There’s the historian factor as well.

They’ve found that African Elephant populations are largely constrained by water availability. Creating artificial watering holes is helping restore elephant populations better than most other attempts.

But the matriarch is typically one of the oldest female members of the group, and elephants remember watering holes that they haven’t visited since they were young. During a drought they will check all of these secondary and tertiary water sources. If that elephant is killed by poachers, the herd may lose the last remaining record of water resources and suffer for it.

I also recall watching a documentary about a troupe of primates. They adopted a young male kicked out of another troupe. Nothing remarkable about him until, again, a drought year. Turns out not all knowledge of edible foods is instinctual. They discovered him eating a fruit none of the tribe had eaten before. When he didn’t die they all started eating it too.

So I think we underestimate the value of record keeping with respect to longevity and inter-group mixing. It’s not all genes and safety in numbers.

I don't think its that simple. If we look back far enough, it was more like the man/men hunted or gathered and women took care of kids, fire and cooking.

If I look at less distant ancestors, they all worked in the fields, and so did grandparents (who were not as old as these days when 15 was a good age to start bearing kids, so 35 years old granny was normal). So it again falls mostly on women. Grandparents, those still living, much less.

I find the grandparents hypothesis compelling at first glance but it sort of begs the question, why don't we live indefinitely in the first place? There are obvious answers, we make tradeoffs to improve performance early in life at the expense of long term function, but it doesn't seem like the reproductive benefits of caring for a grandchild that only shares a 1/4 of your DNA necessarily tips the scales of selection towards longevity. Especially when, in theory, men remain fertile their entire lives and thus there should have always been some selection for longer life spans. You would expect the reproductive benefits of a 70 year old caring for their own child might be at least comparable to 70 year old caring for a grandchild.