According to a study out of the University of Utah and published by the Journal of Experimental Biology, human hands evolved over time to be just as useful in conflict as they are in use of tools. In other words, they were made equally well to build and destroy.

From The Telegraph:

"The role aggression has played in our evolution has not been adequately appreciated," said Professor David Carrier, from the University of Utah.
"There are people who do not like this idea but it is clear that compared with other mammals, great apes are a relatively aggressive group with lots of fighting and violence, and that includes us. We're the poster children for violence."
The forces of natural selection that drove hands to become nimble-fingered also turned them into weapons, Prof Carrier believes.
"Individuals who could strike with a clenched fish (sic) could hit harder without injuring themselves, so they were better able to fight for mates and thus be more likely to reproduce," he said.

So when it comes down to it, we evolved to punch things better so we could have more sex. Works for professional fighters, I guess.

Glib comments aside, the article continues by going further in depth as to the mechanics of force as delivered by the human hand, noting that the force of a punch, over the same amount of surface area, is three times that of a slap, and the structural advantages a fist has to producing force, over a slap.

For the rest of the article, check out The Telegraph.