If you truly wants to bring biology into this then you have to understand that our forefathers were poly nonetheless, but that version of Poly was Polygynous.one male to several females. Scientists think this because when we spot polygyny in other animals, the males tend to be much larger than the females. (So, there’s one big male who fights other men to keep his harem.) And it’s estimated that among our ancestor
Australopithecus afarensis, who lived 3 million years ago, the female was about two thirds the height of the male.
polyandry – that is, one female and many male partners was never been a part of Human evolution. Polyamorus means allowing both types of sexual configuration, When it's already been proved that polyandry had no place in human history. So arguing in favour of it to call it natural is disingenious. It has also been proved that polygamy should have no place in our modern society. (If you want details then watch the documentary "Sister Wives" on Hulu)
So this argument in favour of Polyamory being natural doesn't stand.
Also The tribe you are talking about are the losers of this evolution game. As a species we don't take notes from losers.