Ferrets don't often figure in studies of exercise, perhaps because they don't exercise much. They slink like fog through tunnels, sprint briefly over open ground and spend much of their time sleeping. They are, in biological terms, what's called a non-cursorial species, meaning that they are reluctant and lousy distance runners.

Which is why they were ideal subjects for an experiment conducted at the University of Arizona in Tucson looking at whether humans and other species evolved to like running.

Many anthropologists and distance runners believe that running guided the evolution of early humans. We ran in search of dinner and to flee 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 millenniums "and not evolve other strategies for survival?" asked David A. Raichlen, a professor of anthropology at the University of Arizona, who led the study, published in The Journal of Experimental Biology. "We wondered if natural selection might have used neurobiological mechanisms to encourage exercise activity," he said.

Specifically, Raichlen 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 Raichlen wondered if the endocannabinoids had had a more momentous role in the development of the human race 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?

To test that idea, Raichlen and his colleagues decided to compare the endocannabinoid response to running in species that do or do not historically run — to see, in other words, which animals experience a runner's high.

Ferrets were chosen to represent the nonrunners (mostly because, Raichlen said, "we could adopt them out into the community afterward," unlike other local non-cursorial animals like possums and skunks).

Humans and dogs became the designated cursorial, or distance running, species. The scientists recruited 10 recreational runners and eight dogs of various breeds.

They then took blood samples from all of the humans and animals and, after some preliminary, gentle training ("using positive reinforcement," Raichlen said), had each person or animal run on a treadmill for 30 minutes at a pace equivalent to about 70 percent of his, her or its maximum heart rate.

On a separate day, the people and dogs walked for 30 minutes on the treadmill, while the ferrets, which had found walking on the treadmill difficult to master, rested for 30 minutes in their cages.

The scientists drew blood after each session. They checked all of the samples for endocannabinoids.

It turned out that, as expected, the humans had shown significantly increased levels of endocannabinoids after running. So had the dogs, suggesting, for the first time, that they too experience a runner's high.

But neither species had developed increased endocannabinoid levels after walking.

And the ferrets didn't show higher endocannabinoid levels after either session. They gained, it seems, no neurobiological pleasure from running.

What these findings suggest, besides that ferrets will not make ideal training partners for marathon runners, is that a "reward response" to aerobic activity "appears to be part of our evolutionary history," Raichlen said.

Liking to run, it seems, may have helped to make humans who they are.

So why then, in actual practice, do so few humans today run? (Dogs are another matter; mine has to be constrained from tearing off and lolloping for miles.)

"That's the million-dollar question," Raichlen said. "It appears from our study that we have the evolutionary drive" to exercise. But modern man has learned to ignore it.

Of course, there are limitations to the study and what it can tell us about why so many of us in the developed world tend not to move much. The human volunteers were all fit, for one thing, unlike most modern humans. They may have been uniquely motivated to stay active from an early age and may not be representative of your typical human, present or past.

It's also a bit difficult to draw conclusions based on comparisons between people and ferrets. "Ferrets are weird," said David J. Linden, a professor of neuroscience at the Johns Hopkins University School of Medicine and author of "The Compass of Pleasure: How Our Brains Make Fatty Foods, Orgasm, Exercise, Marijuana, Generosity, Vodka, Learning, and Gambling Feel So Good." "They live in burrows and sleep 18 hours a day."

Still, the new study is provocative.

"Our results are very preliminary," Raichlen said. "But if they have a message, it's that our evolutionary history appears to have included this kind of endurance activity and rewarded it. And as a result, we continue to have a biological imperative" to move.