Strictly speaking they don't need to be; they "just" need to be common enough among human cultures for that to exert distinguishable selection pressure, and successful enough that the groups that come up with the idea don't end up autodarwinating. Though the latter is group selection pressure of a kind, too.

I'd rather stay agnostic on whether or not this is the case; we have very little reliable data on culture under non-marginal paleolithic conditions. I haven't heard of any conclusive skeletal evidence for war in that era (murder yes, war no), but this isn't my field so I could easily be missing some.