http://krwgnews22.com/2015/10/24/human-opposable-thumbs-evolved-for-punching-study/
They found that humans can safely strike, using 55 percent more force, with a closed fist than with a open one, and with more force with an open fist than by slapping. Carrier and his team believe that, at the same time the hand evolved to perform manual chores, it also evolved to be used as “a club during fighting.”
The fisted punch protects our metacarpals – the fingers in our hands – that are the bones that break most often when someone punches a wall or someone else.