Big Steps in the Story of Big History – Part 6

Fire, and cooking with it, sparked an explosion that blew up the human brain.

Brian Villmoare, in his book The Evolution of Everything: The Patterns and Causes of Big History, writes:

Although the evidence is somewhat controversial, and research continues on the topic, Homo erectus has been attributed with the first use of fire by a human species. The importance of fire for later humans is difficult to overstate. Fire allows many things. The first is obviously warmth – we see later humans, such as Neanderthals and Homo sapiens, succeeding in close proximity to glaciated environments in Europe and Asia, where it must have regularly been well below freezing for long periods of time. It is difficult to imagine that this would have been possible without the mastery of fire. Fire is also important for diet. Cooking meat kills parasites, and would have therefore been important for the health of the population. Cooking also enables us to eat foods that are otherwise very difficult to chew and digest (imagine eating a raw potato), because the heat breaks down the tougher, more fibrous material. Finally, fire has the potential to change animal behavior. A brushfire might be used to flush game from bushes for hunting or trapping, or to keep predators, such as lions, from a campsite.

But much more than that, fire allowed cooking, which so softened foods that it freed the human body to devote more calories to the production of neurons.  As Richard Wrangham, an evolutionary biologist who studied under Jane Goodall, writes in his book Catching Fire: How Cooking Made Us Human:

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 … How lucky that Earth has fire. Hot, dry plant material does this amazing thing: it burns. In a world full of rocks, animals, and living plants, dry, combustible wood gives us warmth and light, without which our species would be forced to live like other animals. It is easy to forget what life would have been like without fire. The nights would be cold, dark, and dangerous, forcing us to wait helplessly for the sun. All our food would be raw. No wonder we find comfort by a hearth … 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. Fossil evidence indicates that this dependence arose not just some tens of thousands of years ago, or even a few hundred thousand, but right back at the beginning of our time on Earth, at the start of human evolution, by the habiline [An early member of the genus Homo, known from fossils in Africa dating from 2 million to about 1.5 million years ago] that became Homo erectus.

The process of digestion requires the burning of calories, so if digestion became easier, it would free up calories to be used for other purposes:

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. In the past, the unusual size of these body parts has mostly been attributed to the evolutionary effects of our eating meat, but the design of the human digestive system is better explained as an adaptation to eating cooked food than it is to eating raw meat. 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. The difference in mouth size is even more obvious when we take the lips into account. The amount of food a chimpanzee can hold in its mouth far exceeds what humans can do because, in addition to their wide gape and big mouths, chimpanzees have enormous and very muscular lips. Our second digestive specialization is having weaker jaws. You can feel for yourself that our chewing muscles, the temporalis and masseter, are small. In nonhuman apes these muscles often reach all the way from the jaw to the top of the skull, where they sometimes attach to a ridge of bone called the sagittal crest, whose only function is to accommodate the jaw muscles. 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 MYH16, is thought to have spread throughout our ancestors and left our lineage with muscles that have subsequently been uniquely weak. Our small, weak jaw muscles are not adapted for chewing tough raw food, but they work well for soft, cooked food. Human chewing teeth, or molars, also are small— the smallest of any primate species in relation to body size. Again, the predictable physical changes in food that are associated with cooking account readily for our weak chewing and small teeth. 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 97 percent of other primates. The high caloric density of cooked food suggests that our stomachs can afford to be small. Great apes eat perhaps twice as much by weight per day as we do because their foods are packed with indigestible fiber (around 30 percent by weight, compared to 5 percent to 10 percent or less in human diets). Thanks to the high caloric density of cooked food, we have modest needs that are adequately served by our small stomachs. Below the stomach, the human small intestine is only a little smaller than expected from the size of our bodies, reflecting that this organ is the main site of digestion and absorption, and humans have the same basal metabolic rate as other primates in relation to body weight. But the large intestine, or colon, is less than 60 percent of the mass that would be expected for a primate of our body weight. The colon is where our intestinal flora ferment plant fiber, producing fatty acids that are absorbed into the body and used for energy. That the colon is relatively small in humans means we cannot retain as much fiber as the great apes can and therefore cannot utilize plant fiber as effectively for food. But that matters little. The high caloric density of cooked food means that normally we do not need the large fermenting potential that apes rely on. 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. In the case of intestines, physical anthropologists Leslie Aiello and Peter Wheeler reported that compared to that of great apes, the reduction in human gut size saves humans at least 10 percent of daily energy expenditure: the more gut tissue in the body, the more energy must be spent on its metabolism … [O]ur mouths, teeth, and jaws are clearly not well adapted to eating meat unless it has been cooked. Raw wild meat from game animals is tough, which is partly why cooking is so important.

