The modern scourge of obesity could owe more to the lifestyle of our ancient ancestors than previously thought, according to scientists.
Anthropologists claim our urge to eat supersized portions can be traced back to the dramatic growth of the human brain and body that occurred in the face of environmental changes two million years ago.
The expansion of the human brain, which coincided with the arrival of the first hunter-gatherer economies, required early humans to bolster their diets by seeking out more energy-rich food, researchers said.
William Leonard, a professor of anthropology at Northwestern University in Illinois, said the transition from a highly active subsistence level of living to today's more sedentary lifestyle has played a major role in the rise of obesity, because while we still eat energy rich diets, we are far less physically active.
Today, roughly a quarter of the energy we burn up while resting is used by our brains. Other primates, such as chimps and gorillas, use between a half and a third of that. To make sure our brains have enough energy, humans eat diets that are much richer in calories and nutrients than 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," said Leonard. To find that food, our ancestors had to hunt over large areas of land, meaning they burned up a lot of the calories they consumed.
With the shift to less active lifestyles, humans now have a diet that contains far more calories than they need to survive, Leonard will claim at the American Association for the Advancement of Science annual meeting in Chicago.
"Think about our ancestors. Human hunter-gatherers typically moved eight miles a day in the search for food. In contrast, we can simply pick up the phone to get a meal delivered to our door," Leonard said.
The decline of physical exercise in modern lifestyles contributed to other long-term health problems beyond obesity, including diabetes and cardiovascular disease, he said.