But protruding chins don't really take punches that well, do they? The prototypical knockout punch is one that suddenly knocks the chin to the side, rotating the skull more quickly than the brain, bruising the brain both when the skull makes contact and when the brain passes the skull and makes contact on the other side.
Not to argue, but is being knocked out bad? It seems like a common occurrence to a fight is go to unconscious. Some people have strong chins. Would they do better in a no rules fight historically, if they didn't go down?
Getting knocked out in an athletic competition is one thing. Getting knocked out when a neighboring tribe is attempting to kill all your men and old folks, enslave everyone else, and steal everything of value, would probably have been something else.
Inter-tribal resource wars might be fought by primitives, but they're not fought by idiots. Nor are they fought by chivalrous knights. The throats of unconscious warriors will be cut. Even if a throat was missed in all the chaos, what is the evolutionary advantage to a male of living on after all fertile females have been abducted?