I don’t think anyone is arguing that they don’t exist. It’s just a question as to why they’re prominent in humanity, and not other animals.
That is the most horrible image list I think I have ever seen. Eww. Anyway, one thing it shows is that there actually aren’t many animals with sclera showing - most of the ones that are showing are because a human is holding their eyes open. Most of the exceptions seem to be dogs.
I think a lot of people forget (or don’t get) how natural selection works. Traits get selected for if they: 1) help you survive; or 2) Help you produce more viable offspring. Some traits just get lucky and spread because they occur in conjunction with actually useful traits, or because their bearers are aided by circumstances.
I can’t imagine more visible sclera having made the difference between life and death in our ancestors. Sexual selection might make sense – perhaps it was considered an attractive trait. Or maybe it had something to do with our increasing brain size, which changed the proportions of our skulls, showing more of the whites of our eyes as a side-effect.
There was a good thread on why are eyes/pupils round or not, and if not, in which orientation the oval/slit is.
i have no time now, but do a search keywords goat (pupil OR oval OR iris).
Also, if you or someone finds it, could you post it?
Horses roll their eyes to show the sclera to express anger or fear.
It’s mostly how wide the eyes open, not the actual size.
Note how humans have relatively over-developed muscles around the eyes that serve to widen the range of facial expressions available, while most other animals can only open and close them.
We also have tiny, mostly useless ear muscles, while other species have much more developed ones, used both for rotating their ears and for expressing emotion.
Really? Cuing off of another person’s eyes while otherwise motionless as an indication of interest seems like a very advantageous means of silent communication in a group hunting situation.
So the people who had small sclera and their families starved to death because they couldn’t hunt? Their companions couldn’t have figured out what to do if they couldn’t see the whites of one of the hunters’ eyes? Somehow wolves and lions, etc. manage to be pretty good group hunters despite their eyes.
Sorry, I just don’t see it making much of a difference that way. There are hand signals, calls, and just looking at the iris or head direction, all of which you could use as cues instead.
You have an overly-simplistic view of how natural selection works.
Evolution is about successful breeding, not life and death. And I imagine there’s all sorts of situations where silently indicating with your eyes where something is or that you are interested in something is useful. A woman smiling at a man while looking at him for example; she’s communicating that she’s interested in him, personally, and not the three other guys he’s standing next to. Or if I hold something up and the person I’m talking to glances at it, they don’t need to waste time saying they’ve seen it. Humans are a very communicative species, and this helps that communication work more smoothly.
Humans sweat much more than most other species I understand. We have more eccrine sweat glands than most.
…relative hairlessness (for a non-aquatic mammal, at least), bi-pedal gait, the famous opposable thumbs, continuous female availability for sex (anatomically, that is),…
That was enlightening, thank you.
Yeah, that was my point in post #24 – sexual selection makes more sense as the explanation.
I thought the sweating thing was an advantage in long-distance running (“persistence hunting”), at which we apparently are the best on the planet.
Col. Prescott: “Don’t fire until you see the whites of their eyes!”
Soldier: “When we can see the whites of* their eyes, they *can see the whites of our eyes! Or hadn’t you thought of that?”
What if the Redcoats at Bunker Hill had been suffering from pinkeye? 
It is. Also, running on 2 legs is more energy-efficient than on 4.
Humans have highly visible sclera because humans without highly visible sclera look really damn creepy and are unlikely to find anyone willing to mate with them.
Which is an interesting point. If you have ever had a pet cat, you can read a fair bit from them by looking at their ears, which would serve exactly the same purpose in hunting as reading eye movements.
While man’s best friend sometimes has a visiblesclera, our closest relative http://media4.s-nbcnews.com/j/msnbc/Components/Photos/061108/061108_chimphuman_hlg_2p.grid-6x2.jpg.
Isn’t the major genetic difference between human a chimp the fusion of chromosomes?
Chromosome fusion
Could this trait have been caused by the chromosome fusion and later been selected sexually?
