On wikipedia there is a theory that posits that those with ADHD/ADD are born with genes that were used during hunter/gatherer periods. The reason for the hyperactivity and impulsiveness comes from the need to constantly be searching for food/prey. It seems reasonable, you have these periods to be constantly searching for berries/nuts and you need that extra energy to make sure you have enough for you/family/village. ADHD can also have hyperfocus which is useful for hunting animals, which requires quick reflexes and extended focus on successfully getting the prey. That's one theory anyway.