Raw food takes a very long time to chew and digest.  As Wragham writes:

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 … 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. We fare poorly on raw diets, no cultures rely on them, and adaptations in our bodies explain why we cannot easily utilize raw foods. Even vegetarians thrive on cooked diets. We are cooks more than carnivores … The most direct studies of the impact of cooking measure digestibility, meaning the proportion of a food our bodies digest and absorb. If the digestibility of a particular kind of starch is 100 percent, the starch is a perfect food: every part of it is converted into useful food molecules. If it is zero percent, the starch is completely resistant to digestion and provides no food value at all. The question is, how much does cooking affect the digestibility of starchy foods? … Our digestive system consists of two distinct processes. The first is digestion by our own bodies, which starts in the mouth, continues in the stomach, and is mostly carried out in the small intestine. The second is digestion, or strictly fermentation, by four hundred or more species of bacteria and protozoa in our large intestine, also known as the colon or large bowel. Foods that are digested by our bodies (from the mouth to the small intestine) produce calories that are wholly useful to us. But those that are digested by our intestinal flora yield only a fraction of their available energy to us—about half in the case of carbohydrates such as starch, and none at all in the case of protein. Studies of ileal digestibility [a measurement of the difference between the amount of amino acids ingested and the amount of amino acids recovered for use by the body] show that we use cooked starch very efficiently. The percentage of cooked starch that has been digested by the time it reaches the end of the ileum is at least 95 percent in oats, wheat, potatoes, plantains, bananas, cornflakes, white bread, and the typical European or American diet (a mixture of starchy foods, dairy products, and meat). A few foods have lower digestibility: starch in home-cooked kidney beans and flaked barley has an ileal digestibility of only around 84 percent … Comparable measurements of the ileal digestibility of raw starch are much lower. Ileal digestibility is 71 percent for wheat starch, 51 percent for potatoes, and a measly 48 percent for raw starch in plantains and cooking bananas … [A]s starch granules are warmed up in the presence of water they start to swell—at around 58oC (136oF) in the case of wheat starch, a well-studied and representative example. The granules swell because hydrogen bonds in the glucose polymers weaken when they are exposed to heat, and this causes the tight crystalline structure to loosen. By 90oC (194 oF), still below boiling, the granules are disrupted into fragments. Gelatinization happens whenever starch is cooked, whether in the baking of bread, the gelling of pie fillings, the production of pasta, the fabrication of starch-based snack foods, the thickening of sauces, or, we can surmise, the tossing of a wild root onto a fire. As long as water is present, even from the dampness of a fresh plant, the more that starch is cooked, the more it is gelatinized. The more starch is gelatinized, the more easily enzymes can reach it, and therefore the more completely it is digested. Thus cooked starch yields more energy than raw. Animal protein has been almost as important as starch in diets throughout our evolution, and it remains a strongly preferred food today.

Wragham recounts how an unfortunate event led to some interesting discoveries regarding the body’s ability to more easily digest cooked foods:

