Humans are the only animal to have these weird little tufts of hair above their eyes. I find the question of why this is to be a surprisingly interesting one, worthy of a long and rambling post about human evolution.
The received wisdom is that the eyebrows evolved to stop sweat from dripping into the eyes. I want you to question this. In order for a feature to evolve, it has to give a selective advantage. In the case of sweat dripping into the eyes this is presumably because a great many early proto-humans, while being chased by sabre-toothed tigers on hot days, tripped over logs because sweat got in their eyes, and were eaten.
It’s not entirely implausible that this could have been a significant cause of death for hominins living in sub-Saharan Africa. But could it really have been significant enough to cause the evolution of an entirely new anatomical feature? The bottom line is that millions of individuals, maybe a lot more, would have had to die by having sweat drip into their eyes in order for evolution to select for eyebrows. Given that we humans can use our hands to wipe sweat from our foreheads, this whole story doesn’t seem very likely to me.
More importantly, this idea leaves a lot unexplained. We don’t just have little tufts of hair on our faces, we also have muscles whose only purpose is to move those tufts around. This gives a clue towards what I think is the real purpose of eyebrows: they’re all about communication.
It’s not just eyebrows. The human face is, in general, weird. Let’s take a moment to consider the faces of other mammals. Think about the face of a cat, a dog, a mouse, a horse, a bear, a kangaroo. Feel free to throw in a few of your own. And let’s not forget the faces of our closest relatives, the apes: think of a gorilla’s face, an orang utan’s, a chimpanzee’s. Then, in the context of these other faces, picture a human being.
The first thing that stands out is the lack of hair, pretty unusual among mammals. Then there’s that enormous bulging forehead (easy enough to explain, given the unusual size of the human brain) and the strangely flattened snout (which can also be explained for the same reason – it’s very hard to fit the human’s giant head through the birth canal, and the complications of childbirth are most definitely an evolutionarily significant cause of human mortality). Next is that odd region of long hair on the top of the head – also unusual, though horses have something similar. But then we get into the subtler unusual features: the eyebrows, the whites of the eyes, those weird inside-out lips – and, beneath the skin, a whole load of unique muscles that don’t exist elsewhere in the animal kingdom, and whose only purpose seems to be to move the skin of the face around, contorting it into various different shapes.
Did these features arise because they were needed for survival in the savannah? I think it’s fair to say the answer is no. It’s pretty impossible to imagine a scenario where someone gets eaten by a lion due to an inability to wrinkle the forehead.
But then, evolution doesn’t only proceed by survival of the fittest. In order to pass on your genes you need to do more than just survive. You also have to find a mate, which gives rise to so-called sexual selection. The peacock’s tail is an obvious example: he has it not because it helps him find food or avoid predators (on the contrary, it seems to be an enormous hindrance) but because peahens take it into account when deciding who to mate with. (The question of how this situation arose is a rather interesting one, but best left for another time.)
If you’re a human, you have to do even better than that in order to pass on your genes. Once you’ve found a mate and had a baby, you have to successfully enlist the assistance of society in bringing up the child. Human babies need a lot of time and resources, and unless you’re lucky enough to be the only family living in an extremely abundant environment, you’re not going to be able to provide all that without the help of a tribe. It’s this kind of selection, which might be termed social selection, that I suspect is behind the unusual features of the human face.
You see, a lot of the odd features of the human face have an interesting thing in common. They seem to be about passing on information – the whites of the eyes show where someone is looking, the lips make it obvious whether you’re smiling or frowning, and the eyebrows play an important role in expressing all sorts of emotions – but the information they pass on often seems to be something we’d rather keep to ourselves.
The whites of the eyes are the clearest example. If, like most animals, our eyes were all one dark colour, we’d find it a lot easier to stare at someone without them noticing. It’s almost impossible for a human to deceive another about where they’re looking. Our many facial muscles play a similar role, albeit more subtly: they make it very hard to lie. If you want to get away with saying one thing while meaning another, you have to learn to override all sorts of subconscious communication mechanisms whose very purpose seems to be to give the game away.
Why would this be? I’m not saying I have a definite answer – as much as anything it’s just an interesting observation – but one possible story might go something like this: at some point, some powerful evolutionary force started increasing the size of the human brain. (I don’t want to speculate on what this force was, exactly, but we know it was powerful because it was enough to counter the very real evolutionary pressure for smaller head sizes and consequently easier births). Whatever the reasons for its existence, our large brain gives us a very general form of intelligence. However, this can sometimes come at a social cost. Left to our own devices, as developing adolescents, we might come to the incorrect conclusion that our interests are best served by lying and cheating at every turn. But then society would turn against us and we’d be on our own when it came to trying to bring up a child. (Either that or we’d just get killed in a fight.) Because of this, evolution stepped in by giving us a whole series of specific mechanisms to prevent us from taking this path.
Or something like that. I’m not sure if I’ve got the details right, but it definitely seems that there’s some kind of biological conflict between our conscious ability to deceive and the many physiological mechanisms that make it difficult – and the evolutionary origin of this conflict is interesting to speculate about.