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. 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 evolved. The 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. 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.
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).
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.
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.
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. 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.
- 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 one, 8(11), e80753.
- 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.
- Ostner, J., Vigilant, L., Bhagavatula, J., Franz, M., & Schülke, O. (2013). Stable heterosexual associations in a promiscuous primate. Animal Behaviour
- Kramer, K. L. (2014). Why what juveniles do matters in the evolution of cooperative breeding. Human Nature, 25(1).