Peoples who have created anything remotely resembling a functioning society don't act with this brutality and frequency towards their own kin and tribes, no matter the circumstance. Either the Papua New Guineans and others like them have gone "backwards" from a common ancestor (insert pedantic comment about how there is no such thing as devolution), or the kin-killing irrationally violent genes were always there and the rest of us eliminated most of the people that carried them because they were preventing us from operating even the most primitive of villages.