A single punch can be lethal, so why doesn't a special case (albeit myth) of it make evolutionary sense?

A single punch can be lethal, but not with anything like the frequency that you could be subject to this kind of impact - it's an obvious place to punch someone, and very similar to what happens when you fall on your face. We know that skull shape is something that evolution can and does change in relatively short timeframes. There's no "technical debt" explanation, particularly if the claim was that this is something unique to humans.

What convinced you otherwise? I think the same person can profess either genuine or faked altruism depending on the situation.

Mainly moving from a situation in which I faked it to one in which I genuinely enjoyed being altruistic - but also observing changes in I guess how behaviour seemed to change with observation, which seemed to suggest that my peers also underwent the same change.