A system for hastening chewing and digestion by processing the meat would have greatly reduced the problem. Chimpanzees have a primitive form of processing meat. By adding tree leaves to their meat meals, they make chewing easier. The chosen leaves have no special nutritional properties, judging from the fact that the meat eaters pick leaves from whatever species of tree is nearest when they settle down to eat their prey. The only obvious rule governing their choice is that the leaf must be tough: they take only mature tree leaves, not young tree leaves or the soft leaves of an herb. Sometimes they even use long-dead leaves from the forest floor, mere brown skeletons devoid of nutrients. An informal experiment in which friends and I chewed raw goat meat suggested that the added leaves give traction. When we chewed thigh muscle together with a mature avocado leaf, the bolus of chewed meat was reduced faster than when we chewed with no added leaf. Australopithecines probably used similar practices when they caught gazelle fawns or other small mammals.
Habilines had access to more advanced techniques. Their bones are found close to stone hammers, fist-size spheres whose shapes provide vivid testimony of their repeated use. Habilines probably used the hammers partly to smash prey bones to extract the marrow. They also doubtless used the hammers to crack open nuts, as West African chimpanzees do, as well as to make other tools. In addition to these practices, 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. Because raw unprocessed meat is difficult to chew and digest, I suspect this was one of the most important cultural innovations in human origins, enabling habilines to increase the nutritional benefit of meat and the speed with which they could eat and digest it. Tenderizing meat would have reduced the costs of digestion by cutting the time that meat was in the stomach, and thus allowed habilines to divert energy toward their brains.
Dietary shifts toward roots, meat eating, and meat processing thus can explain the growth in brains from a chimpanzee-like ancestor at six million years to the habilines around two million years ago. From then on, the increases in brain size were more continuous. The habiline cranial capacity of 612 cubic centimeters (37 cubic inches) rose by over 40 percent to reach an average of 870 cubic centimeters (53 cubic inches) in the earliest measured
Homo erectus.
The significance of this rise is complicated by a parallel growth in body weight, from the lowly 32 to 37 kilograms (70 to 81 pounds) of habilines to a substantial 56 to 66 kilograms (123 to 145 pounds) in
Homo erectus
. Unfortunately, body weights are hard to estimate accurately from bones, and the number of specimens is small, so how much larger relative to body weight the brains of the first
Homo erectus
were than those of habilines, or whether they were relatively larger at all, is uncertain. However,
Homo erectus
brains continued to increase in size after 1.8 million years ago, averaging almost 950 cubic centimeters (58 cubic inches) by 1 million years ago. Given the evidence and arguments I have offered that
Homo erectus
originated as cooks, the expensive tissue hypothesis suggests their eating cooked food caused their brains to grow. Once cooking began, gut size could fall and the gut would be less active, both trends reducing the cost of the digestive system.
The fourth notable increase in cranial capacity occurred with the emergence of
Homo heidelbergensis
after eight hundred thousand years ago. The increase was again substantial, leading to a brain occupying around 1,200 cubic centimeters (73 cubic inches). This was the impressive rise that Aiello and Wheeler attributed to the invention of cooking—mistakenly, I believe. It remains a mystery, inviting speculation.
More efficient hunting is a possibility. Hartmut Thieme’s evidence of group hunting four hundred thousand years ago in Schöningen suggests a marked improvement in hunting skills over earlier eras. This raises the possibility that meat intake, and perhaps therefore the use of animal fat, rose significantly before this time and played a role in the evolution of
Homo erectus
into
Homo heidelbergensis
.
Alternatively, cooking surely continued to affect brain evolution long after it was invented, because cooking methods improved. Laying a food item on the fire presumably was the main early method. Such techniques have been used by generations of campers and have been recorded by hunter-gatherers in recent times for foods that are easy to cook. The Aranda foragers of central Australia gather pea-size corms of sedges by digging them from flat ground near rivers. One method of cooking consists merely of laying them on hot ashes for a short time, then rubbing them between the hands to remove the light shell before eating them. !Kung San hunter-gatherers of Africa’s Kalahari Desert cook tsin beans, one of their more important foods, by simply burying them in hot ashes. Putting an animal on a fire to roast can work fairly well, especially if the hairs have been singed off first. Marrow can be cooked with similar efficiency by roasting a complete bone in fire, then using stones to crack it. The marrow flows out like warm butter.
More complex ways to roast presumably would have accumulated slowly, often specific to particular foods. Take mongongo nuts eaten by !Kung hunter-gatherers. Mongongo nuts are a highly nutritious staple, often providing the !Kung with their major source of calories for weeks on end. To cook them, a woman mixes the coals from a dying fire with hot, dry sand. She then buries scores of nuts in the hot pile without allowing the nuts to touch any of the live coals. After a few minutes she kneads the pile to ensure that the nuts are evenly heated, adding more coals as needed. When the nuts are done, she hammers each one to split it, then eats the seeds inside or keeps them for further cooking. We do not know when such a sophisticated method appeared, but it seems likely to have contributed to raising the energetic quality of food, reducing the time the digestive system was active, and so lowering the total costs of digestion and allowing more energy for the brain.
