Modern humans are one of the most social animals. We also have one of the biggest brains. The social brain hypothesis argues this isn't a coincidence. It suggests that socialising is beneficial but intellectually demanding. This drove the evolution of large brains to help us with our social skills and really exploit the benefits of living in a group.
On the surface, this seems reasonable. It's easy to come up with a myriad of reasons why socialising might have been evolutionarily beneficial for our ancestors. However, few of these have been proven to lead to more children. Which a trait has to do if it is to evolve, or else the genes are lost forever. Such a relationship between socialising, sex, and evolution has been found in Macaques, baboons, and many other primate species.
But what about us? Humans might be following the pattern set out by other primates, or we could be picking our own route. Until recently data on the subject was scarce. However, new research on African pygmies indicates that socialising, sex, and BMI might all be linked in our species as well.
Walking the tightrope
Evolution is often simplified as survival of the fittest. However, the theory is actually bit more complicated than that. Whether a trait leads to more successful reproduction is more important than how "fit" it makes someone.
After all, the whole driving force of evolution is passing on genes. Whilst "fitness" and reproduction are often linked (and, in scientific circles, synonymous), this isn't always the case. Some traits can emerge without evolutions help, and some benefits might not offer enough of a reproductive advantage to evolve.
For example, you might think that having a nose is an advantage. It can sniff out smoke, rotten food, and other dangers. Sometimes it can also help hunt down good things too. Like bacon. However, growing and using this nose requires energy, brain power, and time. In order for it to continue to get "fitter, " these benefits must outweigh the costs.
And it turns out they don't. Over the past few million years, our nose has been getting worse. Whilst this might seem counterintuitive, it's actually the "fittest" outcome for us. It turns out having a great nose doesn't really help someone have more babies. So evolution is shifting resources away from the nose to something that does.
Our brain has to walk a similar tightrope. It's hugely costly, requiring a lot of time to grow and a lot of resources to use. In fact, about 1/4 of your daily caloric intake goes straight into your thinking organ. It's easy to think of benefits it could provide that might outweigh this cost. But, like the nose, unless these benefits can be linked to reproduction then they can't explain the evolution of the brain.
Socialising, sex, and BMI
One of the main drawbacks of our large brain is how long it takes to grow. This makes studying the link between traits, evolution, and reproduction tricky. You have to have a lot of patience. And research funding. Additionally, we're now living in a unique environment so even when we do start studying people for hundreds of years there's still some debate over how applicable these results are to our ancestors.
Many explanations for our big brain have sprung up in this vacuum of data. Darwin thought tool use might be why are brains evolved. Others suggest the brain might help us adapt to climate change. Or maybe to deal with heat. And then there's the social brain hypothesis which, as the name suggests, claims socialising might have been the key benefit.
So some researchers went to examine the BaYaka people (also sometimes called the Aka). They're a group of pygmies living the hunter-gatherer lifestyle in the Congo. The experiment was simple: they handed out some honey sticks and watched as they shared them around. Some were given sticks by several different people, accumulating more honey than average. These people were inferred to have a high "social wealth".
Earning this social wealth is hard. Being related to someone is no guarantee you get their honey, nor is having given them honey at a previous date. You need to do a lot of socialising to build up this rapport, but it's worth it. These socially wealthy people also had a higher rate of fertility and greater BMI. The latter may have been helped by all the honey the received, but is still important to consider. Having children puts a lot of stress on the body, so having those extra fat reserves can be a big advantage.
Conclusion
In short, this research establishes that socialising can lead to evolutionary success. Those with the social wealth have more babies, so any traits that help them socialise - like a big brain - will flourish. It seems strong evidence that the social brain hypothesis is correct. Assuming their brains are involved in socialising. And these relationships were present in the past.
Of course, that is a lot of assumptions. So as always, there are some big disclaimers about more work being done. Nevertheless, this is an important first step to finally figuring out the key advantage our brain gave our ancestors. But again, disclaimers are needed about that as well. It may not be the only advantage our brain is responsible for. Other explanations for our large brains - like tool use - weren't tested so can't be ruled ou. Evolution is a messy, multifaceted process so I would be surprised if being social was the only factor involved.
tl;dr: Evidence has been found that links socialising to evolutionary success, potentially explaining why our species got big brains.
References
Chaudhary, N., Salali, G.D., Thompson, J., Rey, A., Gerbault, P., Stevenson, E.G.J., Dyble, M., Page, A.E., Smith, D., Mace, R. and Vinicius, L., 2016. Competition for Cooperation: variability, benefits and heritability of relational wealth in hunter-gatherers. Scientific Reports, 6.
Dunbar, R.I., 2003. The social brain: mind, language, and society in evolutionary perspective. Annual Review of Anthropology, pp.163-181.
Schülke, O., Bhagavatula, J., Vigilant, L. and Ostner, J., 2010. Social bonds enhance reproductive success in male macaques. Current Biology, 20(24), pp.2207-2210.
Silk, J. B. et al. 2010. Strong and consistent social bonds enhance the longevity of female baboons. Curr. Biol.20, 1359-1361
