Talk:Evolution of the eye

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[edit] Unclear statement

Not sure what this means: It is biologically difficult to maintain a transparent layer of cells as sizes, therefore the thicknesses gradually increased. Graft 07:09, 22 June 2006 (UTC)

Neither am I. The "as sizes" part is particularly baffling. I took the line (and much of the rest of the article's end) from Eye#Evolution of eyes. I think that the last two sections both need some significant cleanup and expansion; I couldn't find as many good sources for them as for the early stages of eye development, and the lens-formation stages are tricky to explain in layman's terms, so I gave up for the time being. -Silence 07:19, 22 June 2006 (UTC)
Very nice work, by the way. Graft 13:32, 22 June 2006 (UTC)
Almost 2 years, and no one has figured out what the "as sizes" means. I'm removing it. --Hordaland (talk) 14:06, 27 April 2008 (UTC)

[edit] Evolution of lenses

Completely disagree with the statement "The development of the eye is considered by most experts to be monophyletic". On the contrary, most experts would probably say that it seems to have developed at least 40 times independently. Perhaps the writer means the vertebrate eye.

In response to the request by Silence, I put together the diagram of major stages in the evolution of the eye. However, I'm not 100% happy with one of the diagrams, specifically diagram E, which shows the development of lenses. Some of the diagrams I looked at for reference (specifically this one and this one) appear to state that lenses derived from the cellular humor inside the eye, whereas the article itself states they evolved by "splitting" from the cornea. The diagram is not totally clear in this respect, but I'd like to know which of these is the most accepted. Were the diagrams I referenced out of date? Is there a difference of opinion over how lenses evolved? Did both situations happen in parallel? I'd like to make sure the diagram is as clear and accurate as it can be. I already raised this on Silence's talk page, but decided to throw it open to wider discussion here. ~ Matticus78 12:23, 10 July 2006 (UTC)

[edit] Vision and Intelligent Design

Propose actually citing and quoting a key ID proponent on vision as follows:

Howard Glicksman, M.D. reviews the human body's complex function and its challenges to macroevolution. [1] He extensively reviews the complexity of vision. [2] [3] [4]

Dr. Glicksman challenges macroevolution to explain:

  • the extreme complexity and physiological interdependence of many parts of the eyeball
  • the absolute necessity of many specific biomolecules reacting in exactly the right order to allow for photoreceptor cells and other neurons to transmit nervous impulses to the brain
  • the presence of, not only an eyeball whose size is in the proper order to allow for focusing by the cornea and lens, but also a region in the retina (fovea) that is outfitted with the proper concentration of photoreceptor cells that are connected to the brain in a 1:1:1 fashion to allow for clear vision
  • that vision is dependent on a complex array of turned around, upside down, split-up, and overlapping messages, from over two million optic nerve fibers that course their way to the visual cortex causing a neuroexcitatory spatial pattern that is interpreted as sight
  • that scientists are blind to how the brain accomplishes the task of vision

[edit] References

  1. ^ Glicksman, Howard, (2004) Exercise Your Wonder
  2. ^ Glicksman (2004) Part IV: Vision Part 1–Parts of the Eye October 1, 2004
  3. ^ Glicksman (2004) Part V: Vision Part 2 –The Retina November 1, 2004
  4. ^ Glicksman (2004) Part VI: Vision Part 3 – What Does the Brain See? December 1, 2004

DLH 18:05, 13 July 2006 (UTC)

[edit] Delete Undocumented Assertion

Propose deleting the following section as an assertion without support. The reference to Miller does not appear to mention the eye or vision and appears irrelevant.

Although the eye remains a common and popular argument among laypeople, some intelligent design and creationism advocates have abandoned the eye as an example of "irreducible complexity" because of the relatively thorough understanding of its evolutionary origins biologists now have, instead relying more on mollecular and microscopic structures such as the flagella, though recent developments have shed significant light on these structures as well.[1]

DLH 18:09, 13 July 2006 (UTC)
That sentence, and especially the one immediately following it, seemed to fail a NPOV test. Both sentences discuss evolution vs. ID and creationism, and do not really bring new information to the question of if and how eyes developed through natural selection. It is revealing to see people referred to as "laypeople" of evolution, as if it itself were a religion.