Research on the topic began with a misfortune almost two hundred years ago. On June 6, 1822, twenty-eight-year-old Alexis St. Martin was accidentally shot from a distance of about a meter (three feet) inside a store of the American Fur Company at Fort Mackinac, Michigan. William Beaumont, a young, war-hardened surgeon, was nearby and arrived within twenty-five minutes to find a bloody scene … Beaumont took St. Martin to his own home. To everyone’s surprise, St. Martin survived, and Beaumont continued to house and care for him after he stabilized. In a few months the patient resumed a vigorous life, and he became so strong that he eventually even paddled his family in an open canoe from Mississippi to Montreal. Although the fist-sized wound mostly filled in, it never completely closed. For the rest of St. Martin’s life, the inner workings of his stomach were visible from the outside. The ambitious Beaumont realized that he had an extraordinary study opportunity. He began on August 1, 1825. “At 12 o’clock, M., I introduced through the perforation, into the stomach, the following articles of diet, suspended by a silk string, and fastened at proper distances, so as to pass in without pain—viz.:—a piece of highly seasoned a la mode beef; a piece of raw, salted, fat pork; a piece of raw, salted, lean beef; a piece of boiled, salted beef; a piece of stale bread; and a bunch of raw, sliced cabbage; each piece weighing about two drachms; the lad [St. Martin] continuing his usual employment around the house.” … Beaumont observed the stomach closely … [He] continued his experiments intermittently for eight years. He recorded in detail how long it took foods to be digested by the stomach and emptied into the duodenum. From those observations he drew two conclusions relevant to the effects of cooking. The more tender the food, the more rapidly and completely it was digested. He noted the same effect for food that was finely divided. “Vegetable, like animal substances, are more capable of digestion in proportion to the minuteness of their division . . . provided they are of a soft solid.” Potatoes boiled to reduce them to a dry powder tasted poor, but they were more easily digested. If not powdered, entire pieces remained long undissolved in the stomach and yielded slowly to the action of the gastric juice. “The difference is quite obvious on submitting parcels of this vegetation, in different states of preparation, to the operation of the gastric juice, either in the stomach or out of it.” … The same principles held, said Beaumont, with respect to meat. “Fibrine and gelatine [muscle fibers and collagen in meat] are affected in the same way. If tender and finely divided, they are disposed of readily; if in large and solid masses, digestion is proportionally retarded. . . . Minuteness of division and tenderness of fibre are the two grand essentials for speedy and easy digestion … In addition to “minuteness of division and tenderness,” cooking helped. He was explicit in the case of potatoes. “Pieces of raw potato, when submitted to the operation of this fluid, in the same manner, almost entirely resist its action. Many hours elapse before the slightest appearance of digestion is observable, and this only upon the surface, where the external laminae become a little softened, mucilaginous, and slightly farinaceous. It was the same with meat. When Beaumont introduced boiled beef and raw beef at noon, the boiled beef was gone by 2 P.M. But the piece of raw, salted, lean beef of the same size was only slightly macerated on the surface, while its general texture remained firm and intact.

As Wragham explains:

[N]othing 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 … has an Achilles’ heel: heat turns it to jelly. Collagen shrinks when it reaches its denaturation temperature of 60-70oC (140-158oF), and then, as the helices [plural for helix] start to unwind, it starts melting away.

Cooking, by making foods easier to digest, frees up energy we can use to fuel the electricity that fires our neurons:

[B]rains are exceptionally greedy for glucose—in other words, for energy. For an inactive person, every fifth meal is eaten solely to power the brain. Literally, our brains use around 20 percent of our basal metabolic rate—our energy budget when we are resting—even though they make up only about 2.5 percent of our body weight. Because human brains are so large, this proportion of energy expenditure is higher than it is in other animals: primates on average use about 13 percent of their basal metabolic rate on their brains, and most other mammals use less again, around 8 percent to 10 percent … 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.

Primates spend a startling amount of time having to chew the raw foods they eat.  Cooking freed humans from a life of near-endless chewing:

[It takes a] large amount of time … to eat raw food. Great apes allow us to estimate it. Simply because they are big—30 kilograms (66 pounds) and more—they need a lot of food and a lot of time to chew. Chimpanzees in Gombe National Park, Tanzania, spend more than six hours a day chewing. Six hours may seem high considering that most of their food is ripe fruit. Bananas or grapefruit would slip down their throats easily, and for this reason, chimpanzees readily raid the plantations of people living near their territories. But wild fruits are not nearly as rewarding as those domesticated fruits. The edible pulp of a forest fruit is often physically hard, and it may be protected by a skin, coat, or hairs that have to be removed. Most fruits have to be chewed for a long time before the pulp can be fully detached from the pieces of skin or seeds, and before the solid pieces are mashed enough to give up their valuable nutrients. Leaves, the next most important food for chimpanzees, are also tough and likewise take a long time to chew into pieces small enough for efficient digestion. The other great apes (bonobos, gorillas, and orangutans) commit similarly long hours to chewing their food. 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 42 percent of the day, or just over five hours of chewing in a twelve-hour day. People spend much less than five hours per day chewing their foods. Brillat-Savarin claimed to have seen the vicar of Bregnier eat the following within forty-five minutes: a bowl of soup, two dishes of boiled beef, a leg of mutton, a handsome capon, a generous salad, a ninety-degree wedge from a good-sized white cheese, a bottle of wine, and a carafe of water. If Brillat-Savarin was not exaggerating, the amount of food eaten by the vicar in less than an hour would have provided enough calories for a day or more. It is hard to imagine a wild chimpanzee achieving such a feat. A few careful studies using direct observation confirm how relatively quickly humans eat their food. In the United States, children from nine to twelve years of age spend a mere 10 percent of their time eating, or just over an hour per twelve-hour day. This is close to the daily chewing time for children recorded by anthropologists in twelve subsistence societies around the world, from the Ye’kwana of Venezuela to the Kipsigi of Kenya and the Samoans of the South Pacific. Girls ages six to fifteen chewed for an average of 8 percent of the day, with a range of 4 percent to 13 percent. Results for boys were almost identical: they chewed for an average of 7 percent of the day, again ranging from 4 percent to 13 percent. The children’s data show little difference between the industrialized United States and subsistence societies. In the twelve measured cultures, adults chewed for even less time than the children. Women and men each spent an average of 5 percent of their time chewing. One might object that the people in the subsistence societies were observed only from dawn to dusk. Since people often have a big meal after dark, the total time eating per day might be more than indicated by the 5 percent figure, which translates to only thirty-six minutes in a twelve-hour day. But even if people chewed their evening meals for an hour after dark, which is an improbably long time, the total time spent eating would still be less than 12 percent of a fourteen-hour day, allowing two hours for the evening meal. However we look at the data, humans devote between a fifth and a tenth as much time to chewing as do the great apes. This reduction in chewing time clearly results from cooked food being softer. Processed plant foods experience similar physical changes to those of meat. As the food canning industry knows all too well, it is hard to retain a crisp, fresh texture in heated vegetables or fruits. Plant cells are normally glued together by pectic polysaccharides. These chemicals degrade when heated, causing the cells to separate and permitting teeth to divide the tissue more easily. Hot cells also lose rigidity, a result of both their walls swelling and their membranes being disrupted by denaturation of proteins. The consequences are predictable. By measuring the amount of force needed to initiate a crack in food, researchers have shown that softness (or hardness) closely predicts the number of times someone chews before swallowing. The effect works for animals too. Wild monkeys spend almost twice as long chewing per day if their food is low-quality. Observers have recorded the amount of time spent chewing by wild primates that obtain human foods (such as garbage stolen from hotels). As the proportion of human foods rises in the diet, the primates spend less time chewing, down to less than 10 percent when all of the food comes from humans. Six hours of chewing per day for a chimpanzee mother who consumes 1,800 calories per day means that she ingests food at a rate of around 300 calories per hour of chewing. Humans comparatively bolt their food. If adults eat 2,000 to 2,500 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 2,000 to 2,500 calories an hour or higher, or more than six times the rate for a chimpanzee. The rate is doubtless much more when people eat high-calorie foods, such as hamburgers, candy bars, and holiday feasts. Humans have clearly had a long history of much more intense calorie consumption than primates are used to. Thanks to cooking, we save ourselves around four hours of chewing time per day … Before our ancestors cooked, then, they had much less free time. Their options for subsistence activities would therefore have been severely constrained. Males could not afford to spend all day hunting, because if they failed to get any prey, they would have had to fill their bellies on plant foods instead, which would take a long time just to chew. Consider chimpanzees, who hunt little and whose raw-food diet can be safely assumed to be similar to the diet of australopithecines. At Ngogo, Uganda, chimpanzees hunt intensely compared to other chimpanzee populations, yet males still average less than three minutes per day hunting. Human hunters have lots of time and walk for hours in the search for prey. A recent review of eight hunter-gatherer societies found that men hunted for between 1.8 and 8.2 hours daily. Hadza men were close to the average, spending more than 4 hours a day hunting—about eighty times as long as an Ngogo chimpanzee. Almost all hunts by chimpanzees follow a chance encounter during such routine activities as patrolling their territorial boundaries, suggesting that chimpanzees are unwilling to risk spending time on a hopeful search. When chimpanzees hunt their favorite prey—red colobus monkeys—the colobus rarely move out of the tree where they are attacked. The monkeys appear to feel safer staying in one place, rather than jumping to adjacent trees where chimpanzees might ambush them. The monkeys’ immobility allows chimpanzees to alternate between sitting under the prey and making repeated rushes at them. In theory, the chimpanzees could spend hours pursuing this prey. But at Ngogo the longest hunt observed was just over one hour, and the average length of hunts is only eighteen minutes. At Gombe I found that the average interval between plant-feeding bouts was twenty minutes, almost the same as the length of a hunt. The similarity between the average hunt duration and the average interval between plant-feeding bouts suggests that chimpanzees can afford a break of twenty minutes from eating fruits or leaves to hunt, but if they take much longer they risk losing valuable plant-feeding time … 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 nine-hour commitment to feeding. Eat, rest, eat, rest, eat. An ancestor species that did not cook would presumably have experienced a similar rhythm.

