[quote=@Doivid]
I always find it to be a shakey argument when people appeal to the whole 'human nature' angle, given that that is not at all ironclad, or even well defined, by actual experts in the field. It's very much up in the air at the moment.

It didn't seem to be human nature for the 200,000 or so years before we started farming.
[/quote]

Could've been. I can imagine a circumstance where Glug kills Ug, causing the Ug family to convince the rest of the tribe they need to murder the entire Glug family.