(this is getting pretty tangential, but this is a correct usage of "laypeople" -- see [1] -- and it has nothing to do with religion, except perhaps by distant etymology Montyy0 22:01, 10 December 2006 (UTC))

[edit] Review article in Science

Perhaps you are already familiar with this or it will not be helpful, but I came across a good review article in Science last month: “Casting a Genetic Light on the Evolution of Eyes”. Perhaps those more familiar with this topic might find something useful to add to this article. — Knowledge Seeker দ 03:01, 25 October 2006 (UTC)

[edit] primitive eyes

Unicellular invertebrates?

The earliest predecessor of the eye was a simple patch of photosensitive cells, physically similar to receptor patches for taste and smell, called an "eyespot". Eyespots can only sense ambient brightness: they can distinguish light from dark, but can not distinguish shapes or determine the direction light is coming from. Some organisms covered the spot in transparent skin cells for protection. Eyepatches are found in nearly all major animal groups, and are common among lower invertebrates such as the unicellular euglena. The euglena's eyespot, called a stigma, is located at its anterior end, has a red pigment, and allows the organism to move in response to light, often to assist in photosynthesis.[14][15]

This whole paragraph is a bit of a mess, but I haven't read enough to know how best to fix it. However, since Euglena is unicellular, it is not an invertebrate nor can it have a "patch of photosensitive cells." Also, it is inconsistent in terminology between "eyespot" and "eyepatch," and it's got a very strange voice, i.e. "Some organisms covered the spot in transparent skin cells for protection," rather than, say, "In some animals, the eyespot is covered by transparent skin cells for protection" or something. On a more detailed level, though, there are apparently even phototaxic prokaryotes, so claiming Eugelena as the fundamental precursor appears to be wrong. In overviews in some journals, there appears to be even some debate as to whether the eye arose independently numerous times, and there is a lot of evidence that the opsin proteins were not originally photosensing, and it's hypothesized (but I don't know that it's widely accepted) that prokaryotic rhodopsin evolved in archaea as a primitve form of photosynthesis. [2] and [3] are interesting reading in this area, but the latter needs a Science subscription for the full article.

Montyy0 21:56, 10 December 2006 (UTC)

It's more than a year since Monty pointed out that a patch of photosensitive cells could hardly exist on a unicellular organism. Duh. So I've sort of fixed it. Someone who is more knowledgeable in the field may well be able to fix my fix, but please don't just revert to the original paragraph which included (probably unintentional!) nonsense. --Hordaland (talk) 12:49, 23 December 2007 (UTC)

Thanks! My next target is the Backwards retina business... my recollection is that the vertebrate eye is "inside out" in that the photoreceptors are further back than the layer of supporting neurons and blood vessels and such, and that the cephalopod eye is not, and as a result the vessels and nerves don't have to pass through a blind spot as in vertebrates. Although I've seen some claims that the backwards-ness is a feature rather than a bug, I haven't read anything that seemed convincing that it wasn't just an accident of history. Anyway, if this is being discussed (and the octopus vs human eye seem to be brought up a lot in convergent evolution and creationism vs evolution arguments) it would seem like a good idea to describe how their eyes are morphologically similar but developmentally quite different: I can't find the reference just now, but I believe the cephalopod eye develops as a pit, while the vertebrate eye is an extension of the brain, so developmentally the eyes arise from different tissues. Cephalopods can also see polarization, and one squid, the firefly squid, has some color vision. Cephalopod eyes are also rhabdomeric, and can move around the pigment to accommodate for light levels (as well as adjusting their pupils sizes.)

