Into The Fire

EATING MORE ANIMAL FOODS is one way of boosting the caloric and nutrient density of the diet, a shift that appears to have been critical in the evolution of the human lineage. But might our ancient forebears have improved dietary quality another way? Richard Wrangham of Harvard University and his colleagues recently examined the importance of cooking in human evolution. They showed that cooking not only makes plant foods softer and easier to chew, it substantially increases their available energy content, particularly for starchy tubers such as potatoes and manioc. In their raw form, starches are not readily broken down by the enzymes in the human body. When heated, however, these complex carbohydrates become more digestible, thereby yielding more calories.

The researchers propose that Homo erectus was probably the first hominid to apply fire to food, starting perhaps 1.8 million years ago. They argue that early cooking of plant foods (especially tubers) enabled this species to evolve smaller teeth and bigger brains than those of their predecessors. Additionally, the extra calories allowed H. erectus to start hunting—an energetically costly activity—more frequently.

From an energetics perspective, this is a logical enough line of reasoning. What makes the hypothesis difficult to swallow is the archaeological evidence Wrangham's team uses to make its case. The authors cite the East African sites of Koobi Fora and Chesowanja, which date to around 1.6 million and 1.4 million years ago, respectively, to indicate control of fire by H. erectus. These localities do indeed exhibit evidence of fires, but whether hominids were responsible for creating or harnessing the flames is a matter of some debate. The earliest unequivocal manifestations of fire use—stone hearths and burned animal bones from sites in Europe—are only some 200,000 years old.

Cooking was clearly an innovation that considerably improved the quality of the human diet. But it remains unclear when in our past this practice arose. —W.R.L.

EARLY COOKING of plant foods, especially tubers, enabled brain expansion, argue Richard Wrangham of Harvard University and his colleagues.

plant material and more animal foods.

As to what prompted Homo's initial shift toward the higher-quality diet necessary for brain growth, environmental change appears to have once more set the stage for evolutionary change. The continued desiccation of the African landscape limited the amount and variety of edible plant foods available to hominids. Those on the line leading to the robust australopithecines coped with this problem morphologically, evolving anatomical specializations that enabled them to subsist on more widely available, diffi-cult-to-chew foods. Homo took a different path. As it turns out, the spread of grasslands also led to an increase in the relative abundance of grazing mammals such as antelope and gazelle, creating opportunities for hominids capable of exploiting them. H. erectus did just that, developing the first hunting-and-gather-ing economy in which game animals became a significant part of the diet and resources were shared among members of the foraging groups. Signs of this behavioral revolution are visible in the archaeological record, which shows an increase in animal bones at hominid sites during this period, along with evidence that the beasts were butchered using stone tools.

These changes in diet and foraging behavior did not turn our ancestors into strict carnivores; however, the addition of modest amounts of animal foods to the menu, combined with the sharing of resources that is typical of hunter-gatherer groups, would have significantly increased the quality and stability of hom-inid diets. Improved dietary quality alone cannot explain why hominid brains grew, but it appears to have played a critical role in enabling that change. After the initial spurt in brain growth, diet and brain expansion probably interacted synergistically: bigger brains produced more complex social behavior, which led to further shifts in foraging tactics and improved diet, which in turn fostered additional brain evolution.

A Movable Feast

THE EVOLUTION of H. erectus in Africa 1.8 million years ago also marked a third turning point in human evolution: the initial movement of hominids out of Africa. Until recently, the locations and ages of known fossil sites suggested that early Homo stayed put for a few hundred thousand years before venturing out of the motherland and slowly fanning out into the rest of the Old World. Ear

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