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contemporaries—several skulls, a hand bone complete with fingers, and multiple leg and foot bones that can’t conclusively be connected with the skulls, but at least provide some clues about the creature’s size and gait. Together the evidence tells us that
habilis
, though slight in stature, walked upright all the time and possessed considerably larger brains than the first ancient humans, as spacious as 950 cc, depending on which skull you inspect. The shapes of their heads and jaws indicate that unlike their robust cousins, they didn’t care much for nuts, bark, and berries, but had developed an appetite for meat, and the protein it provided, which may account for their larger brains. (See sidebar “Big Guts vs. Big Brains” p. 21.) Nor did they sport great sagittal crests, or huge, square teeth made for grinding. Their teeth were better at tearing. Chances are they hunted small game in packs, not unlike the way chimpanzees sometimes do. And they helped themselves to savanna carrion and whatever other more adept and deadly predators left behind in the way of their prey’s remains.
    Big Guts vs. Big Brains
    Cows, as we all learned in grade school, have four stomachs. They do because it requires a lot of work to extract enough nutrients from grass to transform it into beef and milk. The same was true of our early savanna–roaming ancestors, at least some of them. Subsisting on a diet of nuts, roots, thistles, berries, and other plants required long intestines and strong stomachs if they hoped to squeeze enough nutrients from them to stay alive.
    As the climate changed in Africa and the savannas became broader and drier, the old jungle ways of gathering low–hanging fruit from nearby trees and not moving very far from day to day simply didn’t work. Fruit and foliage became increasingly rare, and three humans had to cover more distance to gather it, which required still more energy. Ultimately that was not a sustainable survival strategy.
    But if you could get your hands on some meat! Then you were instantly rewarded with much more nutritional bang for your hunting–and–gathering buck. That is precisely what the robust lines of savanna humans did. But this choice paid an additional, unexpected dividend. A diet of meat of any kind (even dining on termites and small rodents) made larger brains possible, and less cowlike intestinal tracts necessary. This is something paleoanthropologist Leslie Aiello dubbed the Expensive Tissue Hypothesis when she first came up with the idea in the early 1990s. What this meant, and what fossil finds reveal, is that as our ancestors began to consume more meat, their bodies could redirect the energy those complex intestinal tracts demanded to the business of constructing larger brains. It was a close question two million years ago which approach might work best. Both experiments were tried, and for hundreds of thousands of years both worked. Ultimately, though, larger brains turned out to be a more effective survival tool than longer intestines, something the fossil record bears out. While australopithecines and the robust members of the human family were relatively small brained, often not much more cerebrally endowed than a chimpanzee,
Homo ergaster
’s brain size ballooned to 900 cc or so. After a run of more than one millionyears, the last of the robust humans finally made their exit 1.2 million years ago.
    This evolutionary path had other ramifications as well. We aren’t as strong as our primate cousins—chimps, gorillas, orangutans—for example. We seem to have exchanged brawn for brains. Richard Wrangham has argued that mastering fire and cooking made meat and other foods of all kinds easier to digest, increasing the protein we could consume and reducing the need for longer intestinal tracts even further. In time bigger brains delivered better weapons, and more strategic ways of hunting. And that likely led to bigger game, more meat, more protein, more cerebral horsepower.