Chimps are our closest living relatives. So it should come as no surprise that they're similar to us in many ways. However, they differ in one key regard: they mature a lot faster. After weaning, a young chimp could easily survive independently in the wider group. Despite this, the juvenile acts a lot like humans and sticks with their family.
This is despite the fact that their mother typically has another kid around this time; and will focus most of her attention on it. So why does the mother put up with it? She'll let the juvenile hang out with her and the new baby for up to five years more.
It's easy to think of many possible benefits mother and juvenile could get from hanging out for so long. However, science isn't just speculation. We have to go out and test it. So some researchers have set out to do just that, investigating why chimps keep their family around.
The family forming hypothesis
Raising kids can be a lot of work, even when they're quick-maturing kids like chimps. This simple fact formed the foundation of the hypothesis for why chimps kept a family around. If the older sibling could do some of the work, it would take a big load of the mother. Hence why she kept the older kid around.
And by "work" I don't mean sending the kid out to bring home the bacon. Like us, they're very social animals and a key part of growing up is socialising, playing and bonding. Again, just like us. If the older kid could do some of that playing and socialising with the new baby it could free up a lot of time for the mother do stuff like eat, sleep, and survive.
So researchers set out to test the idea. Did mothers who kept their older kids around have more "free" time?
Crucially, this increase needs to be relatively big. Having an older juvenile can also "cost" the mother time, so they need to provide a decent benefit to offset this. Mothers will carry young children as they move, so it doesn't slow them down much. But if they're travelling with an older juvenile they'll make the kid walk, slowing down the family's movement.
The reality
So, do mothers who keep older kids around benefit from it? Do they have more free time or resources they can invest in their younger kids? Spoiler: no. Researchers examined 26 years of chimp data from Gombe National Park, Tanzania, and failed to find any link between keeping older kids around and mothers being able to spend more time doing other stuff. They weren't even able to take care of the younger kids more.
However, this research did reveal something else interesting. The younger kids did seem to benefit from this arrangement. No qualitative difference was detected in their lives (although that may be because the research wasn't focused on them), but they did seem to have a higher chance of making to adulthood if they had an older sibling living with the family.
The researchers offer many possible reasons for this. Maybe the added socialisation helps them fare better in chimp groups. Maybe they serve as a good extra set of eyes for finding food or predators. Or perhaps just having more friendly faces around increases everyone's mental health. But again, since the research wasn't focused on the kids it's hard to say for sure.
There did seem to be one benefit to the mothers though. For whatever reason they seemed to recover from birth faster when an older kid was around. Again, since this wasn't the focus it's hard to say why. But for chimps, its clear there is some benefit to keeping the family around.
Human application
Whenever we learn something about chimps it's only a matter of time before we try and use it to understand human evolution. The success rate of this is debatable. However, it is worth noting early hominins would have had a similar rate of development as chimps. This means they would be faced with a similar dilemma after weaning: stick with the family or become more independent as part of the wider groups.
Since this research was unable to identify the exact reason for why chimps might stick around, it's hard to say whether early hominins would follow the same pattern. However, there are some interesting implications if they did.
Crucially, as you may have noticed, humans take a lot longer to mature; partly due to our big brains. We're nowhere near as independent as chimps are after weaning. If we were like chimps and already hanging out with our family for a long period, this might serve as a preadaptation for becoming dependent on that family for a long period. The foundational behaviour is already there.
So it turns out family might not just have been important for day to day survival, but been a crucial part of human evolution. Or not. That's the thing about speculation, it could be wrong.
References
Stanton, M.A., Lonsdorf, E.V., Pusey, A.E. and Murray, C.M., 2017. Do juveniles help or hinder? Influence of juvenile offspring on maternal behavior and reproductive outcomes in wild chimpanzees (Pan troglodytes). Journal of Human Evolution, 111, pp.152-162.
