Evolving with our stomachs

Energy in, energy out
Before you exult in this ascent up the food chain, remember that energy intake is only half the equation. Along with nearly universal access to food, the modern economy has allowed an unprecedented number of Americans to survive using our big, evolved brains and little else. We're getting fat off our own evolutionary success and, says University of Wisconsin paleoanthropologist Henry Bunn, "I don't think biological evolution has really had a chance to react."

A high caloric intake isn't so much to blame as an imbalance between calories in and calories out. Though developing nations generally consume fewer calories than industrialized nations, Leonard found subsistence societies that match developed societies calorie for calorie. The reindeer-herding Evenki people of Siberia consume more than 2,800 calories a day, and far more animal foods than the typical American, yet have lower cholesterol and body mass indicators.

The difference is how we burn energy. In this land of fitness clubs and freeways, we're all mixed up when it comes to expending calories — and our bodies are evolutionarily configured to crave more than we can possibly burn the way we live today.

Paulo Whitaker / Reuters file Members of Brazil's Matis tribal nation still spend much of their days, and most of their energy, foraging and hunting with poison-tipped darts.

Katharine Milton of the University of California, Berkeley, sees this discrepancy each time she visits remote Brazilian tribes like the Matis and Parakana. These societies at least approximate ancient hunter-gatherer lifestyles. Adults endure more than eight hours of strenuous work each day, mostly gathering low-quality food: lots of high-fiber starches like manioc, plus tiny bits of game meat, fruit and nuts.  Obesity?  Almost non-existent.

"I'm sure I'm considered the world's laziest woman in the Amazon basin," she says.

Thinner thighs in six ... centuries
So just mimic an ancient lifestyle, right? Not necessarily. Healthy diets exist throughout the world, Leonard notes, but they've often been developed over centuries as cultures evolve and adapt to available foods. That doesn't work on a fad-diet timeline.

Similarly, the evolution of human traits — especially as Darwin laid it out — can't necessarily turn on on a dime and adjust to new foods.

Peter Stearns believes nothing short of "an adverse evolutionary consequence" — drastically shorter life spans, or perhaps a hampered ability to reproduce — will force large-scale behavioral changes.

Just how adverse?  In a much-debated report released in mid-March, researchers calculated obesity could shorten Americans' lives by two to five years in the next half-century.

Hard to know if that's drastic enough, but short of huge changes in the American diet — and in developing nations, too — you can expect plenty more such headlines.

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