Given these challenges, it is unclear how australopithecines obtained access to the meat of antelope and other game animals. Maybe they found new ways to kill, which would have given them a few minutes or more to cut meat off their prey before they were chased away by the arrival of big carnivores. Or perhaps they discovered how to stand up to the dangerous predators without serious risk of being wounded or killed. A bold group of australopithecines might have confronted the predators with simple spears modified from digging sticks that they had used to obtain roots. That technology would not have been a huge advance from the short sticks chimpanzees use to jab at bush babies hidden in tree holes, as happens in Senegal. Or maybe they threw rocks at their opponents, much as chimpanzees now sometimes scare pigs or humans with well-aimed missiles in Gombe, Tanzania. If they threw rocks, they might have noticed that sometimes the rocks smashed on landing and produced flakes that could be used for cutting.
Whatever the technique, by at least 2.6 million years ago, some groups were definitely getting meat from carcasses that previously only big carnivores would have eaten. Over the next few hundred thousand years, impact notches and cut marks on animal bones caused by stone tools attest to habilines spending long enough in the danger zones to be able to slice the meat off dead animals, from turtles to elephants. The result was a new and immensely beneficial food source. Knowing that habilines were able to cut steaks and that chimpanzees often pound nuts with hammerstones, we can be sure that habilines would have had the cognitive ability to batter their meat before they ate it, and they surely would have preferred their meat pounded.
Habilines must have also eaten substantial amounts of plant food. During periods of food shortage, such as the annual dry seasons, meat would have been particularly low in fat, down to 1 to 2 percent. Plant foods would then have become critical. Habilines’ chewing teeth were similar in size and shape to those of australopithecines, showing a continuing commitment to the same plant foods, including raw roots and corms during the most difficult seasons, and such items as soft seeds and fruits when they could find them. Probably habilines prepared nuts by smashing them to expose the edible seeds, as chimpanzees do. It is doubtful that habilines could process plant foods by any techniques that were much more elaborate than pounding. Almost all the methods hunter-gatherers use to improve the nutritional value of plant foods involve fire, because heat is needed to gelatinize starch. Until fire was controlled, habilines would have been stuck with eating raw plant foods whose caloric value could not be much improved by cold processing.
The breakthrough could have been simple, because it did not require that fire be made from scratch. If fire could be captured, the tending would have been relatively easy. Among hunter-gatherers, children as young as two years old make their own fires by taking sticks from their mothers’ fires. Even chimpanzees and bonobos can tend fires well. The bonobo Kanzi is famous for his ability to communicate with psychologist Sue Savage-Rumbaugh using symbols. During an outing in the woods, Kanzi once touched the symbols for “marshmallow” and “fire.” He was given matches and marshmallows, and he proceeded to snap twigs for a fire, light them with matches, and toast the marshmallows. By the time of habilines
,
brain size had roughly doubled compared with the relative brain size of great apes. 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. In his
Descent of Man,
Charles Darwin mentioned an idea suggested by his archaeologist friend John Lubbock: sparks produced by accident from pounded rocks could have launched the control of fire. Anthropologist James Frazer liked the idea of human fire coming accidentally from hitting rocks together, and so did the Yakuts of Siberia, whose campfire tales recounted how hammering led to controlled fire. Certainly habilines would have seen sparks when they hit stones together to make tools. If they softened their meat by pounding it not only with logs but also with hammerstones, they would have had a second source of sparks. There often would have been dry tinder close by, such as grass or the tinder fungus that many people use today to catch a fire.
Anthropologists caution that the sparks produced by many kinds of rock are too cool or short-lived to catch fire. But when pyrites, a common ore containing iron and sulfur, are hit against flint, the result is a set of such excellent sparks that pyrites and flint are standard components of fire-making kits used by hunter-gatherers from the Arctic to Tierra del Fuego. If a particular group of habilines lived in an area exceptionally rich in pyrites, they could have found themselves inadvertently making fire rather often.
