for the first transition, jump-starting evolution toward humans by shifting chimpanzee-like australopithecines into knife-wielding, bigger brained habilines, while still leaving them with apelike bodies capable of collecting and digesting 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.

How lucky that Earth has fire. Hot, dry plant material does this amazing thing: it burns.

Nowadays we need fire wherever we are. Survival manuals tell us that if we are lost in the wild, one of our first actions should be to make a fire. In addition to warmth and light, fire gives us hot food, safe water, dry clothes, protection from dangerous animals, a signal to friends, and even a sense of inner comfort.

Animals need food, water, and shelter. We humans need all those things, but we need fire too.

Most anthropologists have followed Darwin’s assumption that cooking has been a late addition to the human skill set, a valuable tradition without any biological or evolutionary significance. We use fire, Darwin seemed to imply, but we could survive without it if we had to. The implication was that cooking has little biological importance.

Cooked food does many familiar things. It makes our food safer, creates rich and delicious tastes, and reduces spoilage. Heating can allow us to open, cut, or mash tough foods. But none of these advantages is as important as a little-appreciated aspect: cooking increases the amount of energy our bodies obtain from our food.

The extra energy gave the first cooks biological advantages. They survived and reproduced better than before. Their genes spread. Their bodies responded by biologically adapting to cooked food, shaped by natural selection to take maximum advantage of the new diet. There were changes in anatomy, physiology, ecology, life history, psychology, and society.

Those claims constitute the cooking hypothesis. They say humans are 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.

The Inuit consumed raw food mostly as a snack out of camp, as is typical of human foragers.

In more ordinary circumstances starvation is a rapid threat when eating raw in the wild.

But even hunter-gatherers often live well with little meat for weeks on end, as long as they cook. The Siriono experience suggests that raw diets are dangerous because they do not provide enough energy.

We can think of cooked food offering two kinds of advantage, depending on whether species have adapted to a cooked diet. Spontaneous benefits are experienced by almost any species, regardless of its evolutionary history, because cooked food is easier to digest than raw food. Domestic animals such as calves, lambs, and piglets grow faster when their food is cooked, and cows produce more fat in their milk and more milk per day when eating cooked rather than raw seeds. A similar effect appears in fish farms. Salmon grow better on a diet of cooked rather than raw fishmeal.

The spontaneous benefits of cooked food explain why domesticated pets easily become fat: their food is cooked, such as the commercially produced kibbles, pellets, and nuggets given to dogs and cats.

In humans, because we have adapted to cooked food, its spontaneous advantages are complemented by evolutionary benefits. The evolutionary benefits stem from the fact that digestion is a costly process that can account for a high proportion of an individual’s energy budget—often as much as locomotion does. After our ancestors started eating cooked food every day, natural selection favored those with small guts, because they were able to digest their food well, but at a lower cost than before. The result was increased energetic efficiency.

Evolutionary benefits of adapting to cooked food are evident from comparing human digestive systems with those of chimpanzees and other apes. The main differences all involve humans having relatively small features. We have small mouths, weak jaws, small teeth, small stomachs, small colons, and small guts overall.

Given that the mouth is the entry to the gut, humans have an astonishingly tiny opening for such a large species.

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  percent more than they do.

We also have diminutive muscle fibers in our jaws, one-eighth the size of those in macaques. The cause of our weak jaws is a human-specific mutation in a gene responsible for producing the muscle protein myosin. Sometime around two and a half million years ago this gene, called MYH, is thought to have spread throughout our ancestors and left our lineage with muscles that have subsequently been uniquely weak.

Human chewing teeth, or molars, also are small—the smallest of any primate species in relation to body size.

Continuing farther into the body, our stomachs again are comparatively small. In humans the surface area of the stomach is less than one-third the size expected for a typical mammal of our body weight, and smaller than in  percent of other primates. The high caloric density of cooked food suggests that our stomachs can afford to be small.

Finally, the volume of the entire human gut, comprising stomach, small intestine, and large intestine, is also relatively small, less than in any other primate measured so far. The weight of our guts is estimated at about  percent of what is expected for a primate of our size.

Our small mouths, teeth, and guts fit well with the softness, high caloric density, low fiber content, and high digestibility of cooked food. The reduction increases efficiency and saves us from wasting unnecessary metabolic costs on features whose only purpose would be to allow us to digest large amounts of high-fiber food.

Plants are a vital food because humans need large amounts of either carbohydrates (from plant foods) or fat (found in a few animal foods). Without carbohydrates or fat, people depend on protein for their energy, and excessive protein induces a form of poisoning.

