The euphoric feeling known as "runner’s high" after exercise may have played a role in the evolution of humans' ability to run long distances, according to a study.

Researchers said that the "runner's high," which is triggered by natural neurotransmitter chemicals called endocannabinoids that activate the brain's pleasure circuits in the same way as some drugs like cannabis, developed as an evolutionary 'treat' to motivate endurance exercise in humans and other animals that run long distances.

Researchers say that our early human hunter-gatherer predecessors may have developed long-distance endurance skills because of the "high" that was produced from physical activity.

"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," lead researcher David Raichlen, assistant professor of anthropology at the University of Arizona, said in a university news release.

Investigators tested their theory of the "runner's high" as a built-in motivator for exercise by conducting experiments on two species of natural athletes, humans and dogs, and a more sedentary species that doesn't regularly run in the wild, the ferret.

The study, published in the Journal of Experimental Biology, consisted of 10 human participants to run and walk on a treadmill, and eight dogs and eight ferrets that were specially trained to do the same.

Raichlen and his team collected blood samples before and after all three subject groups had exercised for 30 minutes, and found that after a brief session of physical activity a type of endocannabinoid called anandamide spiked in dogs and people.

However, blood samples from ferrets showed no change in their endocannabinoid levels after exercise.

Researchers also asked to human runners to fill out a mood-evaluating questionnaire and found that and all subjects reported feeling significantly happier after exercise, suggesting that measured levels of anandamide correlated closely to an increase in positive emotional feelings.

"These results suggest that natural selection may have been motivating higher rather than low-intensity activities in groups of mammals that evolved to engage in these types of aerobic activities," Raichlen said.

The experimental results prove that "anandamide-inspired motivation to run was the evolution of an 'endurance athlete phenotype' that played a major role in the survival and reproductive success of our Homo sapiens ancestors," co-researcher Greg Gerdeman, assistant professor of biology at Eckerd College in St. Petersburg, Florida said in a statement.

"This is only one of several lines of evidence suggesting that endocannabinoid signaling may underlie some of the physiological benefits of exercise," Gerdeman added. "Future studies could work to elaborate what kinds of specific exercise regimens might maximize therapeutic outcomes of activating the cannabinoid receptors, such as treatment of depression, for example."