Montyy0 (talk) 06:16, 25 December 2007 (UTC)

Sounds very interesting, hope you follow up.
See also section Circadian rhythms at the bottom of this page. There's more here than meets the eye. (Oh, can you forgive me for that?) Photosensitivity must first have developed for photosynthesis. Thereafter, I strongly suspect that photosensitivity first developed for the sake of circadian rhythms rather than for the sake of vision.
PZ mentions these two uses here (link to pertinent blog entry for the sake of myself and others, as I'm sure you already have it bookmarked) and here, but doesn't get into the which-came-first of it:
PZ: We vertebrates use ciliary photoreceptors in the image-forming part of our eyes; we have rhabdomeric receptors, too, but they're used in a more general way to sense light and dark, and play a role in circadian rhythms.
(BTW I found the source of the nonsense about multicellular patches on unicellular organisms in the article references and have written to <support@thinkquest.org> to ask them to correct or explain it.)--Hordaland (talk) 13:32, 25 December 2007 (UTC)

[edit] Degradation/loss of eyes

Is it worth mentioning the opposite process, where the eyes of an organism are reduced in size, covered over, or disappear completely? The mole is one example given by Darwin. I'm not sure if the opposite process is relevant to the evolution of the eye, but it is enlightening to know the process of eye formation isn't always beneficial to an organism. Richard001 08:35, 27 December 2006 (UTC)

[edit] Paragraph needs context

Despite the precision and complexity of the eye, theoretical analysis of eye evolution, developed by Dan-Erik Nilsson and Susanne Pelger,[5] demonstrated that a primitive optical sense organ could evolve into a complex human-like eye within a reasonable period (less than a million years) simply through small mutations and natural selection.*Pro-intelligent design mathematician David Berlinski[6] criticized these findings, including a criticism that the work contained no computer simulations (something assumed by a number of scientists but disclaimed by the original authors), and criticisms of the scientific establishment in general.[7] The original authors and other scientists subsequently challenged Berlinski's criticisms.

There are a couple of things wrong with this paragraph, not the least that the part in italics is almost information-free, only inferring some dispute, or controversy. David Berlinski has not published in a peer-reviewed journal, and it is misleading to infer a scientific controversy can be made. The criticism regarding a computer simulation is directed toward errors in popular reports of their work, not the original paper. I will replace the above with the following:

In 1802, William Paley claimed that the eye was a miracle of design. Since then, it has often been claimed that the eye is too complex to have evolved in any reasonable time-frame. To examine this claim empirically, Dan-Erik Nilsson and Susanne Pelger demonstrated that a primitive optical sense organ could evolve into a complex human-like eye within a reasonable period (less than a million years) simply through small mutations and natural selection. This paper has not raised any scientific controversy. Pro-intelligent design writer David Berlinski[6] has criticized the findings in the public arena, questioning the basis of the calculations. The original authors and other scientists responded by addressing Berlinski's apparent misunderstandings, including a challenge to submit a paper of his own to a peer-reviewed journal.Trishm 06:58, 3 February 2007 (UTC)

[edit] And the brain?

Shouldn't there be a section explaining how the brain could simultaneously manage to evolve to process new information an eye might send? NigelCunningham 06:22, 8 May 2007 (UTC)

Hmmm, possibly. It might just be worth adding a sentence to note that, alongside the apparatus of the eye itself, supporting infrastructure (brain, eyelids, muscles) would also be co-evolving. No point having a prodigiously complex and expensive eye if the brain using it can't use it to work out up from down. The current article does tend to focus (!) on the eye in isolation (it's implicit that other systems are evolving alongside it, but making this clearer might help). Cheers, --Plumbago 07:57, 8 May 2007 (UTC)
One potentially interesting reference for the this would be Georg Streidter's book Principles of Brain Evolution, as he discusses the principles that lead to brain development across a great number of species, including things like the development of a laminar LGN in mammals, which is the primary way station of visual information from the retina. Edhubbard 16:01, 22 July 2007 (UTC)
I've added PZMyers' review of a new paper to External links. It suggests, as far as this non-scientist can tell, that the eye actually can be considered to be a part of the brain. Or perhaps that it originally was. I recommend the review, at any rate, and don't miss the link to an animation showing the development of the eye. --Hordaland (talk) 11:38, 23 December 2007 (UTC)

[edit] Eyes only evolved once?

