http://www.nytimes.com/2012/08/26/opinion/sunday/debunking-the-hunter-gatherer-workout.html?_r=1&hp

Our findings, published last month in the journal PloS ONE,
indicate that they don’t, suggesting that inactivity is not the source of modern
obesity.

Our findings add to a growing body of evidence suggesting that energy
expenditure is consistent across a broad range of lifestyles and cultures. Of
course, if we push our bodies hard enough, we can increase our energy
expenditure, at least in the short term. But our bodies are complex, dynamic
machines, shaped over millions of years of evolution in environments where
resources were usually limited; our bodies adapt to our daily routines and find
ways to keep overall energy expenditure in check.

All of this means that if we want to end obesity, we need to focus on our diet
and reduce the number of calories we eat, particularly the sugars our primate
brains have evolved to love. We’re getting fat because we eat too much, not
because we’re sedentary. Physical activity is very important for maintaining
physical and mental health, but we aren’t going to Jazzercise our way out of the
obesity epidemic.