We typically believe that the obesity epidemic has been led not only by poor diet, but by our increasingly sedentary lifestyles.
However, research published in the journal PLoS ONE indicated that food choices and portion sizes may play a much larger part in expanding waistlines than lack of exercise.
Figures from the World Health Organisation suggest one in three people worldwide will be overweight by 2015, while obesity will be prevalent among one in ten people.
US, Tanzanian and UK scientists examined the Hadza tribe of Tanzania, who are as close to ancient hunter-gather as the world has, to try determine the difference in their energy expenditure compared to Westerners.
The tribe hunts with axes, bows and digging sticks, rather than modern tools, to catch their prey and collect berries, roots and fruits.
Men traditionally pursue animals over longer distances, while women travel fewer miles gathering fruits.
Although they had far higher activity levels than their Western counterparts, their calorie usage was much the same when size, weight and metabolic rate was taken into account.
This goes against traditional beliefs that exercise is the top weight management tool, with processed and sugary foods more likely to drive up weight than perching on the sofa for a number of hours.
Still, researchers were adamant that exercise should be included as part of a balanced and healthy lifestyle.
Dr Herman Pontzer of the department of anthropology at Hunter College, New York, confirmed the study findings.
He told the BBC: "This to me says the big reason that Westerners are getting fat is because we eat too much - it's not because we exercise too little.
"Daily energy expenditure might be an evolved trait that has been shaped by evolution and is common among all people and not some simple reflection of our diverse lifestyles."