Eyebrows: What are They All About?

by Nathaniel Virgo

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.

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8 Comments to “Eyebrows: What are They All About?”

  1. Good point. But when you say they pass on information we’d like to keep to ourselves, it’s probably only that *sometimes* we’d like to keep it to ourselves – most of the time it probably works to the individual’s advantage to share information. Perhaps another possibility is that eyebrows came first as a primitive form of communication, before our brains had the ability to decide which people we’d like to share this information with and which we’d rather not, but on average it was probably beneficial to share information. (Again I’m just speculating without any evidence here)

    • Yes, that’s just as possible as my hypothesis really. Perhaps we just evolved a whole load of very specific communication mechanisms (including facial expressions and some vocalisations, such as laughter) before we were able to evolve generalised speech, and perhaps it’s just very hard to un-evolve the deep connections between our inner emotional states and our outward facial expressions.

  2. Incorrect. Have you ever had a cat in real life? They have eyebrows, as do dogs. They aren’t as clearly obvious, being generally the same color as the rest of the hair in the animals’ coats, but often sport prominent antennae-like sensor hairs. (Look up studies of how cats know what size holes they can get through, for one; both the regular long whiskers on a cat’s face, and the upper antennae type ones, serve to judge the size of the opening.) Plus, anyone paying attention to those animals will see that interest/emotional state can also be telegraphed via those feelers…. and in neither dogs nor cats do they serve to mop up sweat from their brows.

    • So are you saying that human eyebrows are used for sensing something? I suppose they might be, but it doesn’t seem that likely to me. Of course some animals do have sensory whiskers above their eyes, but I don’t think they’re the same thing as human eyebrows. Apes don’t have sensory whiskers in that position (AFAIK), which means the human eyebrow tuft evolved separately from the cat’s brow whiskers.

      I’ve known a great many cats in my time, but I can’t say I’ve ever noticed them using their brow whiskers for expression – they show their emotions in other ways. But then again, cross-species body language interpretation can be surprisingly tricky, so I’ll admit I could have missed it.

      Some dogs, on the other hand, do seem to move their brows around in quite an expressive way. Uniquely among the animals, dogs can detect where humans are looking (by following the whites of our eyes, like we do). If the evolutionary paths of dogs and humans were intertwined enough for dogs to be able to interpret such details of human facial communication, perhaps they’ve also evolved to mimic our eyebrow movement as well. (That’s just a wild hypothesis-in-the-dark.)

      In short, I guess I didn’t really want to say that humans are the only animal with facial expressions, just that the density of specific information-transmission mechanisms in the human face seems to be much higher than in the rest of the animal kingdom.

      • Re apes eyebrows: or they lost them and we didn’t. Would need more than one case to reasonably claim multiple separate evolutionary origins. Could find out pretty quickly if I wasn’t doing this from my phone.

        I would think that the difficulty in spotting the eyebrows of a cat would be a pretty good basis for the hypothesis that they don’t use them for communication like we do.

        Male nipples. Did anyone mention them?

  3. @Lucas (for some reason I can’t reply directly to your comment), a quick glance at this clade diagram ought to sort that one out: http://en.wikipedia.org/wiki/Primate#Historical_and_modern_terminology – chimps, gorillas, orang utans, gibbons, old world monkeys and new world monkeys would each have had to lose their eyebrows separately in order for that to work. Evolutionarily speaking, humans and apes are not two separate classes – we’re just one species of ape among many.

    • Oh, and to play devil’s advocate, it could be that cats are much better at perceiving other cats’ eyebrows than humans are. We could just lack a special cat-face-processing part of the brain. Whiskers in general are an important part of cat body language – my Mum once bought a scare-cat, which was a sort of metal cut-out of a cat, to stop the neighbour’s cat from stealing food. It looked like it was smoking a cigar, the cigar being an exaggerated representation of it’s whiskers. Apparently without that, cats will mostly just ignore it.

  4. Well, if all the others on the diagram (hence needing more examples) don’t have them it would seem like a separate evolution. Not convinced that is the case, of if having/not having eyebrows is the right way of looking at it.

    From looking at some monkey eyebows (they have them, they’re quite clear from the right angle) it seems that ‘not banging your head on stuff’ is a pretty good reason for our ancestors, especially with their more prominent supraorbital ridge. Then, we could have spandrelled them as communication tools more recently.

    As the for the second point: I have often notices cats whiskers, I’ve never noticed their eyebrows. I believe my sensory system to reflect that of a cats. I don’t think it is reasonable to expect the trend to reverse without a reason (which there most likely isn’t). Not saying they don’t use them, just it seems pretty unlikely.

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