Evolving with our stomachs

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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
Patients perform aerobics at the Aimin Fat Reduction Hospital in the city of Tianjin
Mark Ralston / Reuters
Patients perform aerobics at the Aimin Fat Reduction Hospital in Tianjin, China. The hospital is yet another sign of how many Asians have gone from being malnourished to overweight in just a few decades.
The hunt for enough food, and better food, has played a defining role in the history of human evolution.

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"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.

CONTINUED : Energy in, energy out

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