Evolution of the eye

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Major stages in the eye's evolution.
Major stages in the eye's evolution.

The evolution of the eye has been a subject of significant study, as a distinctive example of a homologous organ present in a wide variety of taxa. The development of the eye is considered by some experts to be monophyletic; that is, all modern eyes, varied as they are, have their origins in a proto-eye believed to have evolved some 540 million years ago.[1][2][3] Much of the evolution is believed to have been concentrated in just a few million years: the first predator to gain true imaging would have initiated an "arms race".[citation needed] Prey animals and competing predators alike would be forced to rapidly match or exceed any such capabilities to survive. Hence multiple eye types and subtypes developed in parallel.[citation needed] However, other experts suggest that the sharing of genes instead only shows convenient pathways, with the eye evolving independently in some cases.[4][5]

Eyes in various animals show adaption to their requirements. For example, birds of prey have much greater visual acuity than humans and some, like diurnal birds of prey, can see ultraviolet light. The different forms of eye in, for example, vertebrates and mollusks are often cited as examples of parallel evolution. As far as the vertebrate/mollusk eye is concerned, intermediate, functioning stages have existed in nature, which is also an illustration of the many varieties and peculiarities of eye construction. In the monophyletic model, these variations are less illustrative of non-vertebrate types such as the arthropod (compound) eye, but as those eyes are simpler to begin with, there are fewer intermediate stages to find.

Anthropology prof. Lynne Isbell, UC Davis, suggests that the ability to spot dangerous snakes may have played a major role in the evolution of primate close-up vision with its eye for color, detail, movement and depth of field. She points out: "A snake is the only predator you really need to see close up. If it's a long way away it's not dangerous." Our ancestors and snakes with mouths big enough to eat them appeared at about the same time, while other serious predators evolved much later.[6]

Contents

[edit] History of research

The human eye, demonstrating the iris.
The human eye, demonstrating the iris.

Since 1802, the evolution of a structure as complex as the projecting eye by natural selection has been said to be difficult to explain.[7] Charles Darwin himself wrote, in his Origin of Species, that the evolution of the eye by natural selection at first glance seemed "absurd in the highest possible degree". However, he went on to explain that despite the difficulty in imagining it, it was perfectly feasible:

...if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.[8]

He suggested a gradation from "an optic nerve merely coated with pigment, and without any other mechanism" to "a moderately high stage of perfection", giving examples of extant intermediate grades of evolution.[8]

Darwin's suggestions were soon proven to be correct, and current research is investigating the genetic mechanisms responsible for eye development and evolution.

[edit] One origin or many?

It is a matter of debate whether the "eye" evolved once, or independently in many clades. The genetic machinery employed in eye development is common to all eyed organisms. This may mean that genes have coded for "eyes" in the first place. Alternatively, they may originally have fulfilled a different purpose, and later been co-opted for use in optical machinery.[9]

All light-sensitive organs rely on photoreceptor systems employing a family of proteins called opsins, which, by structural and sequence homology can be shown to be of common origin. Indeed, the seven sub-families of opsins existed in the common animal ancestor. Recent genetic discoveries have provided valuable evidence for the common ancestry of the eye, as the PAX6 gene has been recognized as a universal "master control" gene for production of eyes in species ranging from mice to humans to fruit flies.[10][11][12]

One hypothesis is that the sensory organs evolved before the brain did.[13] The rationale for this is that there is no need for an information-processing organ (brain) before there is information to process. As eyes and other sensory organs acquire more information, the brain evolves in parallel to process the information and transmit it to effector organs, such as muscles.

[edit] Rate of evolution

The fossil record suggests that eyes appeared during the lower Cambrian period (about 540 million years ago). This period saw a burst of apparently rapid evolution, dubbed the "Cambrian explosion". One of the many hypotheses for "causes" of this diversification (backed up by scant evidence) holds that the evolution of eyes initiated an arms race that caused a rapid spate of evolution.[citation needed] Earlier than this, organisms may have had use for light sensitivity, but not for fast locomotion and navigation by vision.

