We (humans) often cooperate because it allows us to do things we can't do alone. So perhaps we evolved cooperation so we could hunt and defend ourselves. If we look at this reduction alone, it looks like a positive sum because all of the humans benefit in this case. If we look at the system holistically, now that we can defend ourselves, some wild animal may struggle to find food and starve. All and all, zero sum.

This is all well and good for humans so long as the group size doesn't get to a point where theyre now competing for some finite resource.