Right now, the article says "The common origin of all animal eyes is now widely accepted as fact, based on the shared features of all eyes.".

In 1992, in response to a Stephen Jay Gould book, John Maynard Smith wrote:

"In Gould's 'replay from the Cambrian' experiment, I would predict that many animals would evolve eyes, because eyes have in fact evolved many times, in many kinds of animal." -- From "Taking a Chance on Evolution", New York Review of Books, May 14 1992, pages 234-236

So which is it? Did ALL eyes evolve from a common ancestor? Or only the eyes of MODERN animals, with some extinct lineages having evolved eyes independently? If eyes evolved from scratch numerous times in multiple lineages, this is a major, encyclopedia-worthy fact. Bueller 007 14:45, 22 July 2007 (UTC)

I think the answer depends, to a certain extent, by exactly what level of specificity we are talking about. For example, in Climbing Mount Improbable Richard Dawkins discusses the "40-Fold Path to Enlightenment", and points out that eyes have evolved in at least 40 different ways, including compound eyes, standard lens eyes like most mammals have, and a variety of other manifestations. However, recent work on the genetics that lead to the development of eyes in individuals note that the same "toolkit gene", Pax-6, is involved in the formation of everything from fly eyes to human eyes (see e.g., Endless Forms Most Beautiful by Sean B. Carroll for an accessible treatment). This would suggest that these different evolutionary pathways for eyes have evolved from a common genetic signal, most likely based on a simple light-sensing spot (without lenses and optics) that would still give an evolutionary advantage over creatures that did not have the ability to sense light and dark. It is on the basis of this shared geneteic basis, and shared features such as the similarities in the light-sensitive rhodopsin molecules, that it is generally argued that eyes (in general) only evolved once, even if the various forms seen today have diverged greatly from that common eye-spot ancestor. Edhubbard 15:58, 22 July 2007 (UTC)
Thanks. That was what I had thought about the evolution of the eye before I read the Maynard Smith quote. But as I recall, I've seen the "eyes have emerged many times" claim repeated in Daniel Dennett's "Darwin's Dangerous Idea", Dawkins' "The Selfish Gene" and some other sources as well. Dennett, for example, used it to strongly suggest that if we were to encounter an alien lifeform that had developed locomotion through a transparent medium, it would almost certainly have eyes, since that seems to be a universal "good trick" that natural selection has stumbled upon independently many times on earth. This seems more true to my interpretation of the Maynard Smith quote (which Dennett himself uses.)
If I'm correct (and I'm hardly an expert) the current evolutionary theory seems to place the original emergence of the ancestor of all modern eyes some time around or before the Cambrian explosion. So the question is, I guess, did some of the Cambrian fauna of now-extinct lineages also develop eyes independently (perhaps in order to compete with the ancestor of the modern eye)? Bueller 007 16:32, 27 July 2007 (UTC)
Hi Bueller, your comment inspired me to go back to my copy of Endless forms... and look at exactly what Carroll had to say, in the hopes that it might clarify things. He notes that Pax-6 is associated with eye (or eyespot) development in creatures from flatworms to flies to frogs to humans, so this is most likely a highly conserved toolkit gene. It's hard to get all the references, since he only uses sparse endnotes (it's a popular audience book, after all) but he notes that the first experiments showing that the first studies that recognized that three different genes, eyeless in flies, small eye in mice and Aniridia in humans, were all part of the same Pax-6 family were:
Quiring et al., (1994). Science, 265: 785-789. and
Halder, Callaerts and Gehring (1994). Science, 267: 1788-1792.
