Sometimes progress really does appear in the numbers. Consider what we spend to fill our tables.
Since 1929, the percentage of disposable income that the typical American spends on food has been steadily dropping from about one-quarter of our available cash to just 10 percent. Quite simply, it takes less for us to feed ourselves than virtually any other people on Earth.
Technology has been revolutionary in its ability to help us obtain and store food. But essential tools like refrigeration and long-haul transit have only been feasible for the past 150 years, barely a speck in the long view of human history.
Even the advent of organized agriculture and the domestication of food animals, which took place about 10,000 years ago by most estimates, was just a tick on the evolutionary clock. Most modern sources of calories simply weren't available to prehistoric hunter-gatherers.
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In other words, we, meaning most residents in highly technological nations like the United States, are truly among the first handful of generations never to worry about where to get our food.
"This is the first time in human history that's been the case for large numbers of people," says George Mason University professor Peter Stearns, author of "Fat History."
Built for famine
Modern life may have solved most of our food-gathering problems, but human evolution has not kept up. Our bodies are still wired for hunter-gatherer biology: Eat all the food you can and store it — in body fat — in case your supply of food runs out, as in the case of famine. A dangerous configuration for a society with all-you-can-eat buffets.
Our ancient ancestors, especially those on the African plains, hunted far leaner animals than we now eat — species closer to modern deer and elk than the typical meat cow. Processed grains and sugars have no counterparts in ancient foods.
And modern taste combinations like concentrated fat (butter) and concentrated sweetness (sugar) rarely co-exist in nature, notes Loren Cordain, professor of health and exercise science at Colorado State University.
"You get fat on the types of foods that have been introduced since the agricultural revolution," says Cordain, author of "The Paleo Diet," which advocates emulating prehistoric eaters. Think South Beach without any grains, salt or sugar, and lots of fish and broccoli.
Cordain argues that when it comes to food, evolutionary mechanisms simply can't match the fast pace of human innovation. The result in the United States, he suggests, is heart disease and Type 2 diabetes, to say nothing of obesity.
Not every anthropologist goes so far, but most acknowledge that rich nations are all but drowning in food. And as developing nations grow, people once threatened by famine are quickly facing the opposite problem .
Remember those dinner-table threats about starving kids in China? The percentage of obese Chinese doubled from 1992 to 2004, and nearly 23 percent are overweight. India faces similar problems.
One major problem is that quick changes to long-established dietary traditions can be unhealthy, even devastating. Arizona's Pima Indians suffered from their genetic adaptability to famine after being introduced to an American diet around World War II. Now they face epidemic obesity and Type 2 diabetes. Sudden wealth and access to processed foods prompted an even more drastic diabetes crisis in the Pacific island nation of Nauru.
Feeding our brains
"What I think we can say conclusively, is that the evolutionary success of our species is ultimately a nutrition story," says Bill Leonard, chairman of Northwestern University's anthropology department.
One crucial step forward, he says, came with the development of the brain of Homo erectus about 1.5 million years ago.
Leonard analyzed the brain sizes of human ancestors and found they started growing rapidly with the first human species, Homo habilis, about 2 million years ago. About 600 cubic centimeters, the Homo habilis brain was far larger than the brains of earlier australopithecine species. Then, between 2 million and 1.5 million years ago, brain size quickly grew to 900 cubic centimeters, surpassing modern apes' brains.
That's an important change because larger brains require more energy. According to a 2003 article by Leonard, Homo erectus devoted 17 percent of calories to its brain; chimps use about 9 percent. We modern humans use nearly a quarter of our resting energy on our brains.
Shrinking stomachs are another piece of the evolutionary puzzle. Homo erectus appears to have had a smaller gut than its predecessors, with a stomach and intestine that grew more compact as higher-quality foods became available.
What kind of grub? Most theories credit the ability to butcher and distribute meat, plus a move from low-nutrition forage plants to fruits and quality grains such as oats and wheat. (One alternate theory credits our learning to cook tubers and other fibrous plants.)
Better foods meant more calories for less work, an evolutionary step favoring hominids that could hunt meat and collect more nutritious plants.
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.
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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