The evolution of the family

Our ancestors began to share the cost of reproduction around the family, but why did this behaviour evolve?

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In humans the entire family helps raise the child. The amount each person contributes varies from culture to culture, but in no society does the mother raise their kids completely alone. This form of “co-operative breeding” by no means unique to us (birds are big proponents of family values), but it isn’t seen amongst any of the other apes. As such it likely evolved independently in our lineage[1]. But why? After all it’s evolutionarily beneficial for the male to have as many kids as possible; which is easiest to achieve through having as many partners as possible.

Last year I talked about 2 papers which identified reasons why the family might have evolvedThe first noted that if infanticide is common it is no longer a viable tactic for the man to swan off to find a hot new Australopithecus. He must stay to ensure his offspring are not murdered by rival males[2]. For a species that is beginning to use spears, child-murder is scarily common amongst chimpanzees; so if our ancestors had taken it to a more extreme level it may have driven males to become monogamous. The other studied Macaques (who are less likely to kill you with a spear) and found that males were more willing to engage in co-operative breeding if they could be sure that the children were their’s[3].

Juvenile chimps are quite good at taking care of themselves

Now 2 new papers have been published which have identified yet more scenarios in which co-operative breeding could have developed. The first modelled the relationship between mothers and their offspring. Nowadays we often think of juveniles as dependent on their parents, increasing the burden of the family. However, in apes juveniles tend to be self-sufficient so it may be the case that hominin juveniles were also capable of surviving independently. This means that the situation is flipped somewhat, and kids have the opportunity to help their parents (with more than just the dishes)[4].

These models showed that it would be very beneficial for both parties if they did so. If the mother is given assistant by the children that are old enough to fend for themselves then there isn’t as much pressure on them. The extra resources also mean that they can reproduce faster, as they regain their energy reserves quicker after each pregnancy. In fact, these models show that it is possible for older juveniles to almost completely pay for (foodwise) younger siblings without harming their own chance of survival. And this would be helpful to the juveniles since they’re related to their mothers, so helping them reproduce would ensure their own genes are being passed on. Evolution likes this. The author argues that this sort of co-operation amongst parents and offspring may have been the precursor to co-operative breeding[4].

The second paper was another model that examined how the cost of raising children may have prompted the development of co-operative breeding. It makes sense that if these costs are so high a single parent would have a low chance of success then the other parent (and maybe even family members) would have to help. So they simulating this scenario and found that, sure enough, the higher the cost of raising children the better it is to work together. Co-operative breeding spreads through the population and BANG, you have the reproduction strategy of modern humans[1].

This post was actually an elaborate excuse to show you this picture

This post was actually an elaborate excuse to show you this picture. CHIMP FAMILY!!!

Based on this the authors concluded that it was likely harsh environments that were responsible for the rise of co-operative breeding. If there is less food around it’s harder to find enough to keep your kids alive and working together becomes a necessity. Yet in their model they represented this by making the cost of raising children higher[1]. But what if that’s actually what happened. What if human children simply needed more resources to grow? This is actually something that happened. As our brains became bigger then the cost of growing them increased, we grew slower and were more vulnerable as babies. In short, the cost of raising a kid increased as the size of the human brain did.

So now we have four scenarios in which co-operative breeding arose: infanticide, males knowing who their children are, children helping and children being costly. As I said in the last post, there is no contradiction here. Evolution is a complex, mucky thing and many factors drive it. It’s likely the cast that most of these factors played a role in the evolution of the family, each contributing a little bit. The next stage is to look at how these forces interact with each other to see how much each contributed to co-operative breeding.


  1. Smaldino, P. E., Newson, L., Schank, J. C., & Richerson, P. J. (2013). Simulating the Evolution of the Human Family: Cooperative Breeding Increases in Harsh Environments. PloS one8(11), e80753.
  2. Opie, C., Atkinson, Q. D., Dunbar, R. I., & Shultz, S. (2013). Male infanticide leads to social monogamy in primates. Proceedings of the National Academy of Sciences.
  3. Ostner, J., Vigilant, L., Bhagavatula, J., Franz, M., & Schülke, O. (2013). Stable heterosexual associations in a promiscuous primate. Animal Behaviour
  4. Kramer, K. L. (2014). Why what juveniles do matters in the evolution of cooperative breeding. Human Nature25(1).

