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Although our physiology has stayed pretty much the same for the past 50,000 years or so, we humans have utterly transformed our environment. Over the past century especially, technology has almost completely removed physical exercise from the day-to-day lives of most Americans. At the same time, it has filled supermarket shelves with cheap, mass-produced, good-tasting food that is packed with calories. And finally, technology has allowed advertisers to deliver constant, virtually irresistible messages that say "Eat this now" to everyone old enough to watch TV.
This artificial environment is most pervasive in the U.S. and other industrialized countries, and that's exactly where the fat crisis is most acute. When people move to the U.S. from poorer nations, their collective weight begins to rise. As developing areas like, for example, Southeast Asia and Latin America catch up economically and the inhabitants adopt Western lifestyles, their problems with obesity catch up as well. By contrast, among people who still live in conditions most like those of our distant Stone Age ancestors--such as the Maku or the Yanomami of Brazil--there is virtually no obesity at all.
And that's almost certainly the way it was during 99.9% of human evolution. For most of the 7 million years or so since we parted ways with chimps, life has been very harsh--"nasty, brutish and short," in Thomas Hobbes' memorable phrase. The average life expectancy was probably well under 30. But much of that dismal brevity can be chalked up to accidents, infections, traumatic childbirth and unfortunate encounters with saber-toothed cats and other such predators. If a Cro-Magnon, say, could get past these formidable obstacles, he might conceivably live into his 60s or even longer, with none of the obesity-related illnesses that plague modern Americans.
Our earliest ancestors probably ate much as their cousins the apes did, foraging for fruits, shoots, nuts, tubers and other vegetation in the forests and savannas of Africa. Because most wild plants are relatively low in calories, it took constant work just to stay alive. Fruits, full of natural sugars like fructose and glucose, were an unusually concentrated source of energy, and the instinct to seek out and consume them evolved in many mammals long before humans ever arose. Fruit wasn't always available, but those who ate all they could whenever it was were more likely to survive and pass on their sweet tooth to their progeny.
Our love affair with sugar--and also with salt, another crucial but not always available part of the diet--goes back millions of years. But humanity's appetite for animal fat and protein is probably more recent. It was some 2.5 million years ago that our hominid ancestors developed a taste for meat. The fossil record shows that the human brain became markedly bigger and more complex about the same time. And indeed, according to Katherine Milton, an anthropologist at the University of California, Berkeley, "the incorporation of animal matter into the diet played an absolutely essential role in human evolution."