Homo erectus
did not exhibit the apelike features of the habilines. In the evolution of
Homo erectus
from habilines, we find the largest reduction in tooth size in the last six million years of human evolution, the largest increase in body size, and a disappearance of the shoulder, arm, and trunk adaptations that apparently enabled habilines to climb well. Additionally,
Homo erectus
had a less flared rib cage and a narrower pelvis than the australopithecines, both features indicating that they had a smaller gut. There was a 42 percent increase in cranial capacity.
Homo erectus
was also the first species in our lineage to extend its range beyond Africa: it was recorded in western Asia by 1.7 million years ago, Indonesia in Southeast Asia by 1.6 million years ago, and Spain by 1.4 million years ago. The reduction in tooth size, the signs of increased energy availability in larger brains and bodies, the indication of smaller guts, and the ability to exploit new kinds of habitat all support the idea that cooking was responsible for the evolution of
Homo erectus.
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. The only nonhuman primate that regularly sleeps on the ground is the largest species of great ape, gorillas. Gorillas are safer on the ground than
Homo erectus
would have been because gorillas live in forests with few predators and they are relatively enormous. The most frequent ground sleepers are adult males, weighing around 127 kilograms (286 pounds). Smaller gorillas often sleep in trees.
The late Pliocene and early Pleistocene periods in Africa were rich in predators. In wooded areas from 4 million to 1.5 million years ago, our ancestors would have found saber-toothed cats. There was
Megantereon
, the size of a leopard, and
Dinofelis
, as big as a lion. In more open habitats there was the scimitar cat
Homotherium
, equally large. An extinct kind of lion and spotted hyena lived alongside our early ancestors, while modern lions and leopards have been present since at least 1.8 million years ago. There were also many large animals such as elephants, rhinoceroses, and buffalo-like ungulates that could stumble unawares onto an unconscious biped. The African woodlands would have been a very dangerous place to sleep on the ground.
Extrapolating from the behavior of living primates in predator-rich environments, the australopithecines and habilines surely slept in trees. Their habitats were well wooded and their upper-body anatomy suggests they climbed well. But what did
Homo erectus
do? The famous “Turkana boy,” a beautifully preserved specimen of
Homo erectus
dated between 1.51 and 1.56 million years ago provides excellent evidence that they climbed relatively poorly. Physical anthropologists Alan Walker and Pat Shipman have described the Turkana boy as committed to locomotion on the ground. His finger bones had lost the curved, robust shape of australopithecine fingers. His shoulder blade had the modern form, giving no indication of being adapted to the stresses of climbing with the arm above the shoulder. The Turkana boy is so well preserved that Walker was able to study the vestibular system of the inner ear, responsible for balance. Species that climb regularly have a large and characteristically shaped vestibular system. The Turkana boy’s is different from that of species that climb, but closely resembles the modern human system.
So the Turkana boy, like other
Homo erectus
, could not have climbed well and he therefore would have found it difficult to make the type of nest great apes sleep in. Chimpanzees take about five minutes to build their nests by standing on all fours where the nest is taking shape, bending branches toward themselves. They break some of the bigger ones and weave the branches together to form a platform that they finish off with a few leafy twigs that serve as cushions or pillows to make it comfortable. Making a nest depends on being able to move around easily on the end of a swaying branch. The long legs and flat feet of humans such as
Homo erectus
and modern people do not allow such agility. For a mother with a small infant, the gymnastic challenges of making a nest would have been particularly difficult given her need to cradle while she swayed in the tree.
Homo erectus
therefore must have slept on the ground. But to do so in the dark of a moonless night seems impossibly dangerous.
Homo erectus
was as poorly defended a creature as we are, unable to sprint fast and dependent on weapons for any success in fighting. Surprised by a
Dinofelis
or a pack of hyenas at midnight, they would have been vulnerable.
If
Homo erectus
used fire, however, they could sleep in the same way as people do nowadays in the savanna. In the bush, people lie close to the fire and for most or all of the night someone is awake. When a sleeper awakens, he or she might poke at the fire and chat a while, allowing another to fall asleep. In a twelve-hour night with no light other than what the fire provides, there is no need to have a continuous eight-hour sleep. An informal system of guarding easily emerges that allows enough hours of sleep for all while ensuring the presence of an alert sentinel. To judge from records of attacks by jaguars, modern hunter-gatherers are safer in camp at night than they are on the hunt by day.
The control of fire could explain why
Homo erectus
lost their climbing ability. The normal assumption is that when long legs were favored, perhaps as a result of the increasing importance of long-distance travel as humans searched for meat, it was harder for humans to climb efficiently, and
Homo erectus
therefore abandoned the trees. But since that argument does not explain how
Homo erectus
could sleep safely, I prefer an alternative hypothesis: having controlled fire, a group of habilines learned that they could sleep safely on the ground. Their new practice of cooking roots and meat meant that food obtained from trees was less important than it had been when raw food was the only option. When they no longer needed to climb trees to find food or sleep safely, natural selection rapidly favored the anatomical changes that facilitated long-distance locomotion and led to living completely on the ground.
