Which came first (eye evolution Q)

[PlainJain]

Stereoscopic vision or two eyes?

It seems that something as intricate as stereoscopic vision would have have came along a little further down the evolutionary line so to speak - there are still lots of organisms that don't have it. So it seem mighty handy that we had two eyes in case that came up. So why have two eyes if you don't yet have stereoscopic vision? Sure, two are better than one (in case you loose one) but then why wouldn't 3 be even better? I remember from the blind cave fish thread (sorry no link) that eyes are so "expensive" to have that they are selected against relatively quickly when not needed. So it seems early life forms would have been better off with just one receptor patch.Wiki (http://en.wikipedia.org/wiki/Eye) tells us:
The earliest eyes, called "eyespots", were simple patches of photoreceptor cells, physically similar to the receptor patches for taste and smell. These eyespots could only sense ambient brightness: they could distinguish light and dark, but not the direction of the lightsource.
Did 'we' need two of those (were there two of those)? None of this is my field of study so I could be way off on anything I've said here, but it seems odd to me that the two eye model developed before the ability to have stereoscopic vision was a possibility.

[Ferret Herder]

My guess is that due to the natural bilateral symmetry (http://en.wikipedia.org/wiki/Bilateral_symmetry#Bilateral_symmetry) in many/most organisms, two eyes/eye-like organs/eyespots came first.

[Valgard]

IANAB but it seems to me that having more eyes would be advantageous - wider field of vision (or at least some amount of light sensing) means you have a better chance of detecting a predator, that kind of thing. Binocular vision could come later.

[panache45]

How could stereoscopic vision have evolved before we had two eyes? We have lots of things in pairs, without having any "stereo" ability.

[PlainJain]

Haha OK, I guess that does sound silly now that you point it out. But I guess my question remains about why have two eye patch sensors if eyes are so evolutionarily expensive.

[Whack-a-Mole]

Haha OK, I guess that does sound silly now that you point it out. But I guess my question remains about why have two eye patch sensors if eyes are so evolutionarily expensive.

Seems like most eyes in the animal kingdom are on the sides of the head (as opposed to ours on the front).

Seems evolutionarily advantageous to be able to see predators/prey on both sides of you rather than just one. With just one you have your chances of spotting food/danger roughly in half from sideways facing eyes.

Put another way an expanded field of view is a distinct advantage over those with a limited field (all else being equal).

[DrFidelius]

In order to stay upright in the mud, having two eyespots works well. When the light is equal in both, you are upright and can snuffle through the mud efficiently. Or, if you happen to find yourself above the bottom, you can find the mud again by just swimming away from the side your eyes are on while keeping the light equal in both eyes.

A single light-sensitive patch on the midline would work as well, but then you would need to orient yourself so that the light is brightest, which requires more processing power than having two inputs get equal.

A single eye is also a potential failure point, as damage to the one eye would take away an organism's entire phototropic sense, while losing one of two eyes would only limit the critter's ability.

[Darwin's Finch]

I remember from the blind cave fish thread (sorry no link) that eyes are so "expensive" to have that they are selected against relatively quickly when not needed.

Unfortunately, that's a common "accepted fact" that simply isn't a fact. See, for example, this paper (http://jhered.oxfordjournals.org/cgi/content/abstract/96/3/185):

We conclude from these studies that eye degeneration in cavefish may be caused by adaptive evolution and pleiotropy.

[Saltire]

Fish and cephalopods, which probably developed eyes fairly early in the history of Earth life, generally have two. However, few have them situated in a way to allow binocular vision. Two eyes with widely divergent fields of view are very handy when watching for things that want to eat you. Many (probably most) 'prey animals' still lack binocular vision for this reason.

Binocular vision is handy for hunting, which is why many (probably most) predators have it. I believe it evolved long after the side-looking, non-binocular version.

[Hari Seldon]

I think that two eyes developed in order the extend the field of vision, an adaptation that we abandoned in order to gain stereo vision. That seems mainly an adaptation to arboreal or aerial life.