Carroll's explanation of the Halder et al. study is especially interesting, as he notes that when small eye (the mouse gene) was transplanted into unusual regions of the flies, so that "it was also turned on in weird places in the fly [like the wings or legs].... The result was the same as the experiment with the fly gene - fly tissues were induced to form eye structures. However, it is important to emphasize that the tissues formed were fly eye structures, not mouse eye structures. So while each gene had similar effects, the final form depended upon the context of the species of the experiment, not the origin of the gene." (p. 67, emphasis in original). I think this is a very nice resolution of the forty-fold vs. single-evolution discussion (and see above on the top of the talk page, too).
In the footnotes, Carroll also notes that Gould himself had a commentary on this work, published as:
Gould, S.J. (1994). Natural History, 103: 12-20.
so, Gould was aware of these findings, but note that this is about five years after the publication of his book on the Brugess Shale and the Cambrian Explosion. Dawkins probably should have said more, but I don't have my copy of Climbing Mount Improbable handy. In any case, when discussing these findings, Carroll includes in his notes a nice laudatory comment about exactly the chapter of Dawkins' book that we were discussing above. If you have a copy, I'd be interested in hearing if there is any mention of Pax-6.
Going further, Carroll discusses the early evolution of life, during the time of the Cambrian, and notes that "[B]ased on the content and similar roles of the shared genetic tool kit in protosomes and deuterosomes, we can confidently add that the common ancestor of bilaterians (an animal that Eddy De Robertis at UCLA has dubbed Urbilatera, meaning primitive bilaterian) had a tool kit of at least six of seven Hox genes, Pax-6, Distal-less, tinman and a few hundred more body-building genes." (p. 143).
So, based on this, the last common ancestor with most of the modern phylogenetic tree seems to already have had Pax-6. Based on this evidence, Carroll discusses what we can infer about the early evolution of eyes:
"Could Urbilatera have had eyes? Well, probably not the large pronounced eyes like those that we find on trilobites later in the Cambrian. Something that had large, complex eyes would probably have turned up by now in the fossil record. But, because the role of Pax-6 and other genes involved in eye development is shared in both major branches of bilaterians, we can deduce that Urbilatera probably had at least some kind of eyespot or light-sensing organ made up of photosensitive cells arranged in some geometry." (p. 144).
So, Carroll doesn't say that there was no other creatures that could have evolved eyes and gone extinct, but his line of reasoning certainly doesn't seem to leave much room for other creatures that we haven't detected. Of course, one difficulty with using the Pax-6 data to infer the presence of eyes in extinct species is that we have no way of looking to see whether Pax-6 would play a role in their development, since we have no way of observing the morphogenesis of an extinct animal. Sorry, this was longer than I intended, but hopefully there is some good material here for other editors to add to the article... or for me to do later, when I have more time. Edhubbard 20:10, 27 July 2007 (UTC)
Great. Thanks for the reply. Clarifies some things. It still hasn't explained the exact nature of the quotes, but your sources are newer than mine (except for Dennett, but he can hardly be expected to stay on top of the very cutting edge of evolutionary biology) so I suppose that Maynard Smith's claim may just represent the common assumption of his day, before the discovery of the common role of the PAX6 gene. Bueller 007 04:46, 28 July 2007 (UTC)

[edit] Circadian rhythms

It's troubling that such a long and interesting article totally ignores the importance of photosensitivity to circadian rhythms! (Except for the half sentence which I just added, simply to get a mention in there.) Photosensitivity may likely have evolved first for this purpose, while it's use in vision came along later. It's now accepted that "knowledge" of the daily light/dark cycle is the basis for all seasonal, tidal, etc. rhythms, and it is common to and necessary for (virtually) all organisms. I don't think an article about the evolution of the eye can be anywhere near complete, when this aspect is ignored; the present article as it stands should rather be entitled Evolution of vision. --Hordaland (talk) 13:03, 23 December 2007 (UTC)

[edit] The development of the eye is considered by most experts to be monophyletic?

