Originally Posted by grindy

Please note that 'the fittest' does not mean 'the strongest' (which is often subconsciously extended to 'the most aggressive').
It means the most adapted, who can therefore reproduce. It can be about strength, but it can also be about cooperation. The latter actually seems to be the thing making humans especially successful. (Not that this cooperation can't be ultimately used to slaughter a competing tribe in an especially efficient way.)