Genetic programming.
According to this theory, different people are genetically programmed to do different things, based on mankind's early evolutionary stages. Some are genetically programmed to be hunter/defenders while others are genetically programmed to be gatherer/nuturers.
Gatherer/Nurturers are those members of the tribe which would stay close to the camp to watch the sick and young while also tending to the crops and any early domesticated animals. The focus requirements of this group of people was to be on many things at one time (think multitasking). These tasks tend to take more patience but less intense focus, and theory says that these traits were selectively bred into various individuals who then passed them down to what would eventually be our generation. These people are NOT ADHD.
Hunter/Defenders on the other hand would be required to go away from the camp in order to find and kill the meat for the tribe. This group is also the ones who would go to war with neighboring tribes. The type of patience this required was different than that required of someone from the aforementioned group. While these people were able to focus, it was only on the hunt that they would be focusing on (similar traits seen in current ADHD children playing a video game). Yes the focus was intense, but it was only on the one task. Again this trait was selectively bred into those people who would then pass the trait down into our generation. These people are ADD/ADHD.
This is one area I have not looked into enough to decide if I really buy into this theory. I am actually more inclined to believe that what we know as ADHD may actually be an evolutionary stage that mankind is going to. Not sure what evolution intends for us to do with it, but that idea makes more sense to me than the selectively bred trait theroy.