I just heard a science program that claimed that hammerheads have essentially 360 degree vision.

[ivn1188]

Scallops have about 60 eyes. It's unknown if they combine their field of vision, but most of the eyes have overlapping fields.

[Superfluous Parentheses]

Seems to me that having two even minimally functional eyes is in some cases better than one eye that works a little better. DrFidelius explained that better already.

Also: not every seeing animal has two eyes. 3 eyes (http://en.wikipedia.org/wiki/Parietal_eye), 4 eyes (http://en.wikipedia.org/wiki/Four-eyed_fish), 8 eyes (http://en.wikipedia.org/wiki/Spider#Sense_organs) - note that in the last case, there is a "main" pair of eyes capable of forming images, and the others mainly detect direction of incoming light/shadows.

[PlainJain]

Two eyes with widely divergent fields of view are very handy when watching for things that want to eat you.
True, but even better it would seem is to have that third eye in back to see what's trying to eat you. I guess bilateral symmetry is probably the answer and binocular vision is just one of those happy accidents.

Also thanks, Darwin's Finch, for that link.

[Silverstreak Wonder]

Yes, good question too is why so many beings have just two eyes, if it is so random a process? I think a bigger problem is going from those patches that were sensitive to light to suddenly here is a lens made of clear material (nothing else is clear) spaced just right to focus everything, how do you get that randomly? In ALL these different evolving creatures??

Why don't we see some creatures with lenses not clear and not the right shape? It is indeed like something "keeps track" in my book, otherwise what stops them from just going away as a random change too, if there is no "good" and "bad" but just random evolution with the fittest surviving? Why don't we see the mistake ones with eyes and ears that do not work? In fact where are some creatures still developing their ears and eyes and they do not work yet?

Is the claim evolution just stops suddenly, if so how does a random process know to stop, by the way? I think there is a lot wrong with evolution. The biggest being what would keep mistakes from just happening over and over forever? Why don't we see new organs just pop up, needed or not, in every so many thousand births of animals or people if this is how the creation process works?

[MrDibble]

Also: not every seeing animal has two eyes. 3 eyes (http://en.wikipedia.org/wiki/Parietal_eye), 4 eyes (http://en.wikipedia.org/wiki/Four-eyed_fish), 8 eyes (http://en.wikipedia.org/wiki/Spider#Sense_organs) - note that in the last case, there is a "main" pair of eyes capable of forming images, and the others mainly detect direction of incoming light/shadows.

You forgot 5 eyes (http://en.wikipedia.org/wiki/Opabinia). And I think Opabinia's a good example because it comes from a time when compound eyes had just evolved relatively recently. Clearly the models were still being played with (although it occured with lots of other animals with paired eyes)

[The Hamster King]

Yes, good question too is why so many beings have just two eyes, if it is so random a process?Bilateral symmetry. Most animals are mirrored left to right because it makes locomotion simpler and cuts down on the amount of information you need to describe the body. ("What you did on the right ... do it again on the left.") So we wind up with two of a lot of things just by default.

I think a bigger problem is going from those patches that were sensitive to light to suddenly here is a lens made of clear material (nothing else is clear) spaced just right to focus everything, how do you get that randomly?Having the lens focus light is a fairly late development in eye evolution. In most cases the lens started out as just a transparent film of thicker skin to protect the light-sensitive patches.

Why don't we see some creatures with lenses not clear and not the right shape?Um ... like cataracts, or near-sightedness?

It is indeed like something "keeps track" in my book, otherwise what stops them from just going away as a random change too, if there is no "good" and "bad" but just random evolution with the fittest surviving? Why don't we see the mistake ones with eyes and ears that do not work?Because not having working eyes tends to get you killed very quickly in most niches in the animal kingdom.

