Facts and presumptions:
- (fact) Most people are right-handed
- (presumption) Some aspect of right-handedness is genetic
- (presumption) For a long time, the greatest selection pressure in Human Evolution has been other humans
- (fact) The spleen is almost always on the left side of the body
- (fact) The spleen tends to be ruptured quite easily in crush injuries (like car accidents) and physical blows
- (fact) If you have a major spleen rupture, you bleed to death internally
Knowing these facts, and using these reasonable presumptions, most people would probably come up with the same theory that I did. As a right-handed blow (from a fist or a club) is more likely to hit the body on the spleen-side than the other side, a right-handed human has a slight advantage in hand-to-hand combat compared to a left-handed individual. This theory also assumes that it is easier to generate left-handed/right-handed differences than right-spleen/left-spleen differences. In other words, the location of the spleen is dictated by many things, and is harder to mutate to the other side, while the left/right-handedness is a (presumably) simpler alteration in motor neuron arrangement.
Of course, this theory is nearly unprovable. It would require a couple million years, a couple million left-spleen people, and a couple million right-spleen people, and two planets like Earth.
I guess it's more of a conjecture.