The most dramatic moment in human evolution took place about 1.9 million years ago, with the emergence of homo erectus. "Something radical happened," says professor of anthropology Richard Wrangham. "The change happened almost too quickly for the fossil record to record its intermediate stages."
The australopithecine creatures who inhabited the African woodlands before that time looked and acted more like chimpanzees than humans. They were short and short-legged; females weighed only about 60 pounds, males about twice as much. They had large faces, mouths, teeth, and guts, and arms and shoulders that were well adapted--like those of chimps--to hanging for hours in trees, where they could both forage for food and sleep. Then a markedly different creature suddenly emerged from one of the australopithecine populations, and within 100,000 years had spread from Africa to Europe and probably Asia. Homo erectus looked so much like modern humans, Wrangham says, that "if you dressed one in clothes, put a hat on her, and walked her down a Manhattan street, she wouldn't draw too many stares."
In a Current Anthropology article published last December, Wrangham and coauthors James Holland Jones, assistant senior tutor in Mather House, Greg Laden of the University of Minnesota, Ford professor of the social sciences David Pilbeam, and research assistant Nancylou Conklin-Brittain advance a new theory to account for the sudden change. They suggest that the discovery of fire and its corollary, cooking, occurred much earlier in prehistory than generally believed--and furthermore, that the innovation of cooking fundamentally reshaped the body. "All humans cook, and cooking has such big effects on what you can digest that it would necessarily have major impact on any species that adopts it," Wrangham explains. "Things like the shape and size of teeth, for example, are closely adapted to what you eat. But until now, no one has suggested any time [frame for] when cooking evolved that coincides with big changes in the morphology of the body."
The cooking hypothesis is bold because scholars routinely link the origin of cooking to archaeological indications of fire--and there is scant evidence for human control of fire earlier than 300,000 to 400,000 years ago. "But," says Wrangham, "fire doesn't leave much behind in the archaeological record." Furthermore, recent thermal and paleomagnetic data obtained from reddened patches of rock in Kenya suggest the existence of hearths as old as 1.6 million years. "We know that the environment was getting drier [1.9 million years ago]," Wrangham says, "so lightning, for example, would start natural fires. One can imagine a root being cooked by chance, then found and eaten by an australopithecine who liked it."
"Most foraging peoples live under nutritional stress," Pilbeam notes, and cooking expands the range of edible food and improves its overall quality. Wrangham explains, "Cooking breaks down indigestible molecules and makes them digestible. Starch in uncooked roots, for example, is often in a crystalline form. Until it's heated, our digestive systems can't use it." That means our ancestors could obtain as much nutrition from one roasted tuber as they could from several raw ones, and therefore needed to eat less. Consequently, over time, they might have developed smaller, flatter guts and the inward-tilting ribs seen in homo erectus. Cooking also softens food, diminishing the need for big teeth that can shear and pulverize. Indeed, with the emergence of homo erectus, "You see a greater reduction in the size of teeth than at any other point in our ancestry," Wrangham says. Smaller teeth also mean smaller faces, as well as much smaller mouths. And controlled fire would also help defend against night predators, a necessity for creatures who were now sleeping not in trees, but on the ground.
Where foraging apes usually pop food immediately into their mouths, the advent of cooking--a time-consuming activity--meant that hominids had to wait while the foods that they had collected were readied for eating. (The first "homes" were probably pantries at hearthsides.) But with waiting comes the possibility of theft. "The female suddenly becomes vulnerable to a new problem--she can easily lose her food to the much larger, dominant Australopithecus males who might steal it, profiting from her hours of foraging effort," Wrangham says. Pilbeam adds, "She would have an incentive to attach herself to a male defender who could guard the food supply. This could be the point at which human monogamy first occurred."
Under these conditions, females would compete with each other for the best male protectors, in terms of both their strength and agility and their ability to summon allies. The researchers speculate that to attract such males to their hearths, these early women became "sexier," evolving an extended period of sexual desirability, including the concealed ovulatory pattern of human females, and copulating throughout the menstrual cycle. "The theft hypothesis portrays the human family as originating in a swirl of sexual and domestic politics around a kitchen hearth," writes Wrangham, who later adds a classical allusion: "Prometheus is said to have created humans by quickening clay figures with fire. If the foraging and mating systems of humans were indeed shaped powerfully by cooking, the ancient Greek myth may have been close to the truth."
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