Because the maximum safe level of protein intake for humans is around  percent of total calories, the rest must come from fat, such as blubber, or carbohydrates, such as in fruits and roots. Fat is an excellent source of calories in high-latitude sites like the Arctic or Tierra del Fuego, where sea mammals have evolved thick layers of blubber to protect themselves from the cold. However, fat levels are much lower in the meat of tropical mammals, averaging around  percent, and high-fat tissues like marrow and brain are always in limited supply. The critical extra calories for our equatorial ancestors therefore must have come from plants, which are vital for all tropical hunter-gatherers. During periods of food shortage, such as the annual dry seasons, fat levels in meat would have been particularly low, down to  percent to  percent. A carbohydrate supply from plant foods would then have been especially vital.

But if early humans had the same small guts as we do, they could not have obtained their plant carbohydrates without cooking.

Hunter-gatherers living on raw food might sometimes have found plant foods of an exceptionally high caloric density, such as avocados, olives, or walnuts. But no modern habitats produce such foods in abundance all year… Furthermore, seasonal scarcities occur in every habitat and would have forced people to use foods of lower caloric density, such as roots.

Anthropology has traditionally adopted the Man-the- Hunter scenario, proposing our species as a creature that was modified from australopithecines principally by our tendency to eat more meat. Certainly meat eating has been an important factor in human evolution and nutrition, but it has had less impact on our bodies than cooked food.

Starchy foods are the key ingredient of many familiar items such as breads, cakes, and pasta. They constitute almost all the world’s major plant staples. In –, cereals such as rice and wheat made up  percent of the world’s food production, and together with just a few other starchy foods (roots, tubers, plantains, and dry pulses) accounted for  percent of the average diet. 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.

The principal way cooking achieves its increased digestibility is by gelatinization.

Cooking increased the protein value of eggs by around  percent.

The Belgian scientists considered the reason for this dramatic effect on nutritional value and concluded that the major factor was denaturation of the food proteins, induced by heat. Denaturation occurs when the internal bonds of a protein weaken, causing the molecule to open up. As a result, the protein molecule loses its original three-dimensional structure and therefore its natural biological function.

Although gelatinization and denaturation are largely chemical effects, cooking also has physical effects on the energy food provides.

But nothing changes meat tenderness as much as cooking because heat has a tremendous effect on the material in meat most responsible for its toughness: connective tissue.

The main protein in connective tissue, collagen, owes its toughness to an elegant repeating structure.

But collagen has an Achilles’ heel: heat turns it to jelly.

Cooked food is better than raw food because life is mostly concerned with energy. So from an evolutionary perspective, if cooking causes a loss of vitamins or creates a few long-term toxic compounds, the effect is relatively unimportant compared to the impact of more calories. A female chimpanzee with a better diet gives birth more often and her offspring have better survival rates. In subsistence cultures, better-fed mothers have more and healthier children. In addition to more offspring, they have greater competitive ability, better survival, and longer lives. When our ancestors first obtained extra calories by cooking their food, they and their descendants passed on more genes than others of their species who ate raw. The result was a new evolutionary opportunity.

Two kinds of evidence thus point independently to the origin of Homo erectus as the time when cooking began. First, anatomical changes related to diet, including the reduction in tooth size and in the flaring of the rib cage, were larger than at any other time in human evolution, and they fit the theory that the nutritional quality of the diet improved and the food consumed was softer. Second, the loss of traits allowing efficient climbing marked a commitment to sleeping on the ground that is hard to explain without the control of fire.

Literally, our brains use around  percent of our basal metabolic rate—our energy budget when we are resting—even though they make up only about . percent of our body weight… animals: primates on average use about  percent of their basal metabolic rate on their brains, and most other mammals use less again, around  percent to  percent.

The high rate of energy flow is vital because our neurons need to keep firing whether we are awake or asleep. Even a brief interruption in the flow of oxygen or glucose causes neuron activity to stop, leading rapidly to death. The constant energy demand of brain cells continues even when times are tough, such as when food is scarce or an infection is raging. The first requirement for evolving a big brain is the ability to fuel it, and to do so reliably.

Among species that have the same relative basal metabolic rate, such as humans and other primates, extra energy going to the brain must be offset by a reduced amount of energy going elsewhere. The question is what part of the body is shortchanged.

Relative to their body weight, primates with smaller guts proved to have larger brains—just the kind of trade-off that had been expected. Aiello and Wheeler estimated the number of calories a species is able to save by having a small gut, and showed that the number nicely matched the extra cost of the species’ larger brains. The anthropologists concluded that primates that spend less energy fueling their intestines can afford to power more brain tissue. Big brains are made possible by a reduction in expensive tissue. The idea became known as the expensive tissue hypothesis.

