Evolutionary link to modern-day obesity, other problems
Published: Thursday, February 12, 2009 - 15:43 in Biology & Nature
Learn more about: brain and body energy imbalances evolutionary link hunting 
and gathering metabolic demands obesity

That irresistible craving for a cheeseburger has its roots in the dramatic 
growth of the human brain and body that resulted from environmental changes 
some 2 million years ago. Higher quality, nutritionally dense diets became 
necessary to fuel the high-energy demands of humans' exceptionally large brains 
and for developing the first rudimentary hunting and gathering economy.

But the transition from a subsistence to a modern, sedentary lifestyle has 
created energy imbalances that have increased rapidly -- evolutionarily 
speaking -- in recent years and now play a major role in obesity.

Activity patterns must get every bit as much attention as consumption of 
unhealthy foods in any attempt to reverse the modern-day permeations of an 
evolutionary trend that now contributes to obesity worldwide, according to 
William Leonard.

Leonard, chair and professor of anthropology at Northwestern University, will 
discuss his work during the 2009 American Association for the Advancement of 
Science (AAAS) meeting in Chicago at a press briefing that will take place at 2 
p.m. Feb. 12 and during a symposium from 8:30 to 11:30 a.m. Feb. 13.

Two million years ago shifts in foraging behavior and dietary quality helped to 
provide the energy and nutrition to support the rapid evolutionary increases in 
both the brain and body sizes of our ancestors.

Today modern humans use nearly a quarter of their resting energy needs to feed 
our brains, considerably more than other primates (about 8 to 10 percent) or 
other mammals (3 to 5 percent). To support the high-energy costs of our large 
brains, humans consume diets that are much richer in calories and nutrients 
than those of other primates.

"While our large-bodied ape relatives -- chimps, gorillas and orangutans -- can 
subsist on leaves and fruit, we needed to consume meat and other energy-rich 
foods to support our metabolic demands," Leonard said.

Staple foods for all human societies are much more nutritionally dense than 
those of other large-bodied primates. "To obtain these higher-quality diets, 
our foraging ancestors would have had to have moved over larger areas than our 
ape relatives, requiring large activity budgets," he said.

But substantial reductions of intense physical activities for adults living a 
modern lifestyle in the industrialized world have dramatically lowered the 
metabolic costs of survival.

The differences between energy in and energy out widen as we increase the 
nutritional density of our diets while reducing the time and energy associated 
with obtaining food. "Think about our ancestors," Leonard said. "Human 
hunter-gatherers typically move 8 miles per day in the search for food. In 
contrast, we can simply pick up the phone to get a meal delivered to our door."

That decline in daily energy expenditures contributes not only to obesity, but 
also to other chronic diseases of the modern world, such as diabetes and 
cardiovascular disease. "In a sense, those modern diseases represent where we 
started early in our evolutionary history," Leonard said.

The data clearly suggest the obesity epidemic cannot be understood solely by 
looking at consumption, he stressed. "Throughout most of our evolutionary 
history, the acquisition of our high-quality diets required substantial 
expenditure of energy and movement over much larger areas than for other 

The imbalance between energy intake and energy expenditure today, Leonard 
concludes, is the root cause of obesity in the industrialized world.

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