Not my field, but the above sentence, I think, needs correcting - firstly, I really doubt any sort of meta-analysis has been done on what opinions are on this. Worse, the citations used to support the statement are inappropiate - three papers by the same group of authors is not conclusive on acceptance of their claims. Finally, papers exist that contradict their statement. E.g. [4] suggests at least two independent evolutions of the eye, despite the shared PAX6 gene. Indeed, I think this subject is deserving of a separate section in the article, to give a fuller discussion of its nuances. Probably I am an inappropiate person to write it, though. Also, lazy. --Fangz (talk) 14:50, 31 January 2008 (UTC)

[edit] Berlinski vs Nilsson

First, 'misgiving' is used in connection with future events, according to the online Merriam-Webster.[5].

Second, it is important who replied to Berlinski.

Third, Nilsson's reply (ref 14 and 15), are not in a peer reviewed journal (something that would justify a 'scientifically proven that B. was wrong), but on a blog. Northfox (talk) 09:20, 5 April 2008 (UTC)

Berlinski's comments were not in a peer-reviewed paper (because they have no scientific grounding). Therefore no response can be published in a scientific paper. It is important that this article does not read as if Berlinski's comments have any weight (they don't), but on the other hand I feel that they should be mentioned, as they were well publicised. Neither Berlinski nor Nilsson's reply are "reliable sources" per Wikipedia, so I'm not sure what the best way forwards is... Suggestions, anybody? Verisimilus T 12:52, 5 April 2008 (UTC)

[edit] Please check Darwin quote

Something wrong with the sentence:

"...if [...]; if further, [...]; and if [...], then the difficulty of believing that a perfect and complex eye could be formed by natural selection, [...], can hardly be considered real."

What can hardly be considered real? Is it the difficulty of believing which isn't real? Perhaps they said things in that way in his day, but it looks strange. --Hordaland (talk) 20:47, 19 April 2008 (UTC)

You've read that correctly. Smith609 Talk 17:00, 1 May 2008 (UTC)
Thanks! --Hordaland (talk) 18:06, 1 May 2008 (UTC)

[edit] Evolution? Really?

Hi, (sorry for the bad english)

I'll be straight, Evolution is a theory. Which i believe is incredibly wrong.

So, don't say "Evolution", say "Theory of Evolution".

For me, Evolution is one of the biggest lies ever made. It's main goal, is that people stop believing in God, and unfortunaly for some, i can prove it.

NOT Even a single Protein would have come by accident, there is a chance on 10^950 that a medium sized protein would "appear". —Preceding unsigned comment added by Solidus18 (talk • contribs) 16:31, 1 May 2008 (UTC)

Hi, please review the link I've just added to the top of this page. You may also wish to view the wikipedia article on Evolution. For what it's worth, you are quite right, no proteins came about by accident. They all evolved. Smith609 Talk 16:59, 1 May 2008 (UTC)

This doesn't seem to be the right place to debate an issue, but just let me point out that a theory of something-or-other is a theory about that something-or-other; and a theory of something is different from that something. The theory of flight is not flight, and the theory of the earth is not the earth. Rather like a map of downtown Chicago is not downtown Chicago; the rules of the road are not the road; the evolution of the eye is not the eye. Evolution is something that happens in the world of life as we know it, and the theory of evolution offers explanations about how that happens. TomS TDotO (talk) 13:00, 7 May 2008 (UTC)

[edit] Video from NCSE

Do you folks think that it would be appropriate to add a pointer to this recent video on the evolution of the eye from NCSE?

http://www.youtube.com/watch?v=fOtP7HEuDYA&eurl=http://www.expelledexposed.com/

TomS TDotO (talk) 12:34, 7 May 2008 (UTC)

[edit] New References

Hi, thanks for adding new references to this article. Do consider using the references to expand the article, or to back up existing facts. References that are not explicitly referred to in the article are better placed under a "Further reading" header. Thanks again for your contributions! Smith609 Talk 13:29, 8 May 2008 (UTC)