Researchers found that western men and women used strikingly similar amounts of energy each day compared with peers from a traditional community from the open savannah of Tanzania.
Despite trekking great distances each day to forage and hunt for game, results showed that the members of the Hadza tribe burned no more calories each day than a group of Americans and Europeans.
Experts have long assumed that our hunter-gatherer ancestors would have used up more energy than we do today, indicating that a lack of exercise could be behind the current obesity epidemic.
But the study in the PLoS ONE journal – the first to directly measure how much energy hunter-gatherers use – suggests that the rate at which humans use up calories remains relatively constant regardless of lifestyle.
Herman Pontzer, of Hunter College in New York, who led the study with colleagues from Stanford and Arizona universities, said: "The vast majority of what we spend our calories on is things you will never see like keeping our organs and immune system going. Physical activity is just the tip of the iceberg.
"If you spend a bit more [energy] on something like physical activity, you spend a bit less on something else but you do not notice it. This study shows that you can have a very different lifestyle, but [energy use] all adds up tot he same level no matter what."
It follows that the modern obesity problem is more likely down to our higher consumption of food than our ancestors, rather than our lower rates of physical activity, he added.
"People argue about why it is that westerners are getting so fat, and at the end of the day it has to be the fact that we are taking in more energy from food than we are burning – but is the big problem that we are taking in too many calories, or that we are not burning enough?
"But even if we had a lifestyle like our ancestors did ...[we] would not burn more calories than we do today. That has not changed a lot, but over the last 50 years we are eating a lot more than we need to be, so that gets to the heart of this issue."
Despite its apparent limited impact on obesity, Pontzer emphasised that exercise has a wide variety of physical benefits and is essential for keeping the body healthy.
The fact that the Hadza spend more of their daily energy output on physical exercise could be behind the good health of older tribe members, who are much more resistant to chronic illnesses such as heart disease than westerners, he said.
"We are not saying that physical activity is not important for health – clearly it is – but it does not appear to be the main cause of obesity."