What The Eyes Say

Compared to other primates, human muscles are rather weak. In hand to hand combat with a chimp, humans don’t stand a chance. It seems that because we are so good at using tools, we could avoid paying for expensive muscles. Similarly, compared to other animals, language lets humans talk more precisely, and talk about things not in our immediate view. So you might expect we’d get worse at non-language communication. You’d be wrong:

Humans are known to have the largest and most visible sclera – the “whites” of the eyes – of any species. This fact intrigues scientists, because it would seem actually to be a considerable hindrance: imagine, for example, the classic war movie scene where the soldier dresses in camouflage and smears his face with green and brown pigment – but can do nothing about this conspicuously white sclera, beaming bright against the jungle. There must be some reasons humans developed it, despite its obvious costs. In fact, the advantage of visible sclera – so goes the “cooperative eye hypothesis” – is precisely that it enables humans to see clearly, and from a distance, which direction other humans are looking. … Chimpanzees, gorillas, and bonobos – our nearest cousins – follow the direction of each other’s heads, whereas human infants follow the direction of each other’s eyes. (The Most Human Human, p. 39).

Note that most of what we learn via looking at each other’s eyes is hard to verifiably say via language. You might feel he is laughing at you with his eyes, but it will be hard to make that laugh the basis of a group response – others probably didn’t see his eyes at the right moment, or might interpret what they saw differently.

Language was a big innovation, but my homo hypocrites hypothesis is that we humans are now actually post-language in important ways. Language let us express and enforce social norms, but we’ve since developed powerful capacities to coordinate outside the scope of language, to evade those norms. The whites of our eyes seem a key part of that norm-evading capacity.

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  • michael vassar

    This is the first ‘homo hypocrites’ claim that I would agree with, mostly because I have a much lower prior against evolutionary physiology claims than against evolutionary psychology claims (and also because I expect hypocrisy to emerge by default without needing adaptive support). Unlike so many hypocritical behaviors, which I think can emerge from conditioning, this is a simple adaptation that could enable such emergence to take place more easily, and where it’s easy to see how it could be selected for and could have large fitness effects, which is key. Central is its simplicity. Large eye whites are simple and I would expect evolution to hit upon that solution.

  • jsalvatier

    Isn’t this group selection? I know it’s possible, but wouldn’t it have to be a remarkably strong advantage?

    • http://www.uweb.ucsb.edu/~criedel/ Jess Riedel

      It’s clearly an individual advantage to be able to read the eyes of others in your group. Once this becomes a widespread ability (however imperfect it maybe before the whites of the eyes develop), it seems plausible that it is in the watched individual’s interest to be able to give clearer (though still deniable) signals. Likewise, producing and understanding language needn’t rely on group selection effects. It’s usually beneficial both to understand and be understood.

      Of course, it’s conceivable that the advantage of being able to hide where one is looking would outweigh the benefit of clear signaling. Perhaps in humans, where intelligence allows more profound communication, the balance tilted one way (toward readable eyes) while in other primates it tilted the other (toward unreadable eyes).

  • dWj

    The point about infants seems particularly important here; perhaps this is related to the fact that our babies are born less well-formed than those of other primates. How does neural plasticity compare between us and other primate species, especially in the young? At least in humans, the model is largely (not to get quite too “blank slate” here), “throw an initially highly malleable creature into the world primed to learn from its social group”, and waiting to learn language before you can learn anything else is just terribly inefficient. (Especially if we are, as you say, post-language, anyway.)

  • Evan

    i think you may be reading a bit too much into this. couldnt an alternative explanation be that humans just shift their eyes more than other animals when looking at different things? with such large heads (due to large brains), moving our head to change our field of view may place a lot of stress on our neck. it would be a smaller burden to just move our eyes. over time, humans that were able to save energy while scanning the plains would be selected for. the ‘cost’ of having a visible sclera would be minimized by living in a group and having many pairs of eyes available to spot potential danger/predators

    you can see the inverse of this in creatures where high visual acuity is important. owls have such large eyes that they cannot move them at all and have developed the ability to twist their necks 180°

  • MPS

    Perhaps this is known by others but without knowing either way, I would have guessed that visible sclera — and things like blushing and maybe tears and such — that these things evolved in humans before they developed language.

    Post-language evolution could then simply not have been selective enough to revert. Or maybe they were good traits even after language developed to supercede some of their purposes. (And in this regard maybe they fit in with your hypothesis, even if they didn’t develop for that reason.)

  • http://hertzlinger.blogspot.com Joseph Hertzlinger

    I’m reminded of the following passage from The Hot Rock by Donald Westlake:

    It was Prosker’s eyes. The mouth laughed and said words and lulled everybody, but the eyes just hung back and watched and made no comment at all.