Wragham also describes how fire allowed early humans to climb down from trees:

Even the reduction in climbing ability fits the hypothesis that Homo erectus cooked. Homo erectus presumably climbed no better than modern humans do, unlike the agile habilines. This shift suggests that Homo erectus slept on the ground, a novel behavior that would have depended on their controlling fire to provide light to see predators and scare them away. Primates hardly ever sleep on the ground. Smaller species sleep in tree holes, in hidden nests, on branches hanging over water, on cliff ledges, or in trees so tall that no ground predator is likely to reach them. Great apes mostly build sleeping platforms or nests.

And the benefits of fire didn’t end there:

[T]he opportunity to be warmed by fire created new options. 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. After a chimpanzee has performed a five-minute charging display, he sits exhausted, panting and visibly hot, beads of sweat glistening among his erect hairs as he uses increased air circulation and sweat production to dissipate his excessive heat. Most mammals cannot evolve a solution to this problem, because they need to retain an insulation system, such as a thick coat of hair. The insulation is needed to maintain body heat during rest or sleep, and of course it cannot be removed after exercise. At best it can be modified, such as by hair being erected to promote air flow. 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 … Even our ancestors’ emotions are likely to have been influenced by a cooked diet. Clustering around a fire to eat and sleep would have required our ancestors to stay close to one another. To avoid lost tempers flaring into disruptive fights, the proximity would have demanded considerable tolerance. A process similar to domestication could then have led to an evolutionary advance in ancestral humans’ social skills. In animals, more tolerant individuals cooperate and communicate better.  If the intense attractions of a cooking fire selected for individuals who were more tolerant of one another, an accompanying result should have been a rise in their ability to stay calm as they looked at one another, so they could better assess, understand, and trust one another. Thus the temperamental journey toward relaxed face-to-face communication should have taken an important step forward with Homo erectus. As tolerance and communication ability increased, individuals would have become better at reaching a mutual understanding, forming alliances, and excluding the intolerant. Such changes in social temperament would have contributed to a growing ability to communicate, including the evolution of language.

Other researchers have speculated that the even simpler process of external fermentation may have kick-started the caloric savings that led to smaller digestive systems and larger brains even before human ancestors mastered fire:

Rather than relying on the microorganisms within the gut, external fermentation is carried out by organisms in the environment or on the surface of the organic material itself. Like internal fermentation, external fermentation increases the bioavailability of ingested nutrients, specifically, the absorption of macronutrients and micronutrients. In addition, external fermentation contributes to the health and efficacy of the host’s gut microbiome, in turn, facilitating nutrient absorption …  [T]he ingestion of externally fermented foods provides four critical components to digestion and absorption. First, it increases the digestibility of foods; second, it increases the bioavailability of micronutrients; third, it supports gut fermentation by contributing to host microfloral diversity; and lastly, it supports immune function and resistance to disruption of the gut microbiome. These benefits would have been adaptive advantages for our early ancestors and could have played a key role in human brain evolution … Forethought and mechanistic understanding are not requirements for the initial emergence of external fermentation. Our early ancestors may have simply carried food back to a common location, left it there, and intermittently eaten some and added more. Re-use of a storage location could have promoted the stability of a microbial ecosystem conducive to fermentation. As new food items were brought back and added to the cache, they could have become inoculated with the microorganisms already present in the location (or on the hominids themselves) … Socially-transmitted practices such as the re-use of the same storage locations, containers, or food-processing tools would have further promoted the initiation of fermentation and the stability of ongoing ferments. Over time, additional facilitation may have come from culturally reinforced norms, such as superstitions about where food must be stored or how long it must rest before being eaten. As brain size and cognitive capacity increased, understanding of the proximate causes and consequences of fermentation could have progressed in a gradual fashion … Unlike other proposed dietary modifications, a transition to eating fermented foods does not require great leaps in cognitive ability. It does not require advanced planning, as hunting, particularly hunting in groups, would. It does not require the acquisition of a difficult technology, as in fire for cooking. It can more directly explain, than tubers, meat, or cooking, how colon fermentation could be replaced through dietary changes.  Fermentation accounts for all the benefits that cooked food offers: softer food, higher caloric content, greater bioavailability of nutrients, and protection from pathogenic microorganisms. Fermentation solves several problems. It does not require special materials beyond a place to store food (a hollow, a cave, or a hole in the ground could work). It does not require overcoming fear—there is a low barrier to entry. It can be stumbled upon rather than requiring planning and tool use. And it does not require, initially, long-term planning, focused attention, or sophisticated social coordination.

In the next essay in this series, we’ll examine the origins of government.

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