Humans have a rather bizarre relationships structure in which multiple, monogamous families live together. Whilst there may well be an “alpha” male he does not monopolise the females, instead allowing his subordinates to live within and mate with those in his group.
This strategy is interesting for two main reasons; firstly because it is unique within the primate family. Whilst it is not uncommon for multiple family groups to live together they aren’t monogamous, freely breeding with anyone in the troupe. Thus the development of this behaviour in humans is a curiosity waiting to be explained.
Secondly because it is one of the reasons why Homo sapiens is so successful. Whilst there is little male competition within our species – even between different groups – our closest relatives, Pan troglodytes, is a lot less tolerant of other males and so inter-group encounters are almost always hostile. The extinction of several chimpanzee troupes is believed to have been caused by these conflicts.
Humans, being less volatile (I bet you never thought you’d hear our species being called that) are able to co-operate with others a lot easier and use this power to form stronger and better groups leading to the eventual formation of complex civilisation, society and all that good stuff.
Now, having established that this phenomena is interesting it makes sense that someone would like to study how it developed. But how? As regular readers will note there are issues with studying past behaviour, primarily the fact it doesn’t preserve very well. Or, in this case, at all.
However, human success isn’t the only thing that has been influenced by this trait. The human body has as well. Much of other primates’ sexual dimorphism stems from the fact that the males compete with each other.Homo sapiens on the other hand have lost a lot of this dimorphism.
Whilst there are still some rather distinct differences between males and females – a fact any who hasn’t spent their lives under a rock, at the bottom of a mine on Mars will know – there aren’t as many differences as say…between gorilla males and females*.
Gorilla males are considerably larger, more muscular and have bigger canines than their feminine counterparts because of all the male/male competition they have to do. Humans on the other hand have less of a difference, to the point where it is impossible to differentiate between the canines of the different genders.
These are physical traits that would be detectable on the skeleton. And skeletons preserve. So we must summon the archaeologists!
Figuring that less sexual dimorphism in “combative” traits = less male/male competition and less male/male competition = a more modern human style group organisation; some researchers had a look at the skeletons of various hominins and concluded that a relatively modern troupe structure was present by Sahelanthropus.
Which is rather surprising given that Sahelanthropus is arguably the first member of the hominin lineage (i.e. the first human ancestor since we split with the chimpanzee family). So maybe it is the chimp’s group structure which is the newer development? Well parsimony suggests that is not the case given how prevalent male/male competition is in other primates.
More “evolution” has to occur for a weak male/male competition common ancestor to then evolve into several strong male/male competition species than for a strong male/male competition common ancestor to evolve into a weak male/male species once. Thus the latter explanation is preferred.
So it would seem our lineage did evolve weak male/male competition and we did not just inherit it, we just evolved it quite early in our development. So the question is why? Luckily for us the researchers also ran some models to try and figure out why such a group system would develop.
Some long, confusing calculations later and they’re suggesting that the human way of doing things will evolve when having lots of males in the group is advantageous enough to the benefits to the alpha male of dominating all the women. This is because our system allows for more men to be present. So in what situations is having lots of men around preferable?
Risky ones! The more men you have the larger a territory you can control. The large a territory you control the more food you can gather. And having more food is an excellent way to offset risk. So, what risky situations might early hominins have encountered?
Well, it is believed that we diverged from the chimp lineage when Africa underwent some climate change which considerably altered the environment our ancestors inhabited, forcing us to follow a different evolutionary path to chimps (whose environment remained pretty much the same). And if a considerable alteration to your habitat isn’t risky then I don’t know what is!
Of course, whether monogamy had developed this early is a different story. All this data needs to explain it is limited male/male competition. Whilst being monogamous does result in this, so do a whole host of different things.
However, the researchers also identified a situation in which monogamy could develop – when the cost of female promiscuity is great. One such situation in which there is great cost to promiscuity is when there is a large chance of catching an infection and humans have a rather large collection of venereal disease when compared to other primates, which would mean would be such a cost within our species.
That said, the data about when such diseases developed is rather lacking and so creating a time line of the development of monogamy and answering these interesting questions is still beyond researchers. And lets not forget that these diseases might not be sufficient to hamper female promiscuity and there may yet be other unidentified factors at play.
So the take home message is really just that male/male competition was weak in early hominins, probably because climate change placed them in a risky environment where this strategy is advantageous.
And venereal disease may well be the foundation of all human society.
More research on the evolution of monogamy is discussed here.
|Nakahashi W, & Horiuchi S (2012). Evolution of ape and human mating systems. Journal of theoretical biology, 296, 56-64 PMID: 22155135|
*Fun fact of the day: One species of gorilla is called gorilla, making their taxonomic classification “Gorilla gorilla.” This species has a subspecies, also called gorilla; or to give them their full name Gorilla gorilla gorilla.
Also, comment if you read all the way to the end!