  • William Newman

    You write “It seems that because we are so good at using tools, we could avoid paying for expensive muscles.” Partly so, but it’s not purely a case of being outperformed in all aspects of crude performance. We have indeed lost expensive advantages in crude performance that chimps have. Mostly what we have gained to make up for it is indeed subtle soft stuff like brains and dexterity. Not entirely, however: we seem to have gained a few crude performance advantages, too. In particular, we can throw things faster than chimps can, which was a significant issue in combat up through Roman Empire levels of technology. My nonexpert understanding is that this advantage is in part the upside of a design tradeoff in muscle attachment points, increasing our maximum speed at the expense of chimp-like maximum force. And I don’t know whether a human can sprint as fast as a chimp, but I’m pretty sure a human can outrun a chimp over long distances. (And does the human running advantage increase if both contestants are required to carry five foot spears as they run? My guess is yes.)

    • Chris T

      And I don’t know whether a human can sprint as fast as a chimp, but I’m pretty sure a human can outrun a chimp over long distances.

      Humans have traded strength for endurance. We have very few peers in the animal kingdom when it comes to potential endurance.

      Fun fact: some African tribes used to capture and train cheetahs by constantly chasing them until the cheetah was too exhausted to continue running.

    • http://panglott.blogspot.com/ Panglott

      My intuition has been that chimps are stronger mostly because they still brachiate, so they need more power in their arms.

      Is chimp leg strength comparable to human leg strength?

  • Kevin Dick

    Being a devotee of hand-to-hand combat, I would challenge your claim that a human “wouldn’t stand a chance” against a chimp. Certainly, an untrained human wouldn’t. But I think I highly trained, competitively selected one would.

    • Internet Tough Chimp

      oooh oooh eeeeee ! !

    • Evan

      what about against a highly trained, competitively selected chimp? (just being devils advocate here…)

    • Robert Koslover

      Maybe you should fight this chimp?

  • http://twitter.com/ramonehanson Ramone Hanson

    Robin,

    How can you be so sure of the time course here? “Language let us express and enforce social norms, but we’ve since developed powerful capacities to coordinate outside the scope of language, to evade those norms. The whites of our eyes seem a key part of that norm-evading capacity.”

    Seems to me that we clearly could have developed large eyes at the same time as we were developing the neural circuits for language (e.g. Broca’s area). But I do agree with you on one thing: the eyes are the window to the soul. Love,

    Ramone

  • Constant

    Chimpanzees, gorillas, and bonobos – our nearest cousins – follow the direction of each other’s heads, whereas human infants follow the direction of each other’s eyes.

    I’ve done this experiment with my dog many times, after I first noticed it. My dog follows the direction of my head, not my eyes. The way I can tell is simple. My dog wags his tail when I look at him, stops wagging when I don’t. Except: he wags his tail when my face directly faces him, regardless of where my eyes are looking, and stops wagging when my face points away, even if my eyes are still following him.

    I only have the one dog, so I don’t know how well it generalizes.

  • Adam Ozimek

    Stupid question: how do you know a “humans don’t stand a chance” hand to hand against a chimp? Are you talking average human and average chimp, or most humans and average chimp? Or all humans and average chimp?

  • http://entitledtoanopinion.wordpress.com TGGP

    “others probably didn’t see his eyes at the right moment, or might interpret what they saw differently”
    How does the communicator selectively signal to some folks while ensuring that others don’t see in the right moment? With language we speak quietly in a particular direction or when others aren’t around. But as you yourself mentioned, people can tell the direction someone is looking from a decent way off.

  • arch1

    A game called “killer” encourages players to exploit just this ability.

    n people seated in a circle are each dealt a face-down card. Ace designates the “killer,” whose job is to “kill” n-2 players by winking at them undetected. Each non-killer’s job is to definitively catch the killer in the act, or to “die” (turn one’s card up, after a delay so as not to reveal the killer) if winked at.

    It is impressive and a bit eerie to be sitting in a circle of 8-10 players who are dropping like flies for no apparent reason, precisely because the killer is so good at sclera-watching.

    • http://entitledtoanopinion.wordpress.com TGGP

      That’s a good rebuttal of me, I haven’t played that game much but now people who really enjoy it.

  • Philo

    “By allowing anyone who wants to send signals to aliens, we risk hostile aliens destroying us all. Yet even though the cost to discourage such signals seems trivial . . . .” This is a strange hobby horse of yours. First, many of our activities produce signals as by-products; it would be prohibitively costly to prevent such signals. Second, aliens might interact with us to our mutual benefit. (One crucial question is: what are the probabilities of beneficial and of harmful interaction, respectively? The evidential basis for answering this question is awfully thin. Another question: how likely is it that we will encounter aliens of any kind? I would place this probability so low that the whole issue is practically unimportant.)

  • Chris Koresko

    My suspicion is that Evan is right: the simpler and more likely explanation for the large visible human sclera is that it’s just a side effect of having the ability to sweep a large field with the eyes.

    The human brain, due to its size, would likely suffer injury if the head were spun as fast as many animals do theirs. Note that the only animals with brains as large as ours have much more massive heads, which probably limit their maximum angular acceleration to safe values.

    The human lifestyle demands both high visual acuity, so compensating by evolving a large field of view (like a “fish-eye”) is probably not viable. One imagines that the best solution is the one we adopted. The only real downsides I can see off the top of my head (no pun intended) would be a limited maximum pupil size (so poor night vision) and the need for a long, flexible optic nerve.

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