Whites in eyes=advanced species?

I remember watching something on the Discovery Channel a few years ago, about how modern man came to be.

About 1.6 million years ago there was an early human-like creature roaming around which they claim was the first animal having noticeable whites in their eyes, making them more advanced.

How does having noticeable whites make a species more advanced? I would think the bigger the iris the better, it would be much easier to see at night at what not.

The idea is not that advanced species in general have whites to the eyes. Rather, human eyes specifically differ from those of our closest ape relatives in have the sclera (fibrous coating of the eyeball) less pigmented so that it stands out in contrast to the iris and pupil. In apes the sclera is dark so that it does not contrast with the iris and pupil.

One of the main hypotheses for the evolution of this trait is that it has to do with social communication, by making it easier to see where a human is looking.

See here:
Did evolution make our eyes stand out?

The sclera is white in LOTS of animals. My cats have white sclera that are clearly visible. So did our dog (remember the National Lampoon cover for “Buy This Issue or we’ll Shoot this dog” ? They didn’t have to draw in the whites of the dog’s eyes) . Cows have white sclera as well. I’ve dissected them.

And my point is that the white sclera ARE visible around the iris, especially when the eyeball rotates off dead center. I’ll grant that we humans probably have more visible white space in straight-ahead viewing. But I suspect that the white sclera has been an aid to signalling emotion and viewing direction in our ancestors WAY before we were simians.

By contrast, I note that sclera do not need to be white at all. Many rodents have completely black sclera, and it’s hard to distinguish the pupil. Their eyes look like little black BBs in their heads, and there’s obviously no directional or emotional cuing being indicated by them at all.

A slight hijack…

How do scientists know about eyeball color from 1.6 million years ago? Surely an eyeball would not survive 1.6 million years?

I’m not following the logic of the linked article. Following the gaze of another would be good for the ‘gaze follower’ not the ‘gazer’. Handy, but I fail to see this being selected for (in the sense of non-white sclera being selected against).

They’re probably talking about Homo erectus (or ergaster, as the African species is sometimes called). But there is no physical evidence that any of our ancestors had whites in their eyes like ours. We can speculate that they might have, but we have no way of knowing at this point.

However, our closest relatives, that is, other primates, have dark sclera. Therefore the white sclera of other mammals are not homologous, that is, they do not have the same origin through direct descent. They are evidently of independent origin, and thus convergent. Human white sclera originated from immediate ancestors which had dark sclera, thus a hypothesis for why this change took place is justified. White sclera in humans may or may not have the same function as those of other animals.

Why wouldn’t it be of advantage to the gazer to communicate intent to the another individual? It would assist others in cooperating with the gazer on joint tasks, and thus aid the gazer directly. Remember also that most of the gaze followers will be relatives or at least members of the same social group.

DO they have dark sclera? I was under the impression that they simply have less of the sclera visible.

Even if present-day apes do have dark sclera, this doesn’t prove that humans independenmtlt developed it – We could be the sole primates retaining an ancient trait when our relatives changed theirs.

We could be, but that would require a lot more divergence. It’s a lot less likely that all the other primates (all of whom branched off from our line at different points, remember) independently evolved dark sclera, that it is that we evolved white sclera.

It would be of advantage of course. But given how evolution works (to my layman brain) it doesn’t seem like enough of an advantage to keep gaze followers from propagating their genetic information.

Typing with your tail again? :wink:

So unlikely as to be unreasonable. Think about how many primate species there are. It would be like proposing that we are the only primate species to retain the primitive feature of hairlessness.* Would you agree that that is a reasonable hypothesis?

  • Yes, we’re not really hairless, but let’s just use that for shorthand to describe the distribution of our body hair.

It could be good for the gazer, genetically speaking, in several ways:
First, it could help the two cooperate for mutual advantage. That’s clearly good for the gazer.
Second, even if there’s no direct benefit to the gazer, if the gaze follower is related to the gazer, then helping the follower benefits the gazer’s genes, too. (Helping your children is generally good, genetically speaking).
And finally, even if there’s no genetic connection between the two, helping out someone who might help you out later (even if you don’t benefit immediately) is a good thing, and can be selected for. This is pretty clearly true, if you think about how many animals have warning cries.

If a civilization had a dark sclera, other civilizations would never fire on them. They would be unstoppable.

That’s what the article I linked to states. As far as I know, this is correct.

By far the most parsimonious explanation is that humans evolved it independently. The possibility that all other primate lineages evolved dark sclera at least a dozen times from an ancestor with light sclera is so astronomically unlikely that it can essentially dismissed out of hand, especially since other basal lineages believed to be related to primates (Archonta), including colugos, tree shrews, and possibly bats, as far as I know also have dark sclera.

Just a reminder, too, that we’re not the most “advanced species” on the planet. Cockroaches have at least as good a claim on the title as we do.

Interesting timing. I just popped back into this thread because I remembered no one had addressed that. :slight_smile:

Evolution doesn’t cause species to “advance” except in the sense of changing over time. Species tend to become better adapted to their environment, but the environment is constantly changing (over long periods of time), too.