As summarized in the NY Times:
Many anthropologists and distance runners believe that running guided the evolution of early humans. We ran in search of dinner and away from predators.
But running is costly, metabolically. It incinerates energy. It can also cause injury. A twisted ankle would have removed your typical early human from the gene pool.
So why did our ancestors continue to run over the millennia “and not evolve other strategies for survival?” asks David A. Raichlen, a professor of anthropology at the University of Arizona, who led the study, which was published in The Journal of Experimental Biology. “We wondered if natural selection might have used neurobiological mechanisms to encourage exercise activity,” he continues.
Specifically, he and his colleagues became interested in the evolutionary role of the endocannabinoid system. As the name suggests, endocannabinoids are chemicals that, like cannabis in marijuana, alter and lighten moods. But the body produces endocannabinoids naturally. In other studies, endocannabinoid levels have been shown to increase after prolonged running and cycling, leading many scientists to conclude that endocannabinoids help to create runner’s high.
But Dr. Raichlen wondered if the endocannabinoids had had a more momentous role in the development of mankind as a whole. Had we continued to run, as a species, not because we had to run, but because we had become hard-wired to like it?