The steps to managing fire need not have involved the difficult process of deliberately making it. Here is an alternative scenario: during the tens of thousands of generations between the origin of habilines (at least 2.3 million years ago) and
Homo erectus
(at least 1.8 million years ago), from time to time the sparks resulting from habilines’ pounding rocks could have accidentally produced small fires in adjacent brush. Perhaps cocky juvenile habilines dared to grab the cool end of a branch and tease one another with the smoldering twigs or blazing leaves, much as young chimpanzees playfully bully one another with sticks they use as clubs. Adults learned the effect on one another of waving a burning log. The practice of scaring others with fire was then transferred to the serious job of frightening lions, saber-tooths and hyenas, similar to how chimpanzees use clubs against leopards. At first the fires went out. But over time, when sparks happened to start a fire, habilines learned that it was worth their while to keep it going. They cultivated fire as a way to help them defend against dangerous animals.
There are other possibilities. The climate was become increasingly dry. Natural fires could have become more frequent. Perhaps people walked behind brush fires looking for cooked seeds. Maybe they obtained fire from trees that burned slowly after being struck by lightning; a eucalyptus tree can smolder for eight months. Perhaps there was a permanent natural source somewhere in Africa, like the gas-fired strip of flame that has been burning nonstop near Antalya in southwestern Turkey ever since Homer recorded it in the Iliad almost three thousand years ago.
Repeated experience with natural fire would have been necessary to give individuals the confidence to use it, which would not have happened easily—otherwise, fire would have been controlled by every group of habilines. But if there were a natural source of fire, such as sparks, there would have been no need to learn to make fire, because it could be taken from nature again and again, and eventually from other groups: the chance of a rainstorm extinguishing every fire in a neighborhood would soon have become vanishingly small. Among Australian aborigines, groups that lost their fire from a drenching rain or flood would refresh their supply from neighbors, who would expect something in return, such as quartz flakes or red ocher. Sometimes the trade occurred across a territorial boundary, which made it dangerous, but risk did not prevent the vital recovery of fire.
Keeping a fire lit would have been a big achievement, but logs are easy to keep aflame when people are moving. Hunter-gatherers regularly carry fire in the form of a burning log. As long as the carrier is walking, the fire is well oxygenated and the log continues to smolder. When people stop, they start a small fire within a few minutes by adding a few sticks to the smoldering log and blowing.
An important step in fire’s becoming a central part of human lives was to maintain it at night. Suppose some habilines carried a smoldering log by day to protect against predators, then left it at the base of a sleeping tree when they climbed to make a nest for the night. It would not have been such a big step to give it extra fuel so the log would still be burning the next day—perhaps after seeing this happen first by accident. From there it would have been a smaller step to sitting near the fire to keep it burning, and thereby take advantage of its protection, light, and warmth.
Once they kept fire alive at night, a group of habilines in a particular place occasionally dropped food morsels by accident, ate them after they had been heated, and learned that they tasted better. Repeating their habit, this group would have swiftly evolved into the first
Homo erectus
. The newly delicious cooked diet led to their evolving smaller guts, bigger brains, bigger bodies, and reduced body hair; more running; more hunting; longer lives; calmer temperaments; and a new emphasis on bonding between females and males. The softness of their cooked plant foods selected for smaller teeth, the protection fire provided at night enabled them to sleep on the ground and lose their climbing ability, and females likely began cooking for males, whose time was increasingly free to search for more meat and honey. While other habilines elsewhere in Africa continued for several hundred thousand years to eat their food raw, one lucky group became
Homo erectus—
and humanity began.