Chimpanzees have a cranial capacity of around  to  cubic centimeters (. to . cubic inches). Australopithecines, with the same body weight as chimpanzees or even slightly less, had substantially larger cranial capacities, about  cubic centimeters (. cubic inches).

During seasons of plenty, australopithecines would have eaten much the same diet as chimpanzees or baboons do when living in the kinds of woodland that australopithecines occupied—fruits, occasional honey, soft seeds, and other choice plant items. It was when fruits were scarce that australopithecines must have eaten better than their chimpanzee-like ancestors… items. The most likely alternatives were starch-filled roots and other underground or underwater storage tissues of herbaceous plants. These would have been ideal.

Carbohydrates are stored abundantly in corms, rhizomes, or tubers of many savanna plants and are highly concentrated sources of energy-rich starch in the dry season. These food reserves are so well hidden that few animals can find them, but chimpanzees do dig for tubers occasionally, sometimes with sticks, and australopithecines would have been at least as skillful and well-adapted: their chewing teeth are famously massive and somewhat piglike, suited to crushing roots and corms.

The underground energy-storage organs of plants have a quality anticipated by the expensive tissue hypothesis: they have less indigestible fiber from plant cell walls than foliage, making them easier to digest and therefore a food of higher value. A dietary change from foliage to higher quality roots is thus a plausible explanation for the first increase in brain size, from forest apes to australopithecines five million to seven million years ago.

During the second sharp increase, brain volume rose by about one-third, from the roughly  cubic centimeters ( cubic inches) of australopithecines to  cubic centimeters ( cubic inches) in habilines (based on measurements of five skulls). The body weights of australopithecines and habilines were about the same, so this was a substantial gain in relative brain size. Given the archaeological evidence, the big dietary change at this time was more meat eating, so meat should have made this brain growth possible. To account for such a large increase in brain size, it seems likely that habilines processed their meat.

Stone hammers or wooden clubs could equally have been used for tenderizing meat. After habilines cut hunks of meat off the carcasses of game animals, they may have sliced them into steaks, laid them on flat stones, and pounded them with logs or rocks. Even relatively crude hammering would have reduced the costs of digestion by tenderizing the meat and breaking connective tissue.

An early form of earth oven is the kind of innovation that could have been influential because it would have marked an important advance in cooking efficiency.

Likewise, the use of containers must have made cooking more efficient and might have contributed to reducing digestive costs and thus allowing increases in brain size.

Although the breakthrough of using fire at all would have been the biggest culinary leap, the subsequent discovery of better ways to prepare the food would have led to continual increases in digestive efficiency, leaving more energy for brain growth. The improvements would have been especially important for brain growth after birth, since easily digested weaning foods would have been critical contributors to a child’s energy supply. Advances in food preparation may thus have contributed to the extraordinary continuing rise in brain size through two million years of human evolution— a trajectory of increasing brain size that has been faster and longer-lasting than known for any other species.

Cooking was a great discovery not merely because it gave us better food, or even because it made us physically human. It did something even more important: it helped make our brains uniquely large, providing a dull human body with a brilliant human mind.

Foods soften when they are cooked, and as a result, cooked food can be eaten more quickly than raw food. Reliance on cooked food has therefore allowed our species to thoroughly restructure the working day. Instead of chewing for half of their time, as great apes tend to do, women in subsistence societies tend to spend the active part of their days collecting and preparing food. Men, liberated from the simple biological demands of a long day’s commitment to chewing raw food, engage in productive or unproductive labor as they wish. In fact, I believe that cooking has made possible one of the most distinctive features of human society: the modern form of the sexual division of labor.

The Hadza illustrate two major features of the sexual division of labor among hunter-gatherers that differentiate humans sharply from nonhuman primates. Women and men spend their days seeking different kinds of foods, and the foods they obtain are eaten by both sexes. Why our species forages in such an unusual way (compared to primates and all other animals, whose adults do not share food with one another) has never been fully resolved.

Hunting large game was a predominantly masculine activity in . percent of recent societies.

Even more distinctive of humans is that each sex eats not only from the food items they have collected themselves, but also from their partner’s finds. Not even a hint of this complementarity is found among nonhuman primates.

In foraging societies a woman always shares her food with her husband and children, and she gives little to anyone other than close kin. Men likewise share with their wives, whether they have received meat from other men or have brought it to camp themselves and shared part of it with other men. The exchanges between wife and husband permeate families in every society.