Such improvements in cooking efficiency could explain why there was a steady upward trend in brain size during the lifetimes of the early human species. Brains were notably bigger in late
Homo erectus
than in early
Homo erectus
, and in late
Homo heidelbergensis
than in early
Homo heidelbergensis
. Major dietary breakthroughs such as meat eating and the invention of cooking cannot account for these smaller changes. The steady rise in brain size between the major jumps is most easily explained by a series of improvements in cooking techniques. Perhaps some particularly important advances enabled the prominent rise in brain size with
Homo heidelbergensis.
The same possibility applies to the evolution of our own species,
Homo sapiens
, around two hundred thousand years ago. The gain in brain size was relatively minor, from 1,200 cubic centimeters (73 cubic inches) in
Homo heidelbergensis
to around 1,400 cubic centimeters (85 cubic inches) in
Homo sapiens
. Various modern behaviors are seen for the first time around this transition, such as the use of red ocher (presumably as a form of personal decoration), making tools out of bone, and long-distance trade. Increasing behavioral sophistication could also have happened in cooking techniques.
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. Hunter-gatherers worldwide used earth ovens that employed hot rocks. The ovens do not appear to have been used by the people who expanded out of Africa more than sixty thousand years ago and colonized the rest of the world, since they are not recorded in Australia until thirty thousand years ago. However, it is possible that a simpler design, now vanished and forgotten, may have been used in earlier times.
In recent earth ovens the hot rocks provide an even, long-lasting heat. A typical procedure recorded in 1927 among the Aranda of central Australia involved digging a hole, filling it with a pile of dry wood, and topping that with large stones that did not crack when heated—often river cobblestones that had to be carried from a distance. When the stones were red-hot and fell through the fire, they were pulled out with sticks and the ashes were removed. The hot stones were then returned and covered with a layer of green leaves. Cooks liked to wrap meat in leaves to retain its juices before placing it on this layer, sometimes on top of a plant food such as roots. More green leaves and perhaps a basket mat would be laid on top, water was poured on, and some people added herbs for taste. Finally, the hole was filled with a layer of soil to retain the steam. After an hour or more—sometimes it was left overnight—the meat and vegetables would be ready and superb. The meat was laid on leafy branches, carved with a stone knife, and served. The even heat and moist environment made earth ovens efficient for gelatinizing starch and other carbohydrates, and they offered effective control over the tenderness of meat. This sophisticated cooking technique doubtless increased the digestibility of the meat and plant foods.
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. Pottery is a very recent invention, around ten thousand years ago, but natural objects could have been used as cooking containers long before that. Certain animals come with their own dishes. Shellfish, such as mussels, have been cooked whole in many parts of the world by being thrown into a fire until the valves open. The Yahgan of Tierra del Fuego used mussel shells to catch the drips from a roasting seal or to hold whale oil, which they ate by dipping pieces of edible fungus into it.
It is a small step from such techniques to cooking in a container. Heating in natural containers by early
Homo sapiens
is indicated around 120,000 years ago by evidence that people made a glue from ancient birch tar, which they used to haft stone points on to spears. The glue had to be heated to achieve the desired stickiness, so people must have been cooking with containers by then. Some containers would have needed little imagination. Turtles are a natural convenience food because they can be easily kept alive for days, and whether alive or cooked they are easily carried. If they are turned upside down they even provide their own cooking pot. After their flesh has been eaten, their bodies remain useful. Andaman Islanders from the Bay of Bengal cooked turtle blood in an upside-down shell until it was thick, then ate it at once. Like many Asian peoples they also used bamboo as a container, sometimes for cooking. The Andaman Islanders would clean a length of bamboo and heat it over a fire so all its juices were absorbed. They then packed it with half-cooked pieces of wild pork or other meat and heated it so slowly that the meat swelled without cracking the bamboo. When the bamboo stopped steaming, they removed it from the fire and stuffed the opening with leaves to seal it. The cooked meat could be left for several days. Sadly, many ingenious cooking techniques practiced by early people with plant materials are forever lost to us because they leave no traces.
Development of other methods would have improved the efficiency of cooking and the quality of food. Various special ways of roasting have unknown antiquity. In their cold climate near the Antarctic, the Yahgan developed a two-stone griddle by heating two flat stones in a fire. The stones were then withdrawn, and the larger stone served as a griddle for a steak or layer of blubber, while the smaller was laid on top. This worked so well that the fat was browned and shriveled in a few minutes, a favorite for the hunters. The Yahgan were also fond of sausages. To make a sea-lion blood sausage, they kept the blood that collects in the abdominal cavity of a freshly killed sea lion. They took a soft, still moist piece of gut, turned it inside out, cleaned it, tied it shut at one end with sinews, filled it with air by blowing, tied the other end shut, and left it to dry. When the empty sausage was sufficiently firm, they used a large shell to fill it with blood, tied it shut again, and for safety’s sake jabbed a short, thin stick into each end to prevent the ties from unraveling. They then put the sausage into hot ashes, poking it occasionally to keep it from bursting. The same idea worked equally well with other parts of the gut. They sometimes filled stomachs with blubber or chopped tissues like heart, lungs, or liver. These haggises of the past left no traces, but they remind us that even in the bush, long before such recent inventions as grinding and stone boiling (which started within the past twenty-five thousand to forty thousand years), cooking can involve much more than simple heating.
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. When Charles Darwin called cooking “probably the greatest [discovery], excepting language, ever made by man,” he was thinking merely of our improved food supply. But the idea that brain enlargement was made possible by improvements in diet suggests a wider significance. 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.