Handedness

Why there are left-handed people (i.e. why is there the hand difference)? Left brain hemisphere controls the right side of the body  (and vice versa) and speech is also on the left side (now, again in most people, even left-handers); so it may be that the left side is more capable in order not to need both hemispheres too energy-demanding - just the left one. Yet then, why some people have it on the reverse, why it's not the one way in everyone? It suggest there may be an advantage in being left-handed.
A study has found there are more violent crimes in societies with more left-handers. It's explained that left-handers have more chance to injure, because the attack from the left side isn't predicted.
Other primates don't have the handedness trait. There already were left-handers among cave painters in the stone age.
It seems handedness is in some way genetically determined, but probably not absolutely. People who suffered left hemisphere damage in early life are more likely to be left-handed (that means the right hemisphere takes some functions).
Overall some 10% of population are left-handed.