(3 of 5)
For starters, meat provided a concentrated source of protein, vitamins, minerals and fatty acids that helped our human ancestors grow taller. The first humans were the size of small chimps, but the bones of a Homo ergaster boy dating back about 1.5 million years suggest that he could have stood more than 6 ft. as an adult. Besides building our bodies, says Emory University's Dr. S. Boyd Eaton, the fatty acids found in animal-based foods would have served as a powerful raw material for the growth of human brains.
Because it's so packed with nutrients, meat gave early humans a respite from constant feeding. Like lions and tigers, they didn't have to eat around the clock just to keep going. But more important, unlike the big cats, which rely mostly on strength and speed to bring down dinner, our ancestors depended on guile, organization and the social and technological skills made possible by their increasingly complex brains. Those who were smartest about hunting--and about gathering the plant foods they ate as part of their omnivorous diets--tended to be better fed and healthier than the competition. They were thus more likely to pass along their genes.
The new appetite for meat didn't mean we lost our passion for sweets, though. As Berkeley's Milton points out, the brain's growth may have been facilitated by abundant animal protein, but the brain operates on glucose, the sugar that serves as the major fuel for cellular function. "The brain drinks glucose 24 hours a day," she says. The sugars in fruit and the carbohydrates in edible grains and tubers are particularly good sources of glucose.
The appetite for meat and sweets were essential to human survival, but they didn't lead to obesity for several reasons. For one thing, the wild game our ancestors ate was high in protein but very low in fat--only about 4%, compared with up to 36% in grain-fed supermarket beef. For another, our ancestors couldn't count on a steady supply of any particular food. Hunters might bring down a deer or a rabbit or nothing at all. Fruit might be in season, or it might not. A chunk of honeycomb might have as many calories as half a dozen Krispy Kreme doughnuts, but you might be able to get it once a year at best--and it wouldn't have the fat.
Beyond that, hunting and gathering took enormous physical work. Chasing wild animals with spears and clubs was a marathon undertaking--and then you had to hack up the catch and lug it miles back to camp. Climbing trees to find nuts and fruit was hard work too. In essence, early humans ate what amounted to the best of the high-protein Atkins diet and the low-fat Ornish diet, and worked out almost nonstop. To get a sense of their endurance, cardiovascular fitness, musculature and body fat, say evolutionary anthropologists, look at a modern marathon runner.