In the last century something unexpected happened: humans became sedentary. We traded in our active lifestyles for a more immobile existence. But these were not the conditions under which we evolved. David Raichlen from the University of Arizona, USA, explains that our hunter-gatherer predecessors were long-distance endurance athletes. ‘Aerobic activity has played a role in the evolution of lots of different systems in the human body, which may explain why aerobic exercise seems to be so good for us’, says Raichlen. However, he points out that testing the hypothesis that we evolved for high-endurance performance is problematic, because most other mammalian endurance athletes are quadrupedal. ‘So we got interested in the brain as a way to look at whether evolution generated exercise behaviours in humans through motivation pathways’, says Raichlen.