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.
The only alternative is the traditional theory that cooking was first practiced by beings that already looked like us—physically human members of the genus
Homo
. If this were true, by the time our ancestors adopted cooking,
Homo erectus
had long ago adapted to a soft, easily chewed diet of high caloric density. But as we have seen, cold-processing techniques such as grinding and blending provide relatively poor energy even when carried out by raw-foodists with modern equipment.
For more than 2.5 million years our ancestors have been cutting meat off animal bones, and the impact was huge. A diet that included raw meat as well as plant foods pushed our forebears out of the australopithecine rut, initiated the evolution of their larger brains, and probably inspired a series of food-processing innovations. But according to the evidence carried in our bodies, it would take the invention of cooking to convert habilines into
Homo erectus
, and launch the journey that has led without any major changes to the anatomy of modern humans.
CHAPTER 5
Brain Foods
“Tell me what you eat, and I shall tell you what you are.”
—JEAN ANTHELME BRILLAT-SAVARIN,
The Physiology of Taste: Or Meditations on Transcendental Gastronomy
“
M
an is but a reed, the weakest in nature, but he is a thinking reed,” wrote philosopher Blaise Pascal in 1670. Exceptional intelligence is the defining feature of our species, yet its origins have long been a puzzle. Darwin concluded that intellect would have given advantages in social competition and the struggle to survive, but why humans should be brainier than other species was unclear. Only recently has an explanation emerged. In the view of many evolutionary anthropologists, the pressure for intelligence indeed comes primarily from the advantages of outwitting social competitors, whereas a major reason for species differences is how much brainpower the body can afford. For this reason the quality of the diet has been identified as a key driver of the growth of primate brains. For humans, cooking must have played a major role.
Attempts to explain the evolution of intelligence have sometimes appealed to rather specific advantages. Evolutionary biologist Richard Alexander argues that because humans practice warfare, and brainpower is critical for planning raids and winning battles, higher intellect could have been favored by a long evolutionary history of intense intergroup violence. But this hypothesis is undermined by chimpanzees, which behave in ways similar to warfare in small-scale human societies, but without humans’ braininess. Violence between groups of chimpanzees is like a “shoot-on-sight” policy. Parties of males attack vulnerable rivals from adjacent groups whenever they encounter them, sometimes during incursions deep into the other group’s territory in search of victims. Death rates from these interactions among chimpanzees are similar to those in small-scale societies of humans, yet chimpanzees are much less brainy than humans, and only about as clever as their more peaceable relatives, bonobos, gorillas, and orangutans.
Another suggested explanation for the evolution of intelligence is more ecological than social. This line of thinking proposes that intellect would be favored in species that occupy large home ranges, on the theory that wide-roaming creatures would need exceptional brainpower to mentally map their territories. And indeed, human hunter-gatherers cover huge areas compared to the ranges of apes and monkeys. But the correlation between range size and brain size does not generalize. Species of primates with larger brains are more intelligent, but they show no overall tendency to have larger ranges. The association of intellect and range size in humans looks accidental; that is, there is no evidence for a causal effect of brain size on range size, or vice versa, across primate species.
A more promising approach assumes that numerous kinds of benefits come from being intelligent. Clever species can forage in a variety of creative ways, such as using grasses and twigs to extract insects from holes, or lifting stones as hammers to smash nuts. Big-brained species can also manage complex social relationships. Evolutionary psychologist Robin Dunbar found that primates with bigger brains or more neocortex live in larger groups, have a greater number of close social relationships, and use coalitions more effectively than those with smaller brains.
Brains pay off socially when they beat brawn. Relationships can change daily in primates that live in large groups, such as chimpanzees or baboons. Flexible coalitions in which two or more group members gang up on another group member allow small or individually low-status animals to compete successfully for access to resources and mates. Coalitions are difficult to manage because individuals compete for the best allies, and an ally today may be a rival tomorrow. Individuals must constantly reassess one another’s moods and strategies, and alter their own behavior accordingly. Clever animals can be deceitful too, deliberately hiding their feelings by masking facial expressions, or screaming to pretend they have been attacked when their real motive is to rally supporters to chase a dominant individual away from food. The result is a soap opera of changing affections, alliances, and hostilities, and a constant pressure to outsmart others.
Most animals are not up to the cognitive challenges of juggling social alliances. They compete one-on-one, like chickens, or following simple rules such as supporting members of their own group against outsiders. The exceptions are telling. Birds in the crow family have many of the social abilities of primates and are distinctly large-brained compared to other birds. Bottlenose dolphins form particularly complex and changeable alliances, and have the largest brains relative to body size of any nonhuman. Spotted hyenas live in large groups and use flexible coalitions to compete for power, and consistent with the primate evidence, they have bigger brains than their less social relatives. A similar link of sociality to mental power is found in social insects, whose neural tissue is concentrated not in brains but in ganglia. Darwin noted that colony-living ants and wasps have “cerebral ganglia of extraordinary dimensions,” many times larger than other insects.