Interesting theory. Still, compared to more specialized defensive organs of other animals such as large canine teeth, tusks, horns, and the like, the fist is a pretty poor defensive weapon. A large male chimp can easily best an unarmed human of similar weight with his powerful musculature and crushing jaws, armed with large interlocking canines.

Personally, I suspect that the morphology of the human hand has much more to do with dexterity and tool use than fighting. The same strong hand-shape that makes a good fist can also wield a weapon like a club, axe or spear. A band of armed humans is a whole lot more dangerous!

Or it could just be that my love of Eskrima has biased me a bit...