The principal reason that animals have a brain and plants don't is because we can move of our own volition. Plants can't, and so they don't even need to know where they are. What's the point of knowing that other plants are better situated if you can't join them-or that a logger is approaching if you can't flee?

But if an organism has legs, wings, or fins, it needs a sensory system to let it know about here and there, a decision system to determine if it's better to be here or there, a motor system to go over there if that's the better alternative, and a memory system that will get it back to here. That pretty much explains a brain. The human brain enhances its movement repertoire by being able to walk, run, and sprint, to kick and jump, and to grasp and throw. Perhaps more important, we seek to move with style and grace (the arts) and at a virtuoso level (the Olympics).

Social species also develop communicative movement systems to establish and maintain relationships and enhance survival. Many mammals use an intimate tactile form called grooming or caressing and visual/auditory signal systems that can communicate information at a distance. Human language and music are the most advanced of these systems.