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10 thoughts on “The evolution of the family”

  1. john zande says:

    I saw a very interesting theory the other concerning the origin of sacrifice which i’d be keen to hear your opinion on. As far as I’m aware it was presented by a grad student so i’m not sure if the idea has ever been published. In brief: a predator enters the clans range, their anxiety levels increase. There’s going to be blood. They all know it. The predator strikes, a clan member is brought down. As horrific as this act is it is also cause for tremendous relief for those surving members. It wasn’t “them” who died. This package of relief/pleasure spurred on the preemptive behaviour of sacrifice so as to bypass the (predicted) anxiety by appeasing the danger.

    1. Adam Benton says:

      I’m not sure what the evolutionary component of all that would be. That we retained some cultural/genetic memory of “someone else die = good feelings” and tried to replicate it. Or maybe we developed a nervous system that responds pleasurably to survival, driving us to hunt down that adrenalin rush through weird things, including sacrifice?

      The former would need an awful lot of evidence to graduate beyond the “just so story” style evopsych speculation, and I’m not aware of any. As for the latter, I’m not sure why the whole predator/sacrifice thing would need to be linked. After all, adrenaline junkies get their fix in ways that adrenaline didn’t evolve to deal with, so why must sacrifice be triggering a reaction that evolved in a similar scenario? Couldn’t it be abstracting the whole thing, like a rollercoaster abstracts fear.

      1. john zande says:

        I don’t think they were talking genetic, rather cultural; an adapted behaviour which had socially reaffirming benefits.

        You’re right, though: it’s a desperately difficult thing to move out of the speculation bin.

        1. Adam Benton says:

          I think cultural explanations would run into similar problems as the latter. Going through rituals and/or traumatic/exciting experiences can be great for social bonding and cementing relationships; but why must this behaviour have had its start in a natural equivalent?

        2. john zande says:

          You don’t think the rather extreme nature of human sacrifice (later animal sacrifice in its stead) had a natural kick? It could be entirely cultural, no doubt about it, but given its greater (believed) prevalence in more primitive eras would, in my mind at least, perhaps indicate a environmental cue.

          Either way, it’s always fun to speculate.

        3. Adam Benton says:

          I think it may well have done, but I think it would’ve been something a bit closer to sacrifice, both temporally and similarity of the action. I don’t buy it being an abstraction of predation experiences retained for a significant period of time. It probably started quite soon after the kick (or at least, some iteration of sacrifice did), which was a bit closer to the behaviour. Maybe a battle or something would invoke similar feelings; particularly if you start executing captives afterwards.

          It seems a lot more plausible that the thrill of fighting, of sacrificing captives and maybe associating that murder with victory could develop into sacrifice.

          But still, that’s speculation. Which is probably my favourite bit of the whole thing. Always nice to exercise the imaginatorium.

        4. john zande says:

          I’m with you on that!

  2. Cynthia Echterling says:

    A few other human differences come to mind to make the story more complicated. With humans, unrelated females will help care for children — it takes a village. Grandparents as well as siblings caring for children. Like bonobos we aren’t limited to when we can have sex, which probably would give males an added incentive to come back after they began going off to scavenge or hunt. And a good reason to keep your main squeeze happy.

    1. Adam Benton says:

      I think additional forces and factors will be discovered that add to this story; and no doubt there will be a part 3 at some point examining a new set of papers that add some new pieces to the puzzle. However, I suspect it might be a bit simpler than you’re letting on. Whilst it might take a village to raise a child, what about when we still lived in troupes? Could a more basic, familial-only co-operative breeding have developed and then become more complex as our social lives became more advanced?

      The grandparent thing is an interesting topic I intend to write about at some point.

  3. Jim Birch says:

    “Could a more basic, familial-only co-operative breeding have developed and then become more complex as our social lives became more advanced?”

    This seems more like it. As genetic testing wasn’t available until recently fathers would have had to use association to determine which children to apply the new feed-and-protect urge to. As the web of social cooperation developed and expanded that association would apply to a wider circle of kids. It seems to me that things like the Convention on the Rights of the Child would draw their emotional push from this basic drive – with the “familial group” extended to the entire species – although there are clearly cultural narratives at work here too.

    Mothers appear to have some more older adaptions like the ability to discern their own children by smell or subtleties of their bleats and, prior to monogamous pairing, a much greater genetic investment in their own kids than fathers. (That’s probably why mothers know their who their children are hanging out with, which clothes belong to which one, various bodily function details, and so on, while fathers have just realized there are some smaller people in the house.) Despite this more primal link to their own kids, mothers are clearly able to extend the parentable group outwards to the wider social group and beyond, usually more than men. Coopertition is a marvel, ain’t it?!

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