In fact where are some creatures still developing their ears and eyes and they do not work yet?Because the incremental stages DO work, just not as well as a "fully" evolved eye. A light sensitive patch of skin works better than no eye at all. A light sensitive patch of skin in a pit (which gives you directional information) works better than a flat patch. A pit with a clear cover to protect it works better than a pit with nothing. And on and on. It's not like creatures go through generation after generation with useless partial eyes and then -- BAM -- they have perfect vision. At each step along the way each little change is a tiny improvement over what came before.

Is the claim evolution just stops suddenly, if so how does a random process know to stop, by the way?Nope. Our eyes are still evolving.

The biggest being what would keep mistakes from just happening over and over forever?Bad mutations happen all the time. The creature usually winds up dying young or being out-competed for a mate, and so the "mistake" doesn't get passed on to the next generation.

Why don't we see new organs just pop up, needed or not, in every so many thousand births of animals or people if this is how the creation process works?Because that's not how the process works. A new organ doesn't just "pop up". It appears very, very gradually with tiny changes over time piling up until they add up to something big.

[Der Trihs]

Yes, good question too is why so many beings have just two eyes, if it is so random a process? Because their ancestors developed just two eyes and passed it on; there's not much advantage to developing one or more extra crude eyes when you already have two good ones. And as pointed out, quite a few animals have more than two.

I think a bigger problem is going from those patches that were sensitive to light to suddenly here is a lens made of clear material (nothing else is clear) spaced just right to focus everything, how do you get that randomly? In ALL these different evolving creatures?? First the animal develops a thins layer of tissue over the primitive eye for protection; then over time it becomes thicker and more transparent. Then over time you have various other random changes that are selected for or against; until you have the eye the modern animal is equipped with. Exactly which would depend on the type of eye; not all eyes even have lenses.

Why don't we see some creatures with lenses not clear and not the right shape? It is indeed like something "keeps track" in my book, otherwise what stops them from just going away as a random change too, if there is no "good" and "bad" but just random evolution with the fittest surviving? Because the ones with defective eyes don't survive. Evolution isn't random; natural selection is mindless, not random.

Why don't we see the mistake ones with eyes and ears that do not work? Because they die. And we do, on occasion see them first.

In fact where are some creatures still developing their ears and eyes and they do not work yet? That's not how it works. The primitive eyes work; they just don't work as well. and we see plenty of animals with various types of eyes with various levels of sophistication.

Is the claim evolution just stops suddenly, if so how does a random process know to stop, by the way? Evolution never stops.

I think there is a lot wrong with evolution. The biggest being what would keep mistakes from just happening over and over forever? Why don't we see new organs just pop up, needed or not, in every so many thousand births of animals or people if this is how the creation process works?Again, that's not how evolution works. The organism gradual changes generation by generation; organs don't "just pop up". And there are plenty of mistakes; the creature with such a mistake does not reproduce and the mistake isn't passed to the next generation.

[Punoqllads]

Unfortunately, that's a common "accepted fact" that simply isn't a fact. See, for example, this paper (http://jhered.oxfordjournals.org/cgi/content/abstract/96/3/185):
Geneticists are almost as silly as particle physicists:

"Hedgehog" signals
"Sonic hedgehog" signals
"Tiggy winkle hedgehog" signals

Reminds me that one of the candidate pair for the names of the quarks that are now called "top" and "bottom" were "truth" and "beauty", truth being the rarer of the pair, of course.

[Fubaya]

Yes, good question too is why so many beings have just two eyes, if it is so random a process? I think a bigger problem is going from those patches that were sensitive to light to suddenly here is a lens made of clear material (nothing else is clear) spaced just right to focus everything, how do you get that randomly? In ALL these different evolving creatures??

I've seen demonstrations where they take two pieces of clear membranes, which represent light sensitive patches, and pump water in between them. As they bulge out, the picture these "eyes" see goes from a basic light/dark blur to a perfectly crisp image. There is one demonstration about 2:45 into this video: http://www.youtube.com/watch?v=zI_oGu-2clE

[horsetech]

Geneticists are almost as silly as particle physicists:

"Hedgehog" signals
"Sonic hedgehog" signals
"Tiggy winkle hedgehog" signals

Reminds me that one of the candidate pair for the names of the quarks that are now called "top" and "bottom" were "truth" and "beauty", truth being the rarer of the pair, of course.

