[devfeed]

I can't answer your first question about why left-handed people exist, but I can provide some clues about the proportions:

Left-handedness is a disadvantage in society where everything is designed for the right-handed person. However, there is one advantage to being left-handed: in hand to hand or sword to sword combat, the southpaw has an advantage in being unfamiliar.

This article http://www.economist.com/node/3471297 (Economist, 2004) describes an empirical finding: in societies that were historically more violent, the left-handedness proportion is higher.

Perhaps this is also why left-handed people even exist. They have an advantage in historical person-to-person violent conflicts. This advantage would go away as their proportion gets higher. I presume (this is conjecture) that the minority proportion that it is, is the steady state equilibrium where the violent advantage outweights the other disadvantages of being out of sync with society.