Because the amount of time spent chewing is related to body size among primates, we can estimate how long humans would be obliged to spend chewing if we lived on the same kind of raw food that great apes do. Conservatively, it would be  percent of the day, or just over five hours of chewing in a twelve-hour day.

Six hours of chewing per day for a chimpanzee mother who consumes , calories per day means that she ingests food at a rate of around  calories per hour of chewing. Humans comparatively bolt their food. If adults eat , to , calories a day, as many people do, the fact that they chew for only about one hour per day means that the average intake rate will average , to , calories an hour or higher, or more than six times the rate for a chimpanzee… used to. Thanks to cooking, we save ourselves around four hours of chewing time per day.

The time budget for an ape eating raw food is also constrained by the rhythm of digestion, because apes have to pause between meals. Judging from data on humans, the bigger the meal, the longer it takes for the stomach to empty. It probably takes one to two hours for a chimpanzee’s full stomach to empty enough to warrant feeding again. Therefore, a five-hour chewing requirement becomes an eight- or ninehour commitment to feeding. Eat, rest, eat, rest, eat. An ancestor species that did not cook would presumably have experienced a similar rhythm.

These time constraints are inescapable for a large ape or habiline eating raw unprocessed food. Males who did not cook would not have been able to rely on hunting to feed themselves. Like chimpanzees, 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.

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.

That women tend to cook for their husbands is clear. In  anthropologists George Murdock and Catarina Provost compiled the pattern of sex differences in fifty productive activities in  cultures. Although men often like to cook meat, overall cooking was the most female-biased activity of any, a little more so than preparing plant food and fetching water. Women were predominantly or almost exclusively responsible for cooking in . percent of societies.

Before cooking, we ate more like chimpanzees, everyone for themselves. After the advent of cooking, we assembled around the fire and shared the labor.

Relying on cooked food creates opportunities for cooperation, but just as important, it exposes cooks to being exploited. Cooking takes time, so lone cooks cannot easily guard their wares from determined thieves such as hungry males without their own food. Pair-bonds solve the problem. Having a husband ensures that a woman’s gathered foods will not be taken by others; having a wife ensures the man will have an evening meal.

Either way, the result was a primitive protection racket in which husbands used their bonds with other men in the community to protect their wives from being robbed, and women returned the favor by preparing their husbands’ meals.

Restraint is rare indeed in animal competition over food. Chimpanzees fight over any food that can be monopolized, but the contests are fiercest over meat, producing a fracas that can often be heard more than a kilometer (half a mile) away. Within seconds of a successful predation by a lowranking chimpanzee, a dominant male is liable to snatch the entire carcass from the killer. In a large group, the carcass will be torn apart by screaming males desperate for a share… The most subordinate individuals get little… Overall, females eat much less meat than males, and their low success rate is clearly due to their poor fighting ability.

If the first cooks were temperamentally like chimpanzees, life would have been absurdly difficult for females or low-status males trying to cook a meal. Cooked food would have been intensely valuable.

The idea that cooking led to our pair-bonds suggests a worldwide irony. Cooking brought huge nutritional benefits. But for women, the adoption of cooking has also led to a major increase in their vulnerability to male authority. Men were the greater beneficiaries. Cooking freed women’s time and fed their children, but it also trapped women into a newly subservient role enforced by male-dominated culture. Cooking created and perpetuated a novel system of male cultural superiority. It is not a pretty picture.

Humans are exceptional runners, far better than any other primate at running long distances, and arguably better even than wolves and horses. The problem for most mammals is that they easily become overheated when they run.

The best adaptation to losing heat is not to have such an effective insulation system in the first place. As physiologist Peter Wheeler has long argued, this may be why humans are “naked apes”: a reduction in hair would have allowed Homo erectus to avoid becoming overheated on the hot savanna. But Homo erectus could have lost their hair only if they had an alternative system for maintaining body heat at night. Fire offers that system. Once our ancestors controlled fire, they could keep warm even when they were inactive. The benefit would have been high: by losing their hair, humans would have been better able to travel long distances during hot periods, when most animals are inactive. They could then run for long distances in pursuit of prey or to reach carcasses quickly. By allowing body hair to be lost, the control of fire allowed extended periods of running to evolve, and made humans better able to hunt or steal meat from other predators.

It is very likely that habilines were mentally capable of keeping a fire alive. The big question for the habilines that became Homo erectus is not how they tended fire, but how they would regularly have obtained it.

If you would like to learn more about key innovations in human history, read my book From Poverty to Progress: How Humans Invented Progress, and How We Can Keep It Going.

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