I rarely see people on HN miss the point quite this badly. Reddit, yes, but not here.

The point is: for your scenario to play out, a tribe has to exist (and it has to be close enough for the couple's offspring to find it, but that's a detail). No tribe, no viable reproduction. Hence, the tribe is the minimum viable unit, even if it also occasionally supports an isolated couple somewhere in the vicinity.

Your argument is only true in the trivial sense that sexually reproducing organisms are only viable in larger numbers than a single pair; but that's a distinction that doesn't demarcate anything interesting. Humans are like ants, you say, but your argument also leads to the same conclusion about tigers, bears and other solitary animals in that they need more than a breeding pair to be viable. It descends into tautology.

Yes, humans are somewhat like ants in that they are usually social. Humans are adaptable enough to survive in pairs, though lack of specialization means they won't necessarily live the most comfortable life, so it would be an unusual choice. But there's nothing elective about ants' socialism. So in the end I don't think your argument works.

> your argument also leads to the same conclusion about tigers, bears and other solitary animals in that they need more than a breeding pair to be viable

No. Those species have specialized adaptations that allow them to live in isolation, which humans lack. And it's not just humans. All simians lack these adaptations. Indeed, the vast majority (perhaps all -- I can't think of a single counterexample) of non-carnivore mammals lack them.

Isolation does not mean other groups don't exist. It means lack of contact, and the adults don't need contact and as I have said children can move as happens with many territorial animals.