Everyone knows that exercise is good for the body. But it’s also been well-established that exercise has positive effects on the brain, especially as we age. Less clear has been why physical activity affects the brain. Doctors David A. Raichlen and Gene E. Alexander explain how key events in the evolutionary history of humans may have forged the link between exercise and brain function. And they show how cognitively challenging exercise may benefit the brain more so than physical activity, which makes fewer cognitive demands.