Then there are the transposons that have been named things such as bilbo, baggins, frogger, stalker, etc.

http://flybase.org/static_pages/lists/dmel_te.html

[njtt]

In order to stay upright in the mud, having two eyespots works well. When the light is equal in both, you are upright and can snuffle through the mud efficiently. Or, if you happen to find yourself above the bottom, you can find the mud again by just swimming away from the side your eyes are on while keeping the light equal in both eyes.

A single light-sensitive patch on the midline would work as well, but then you would need to orient yourself so that the light is brightest, which requires more processing power than having two inputs get equal.

A single eye is also a potential failure point, as damage to the one eye would take away an organism's entire phototropic sense, while losing one of two eyes would only limit the critter's ability.

Actually, finding, or avoiding, the light is not quite that simple. It appears you also have to spin round and round as you go http://www.nature.com/nature/journal/v456/n7220/full/nature07590.html

[DrFidelius]

Cool.

I was thinking of benthic proto-chordates, and didn't even consider pelagic organisms.

[md2000]

The story I read once - most animals have eyes on both sides of their heads - i.e. horses - so they can see predators approaching all around. The ones that relly need to concentrate and know what's near them and exactly where, find binocular / stereo vision useful. You see this in dogs or even moreso in cats, who have to jump and grab things. Or monkeys, and hence humans, who have to judge how far away that next branch is when they jump for it. So stereo evolves from panavision for animals where it is an advantage.

The evolution of a light-sensitive patch from simple to directional (in a hole) to protected and multi-point, to maeuverable, is an obvious evolution. There are still some primitive examples where it is not necessary.

As a note, evolution loves to do the accidental doubling-up trick. A lumpy simple organism may accidentally double-up its genes and become a segmented double organism. If this is an advantage (twice the limbs) it may happen again- this is why so many life forms are repeating segmented. Then, each segment may change and specialize for advantage. A lobster is multiple segments where some limbs are just little fins, others are big claws, others are legs; a millipede is mostly repeating leg segments. Our ribs are the same concept.

Our eyes have 2 types of light sensors, rods and cones. Rods are low-ight sensitive and give black-white ability (hence, no colors when it's twilight) While the cones have color sensitivity. The original cones doubled and then again so we have 3 types of cones, red green blue sensitive. SOme people acidentally don't develop one or more set (color blindness); some people, I read, accidentally develop a fourth set. not sure what the result there is, better color sensitivity? So in that regard, color sensitivity may still be evolving.

However, are color cues a survival / better breeding trait compared to earlier times? ("This meat looks green... better not eat...") If not, it's just irrelevant genetic drift.

[njtt]

However, are color cues a survival / better breeding trait compared to earlier times? ("This meat looks green... better not eat...") If not, it's just irrelevant genetic drift.

Good color vision is an important survival trait for monkeys who need to be able to spot the fruit amongst the leaves. For humans, perhaps not so much, but we got to keep it and we find uses for it.

Most primates (including humans) are trichromats (3 cone types), whereas other mammals are, at best, dichromats (just two cone types) and thus cannot distinguish colors that look quite different to non-color-blind humans.

Because women have 2 X chromosomes they carry two copies of genes for cone cell receptor proteins. In their retina,some of their cone cells random express the gene from from one X chromosome and some from the other. Normally this does not make any difference, because the genes are the same, but if one of them has mutated just enough to shift the spectral sensitivity of the cones that use it, she will have 4 cone types (although the new type, I think, is less sensitive overall). I believe there is evidence that this does happen, and that it it does allow these women to distinguish colors that look the same to most of us (I might be able to dig up a site if anyone cares). I doubt whether it is a selective advantage, though, especially as it means that some of their sons will get the mutated gene and not the normal one, leaving them with a form of color blindness. The mothers (and some of their daughters) gain a bit of color sensitivity, but the sons lose out.