David Raichlen runs six mornings a week, enjoying the sunrise while training for the Tucson Marathon in December. “I always run in the morning, because once the day starts, it’s really hard for me to come back and run,” said the biological anthropologist at the University of Arizona.

The sunrise is just one reason to enjoy a morning run. There may also be an evolutionary explanation why humans feel good after exercise.

Raichlen discussed this neurobiological connection on Wednesday night as part of the weekly Happiness Downtown Lecture Series at the Fox Tucson Theatre. UA’s College of Social and Behavioral Sciences sponsors the free series.

Raichlen studies the exercise-induced reward system in the brains of humans and other mammals. About 2 million years ago, our ancestors switched from a vegetarian diet to a hunting and gathering lifestyle that involved an increase in physical activity.

“It suggests that, from an evolutionary standpoint, we are aerobic athletes,” Raichlen said. “More recently, things have sort of changed, and the way we live today allows us to be a little more sedentary and inactive, but our physiology and neurobiology fall within the context of this aerobic, athletic activity.”

Raichlen described how our brains are wired and then conditioned by our environment and activities to receive a reward of happiness from exercising.

The crowd of just under 1,300 at the historic theater and its overflow auditorium listened attentively to Raichlen’s tips on finding the type and intensity of exercise that can make you feel good.

The secret, Raichlen said, is finding the “evolutionary sweet spot,” or physical exercise that is neither too demanding nor too easy. This sweet spot is in the range of intensity where the most endocannabinoids, or chemicals responsible for endurance energy, are released.

Running is not the only activity that stimulates the release of these sustained-energy chemicals. Other endurance activities, such as swimming, cycling or playing a sport, can cheer you up if done regularly.

“(Our ancestors) didn’t have stair climbers and ellipticals, but that doesn’t mean the effect of exercise on the brain can occur only through running and walking,” Raichlen said. “What I think is that all responses to exercise are pretty generalizable to any form of endurance activity, as long as it’s moderate in intensity.”

These beneficial effects were most likely linked with exercise to motivate early humans to start hunting for food some 2 million years ago, when they became hunter-gatherers.

Today, encouraging people to participate more in aerobic exercise is one of the main public health issues, Raichlen said.

“Everything in life is just consistency,” Raichlen told the audience, “whether it’s what you do at work, or running or training.”

The lecture motivated Noella Nazerozich, 28, a massage therapist and student at Pima Community College, “to go work out right now.”

Tim McHenry, a 72-year-old UA retiree, said he already exercises, but “I’m going to increase it now.”

Some people in the audience even changed their feelings about exercise.

“I exercised a lot as a young man,” said Jake Elkins, a retired teacher. “I got over it in the Marine Corps.” A Vietnam veteran, Elkins said he hopes to treat his post-traumatic stress disorder with exercise instead of prescription drugs.Contact UA journalism graduate student Corey Poindexter-Ramirez at cpdex@email.arizona.edu