EPILOGUE
The Well-Informed Cook
Cooking launched a dietary commitment that today drives an industry. The popular foods cooked in giant factories are often scorned as lacking in micronutrients, having too much fat, salt, and sugar, and having too few interesting tastes, but they are the foods we have evolved to want. The result is excess. By the turn of the twenty-first century, 61 percent of Americans were “overweight enough to begin experiencing health problems as a direct result.” With the ready availability of such products as high-fructose corn syrup, cheap palm oil, and intensely milled flour, measured daily energy intake in the United States rose by almost two hundred calories between 1977 and 1995. As a result, more people continue to die in the United States of too much food than of too little, as John Kenneth Galbraith first noted a half century ago. The trends toward easier foods and greater obesity are now found in many industrialized countries. To reverse the decline in health, we should eat more foods with a low caloric density. But few examples can be found in the typical supermarket, because we tend not to like them. We would find it easier to choose appropriate foods if we had a better sense of how many calories we obtain from them. We need to become more aware of the calorie-raising consequences of a highly processed diet.
To do so, we need to better understand nutritional biophysics. Consider meat: the biochemistry of protein digestion is well known. Researchers know precisely what secretions are applied to food molecules at each point in their journey down the alimentary canal. They can say which chemical bonds are severed by which enzymes at which point, how the cells and membranes carry the products of digestion across the gut wall, and how mucosal cells respond to changing pH or mineral concentrations. The detail of biochemical knowledge is exquisite.
Yet this impressive expertise concerns protein, not meat, digestion. Nutritional science is focused so intensively on chemistry that physical realities are forgotten. Researchers treat the food entering the stomach as if it were a solution of nutrients ready for a cascade of biochemical reactions. They forget that our digestive enzymes interact not with free proteins but with a slimy three-dimensional bolus, which after a meal of meat is a messy collection of chewed chunks of muscle, each piece of which is wrapped in multilayered tubes of connective tissue. Structural complexity matters because it affects how easily the food bolus is converted to digestible nutrients, and therefore how many calories we get from our food. As we saw in chapter 3, the rats that gained an extra 30 percent fat in Oka’s experiment had no extra calories in their food. They merely had their diet softened. The Evo Diet, described in chapter 1, was calculated to give the volunteers sufficient calories to maintain weight, yet they lost weight rapidly.
Assessing the energy value of foods is a difficult technical problem. Nutritionists cannot calculate the value of foods directly because foods are too complicated in their composition and structure, and digestive systems treat different foods in different ways. So instead of making precise calculations of exactly the number of calories people can obtain from a given food, nutritionists make rough guesses. They do so according to a set of agreed rules that are not perfect but provide a good approximation, at least for foods that are very easily digested. They call these rules a convention.
For more than a century, the convention that has dominated estimating energy values in foods, and now under-girds the food-labeling system of the Western world, has been the Atwater system. Wilbur Olin Atwater, who invented it, was born in 1844. He was a professor of chemistry at Wesleyan College in Connecticut at the end of the nineteenth century. His admirable aim was to ensure that poor people could use their limited resources to get enough to eat. He set out to discover how many calories different foods provided. Atwater knew that food contained three main items the body uses for energy: protein, fat, and carbohydrate. Using a simple laboratory device called a bomb calorimeter, he recorded how much heat was released when typical proteins, fats, and carbohydrates were completely burned. He found that there was not a lot of variation among the different types of each item. For example, all proteins tended to produce a little more than four kilocalories of heat per gram.
After that, Atwater needed to know two more things. First was how much of the major macronutrients—protein, fat, and carbohydrate—a food contains. Fat was easy, because unlike protein and carbohydrates, fat dissolves in ether. So Atwater chopped foods finely, shook them up with ether, and weighed how much material was dissolved in the ether. That gave him a food’s fat content (or, more strictly, lipid content: lipids include both fats, which are solid at room temperature, and oils, which are liquid). The same method is used today. Protein was harder to index because no test identifies proteins in general. However, Atwater knew that about 16 percent of the weight of an average protein was nitrogen. So he found a way to measure the amount of nitrogen, which gave him the concentration of protein.