Since the fossil record, particularly of the Early Cambrian, is so poor, it is difficult to constrain the rate of eye evolution. Simple modelling, invoking nothing other than small mutations exposed to natural selection, demonstrates that a primitive optical sense organ could evolve into a complex human-like eye within under a million years.[14] David Berlinski, an intelligent design proponent, questioned the basis of the calculations,[15] and the author of the original paper refuted Berlinski's criticism.[16][17]

[edit] Stages of eye evolution

The stigma (2) of the euglena hides a light-sensitive spot.
The stigma (2) of the euglena hides a light-sensitive spot.

The earliest predecessors of the eye were photoreceptor proteins that sense light, found even in unicellular organisms, called "eyespots". Eyespots can only sense ambient brightness: they can distinguish light from dark, sufficient for photoperiodism and daily synchronization of circadian rhythms. They are insufficient for vision, as they can not distinguish shapes or determine the direction light is coming from. Eyespots are found in nearly all major animal groups, and are common among unicellular organisms, including euglena. The euglena's eyespot, called a stigma, is located at its anterior end. It is a small splotch of red pigment which shades a collection of light sensitive crystals. Together with the leading flagellum, the eyespot allows the organism to move in response to light, often toward the light to assist in photosynthesis,[18][19] and to predict day and night, the primary function of circadian rhythms.

[edit] Early eyes

The basic light-processing unit of eyes is the photoreceptor cell, a specialized cell consisting of two molecules in a membrane: the opsin, a light-sensitive protein, surrounding the chromophore, a pigment that distinguishes colors.

The planarium has "cup" eyespots that can slightly distinguish light direction.
The planarium has "cup" eyespots that can slightly distinguish light direction.

The multicellular eyepatch gradually depressed into a cup, which first granted the ability to discriminate brightness in directions, then in finer and finer directions as the pit deepened. While flat eyepatches were ineffective at determining the direction of light, as a beam of light would activate the exact same patch of photo-sensitive cells regardless of its direction, the "cup" shape of the pit eyes allowed limited directional differentiation by changing which cells the lights would hit depending upon its angle. Pit eyes, which had arisen by the Cambrian period, were seen in ancient snails, and are found in some snails and other invertebrates living today, such as planaria. Planaria can slightly differentiate the direction and intensity of light because of their cup-shaped, heavily-pigmented retina cells, which shield the light-sensitive cells from exposure in all directions except for the single opening for the light. However, this proto-eye is still much more useful for detecting the absence or presence of light than its direction; this gradually changes as the eye's pit deepens and the number of photoreceptive cells grows, allowing for increasingly precise visual information.[18]

When a photon is absorbed by the chromophore, a chemical reaction causes the photon's energy to be transduced into electrical energy and relayed, in higher animals, to the nervous system. These photoreceptor cells form part of the retina, a thin layer of cells that relays visual information,[20] as well as the light and daylength information needed by the circadian rhythm system, to the brain. However, some jellyfish, such as Cladonema, have elaborate eyes but no brain. Their eyes transmit a message directly to the muscles without the intermediate processing provided by a brain.[13]

During the Cambrian explosion, the development of the eye accelerated rapidly, with radical improvements in image-processing and detection of light direction.[21] As certain organisms benefited from the dramatic advantages given by full-fledged eyes, many other organisms were forced to evolve similarly advanced eyes in order to compete.[citation needed] As a result, the majority of major developments in eyes are thought to have occurred over the span of only a few million years. In the book In the Blink of an Eye, Andrew Parker discusses a theory that the evolution of the eye was the catalyst for the Cambrian Explosion. [22]

The primitive nautilus eye functions similarly to a pinhole camera.
The primitive nautilus eye functions similarly to a pinhole camera.

The "pinhole camera" eye was developed as the pit deepened into a cup, then a chamber. By reducing the size of the opening, the organism achieved true imaging, allowing for fine directional sensing and even some shape-sensing. Eyes of this nature are currently found in the nautilus. Lacking a cornea or lens, they provide poor resolution and dim imaging, but are still, for the purpose of vision, a major improvement over the early eyepatches.[23]

Overgrowths of transparent cells prevented contamination and parasitic infestation. The chamber contents, now segregated, could slowly specialize into a transparent humour, for optimizations such as colour filtering, higher refractive index, blocking of ultraviolet radiation, or the ability to operate in and out of water. The layer may, in certain classes, be related to the moulting of the organism's shell or skin.

It is likely that a key reason eyes specialize in detecting a specific, narrow range of wavelengths on the electromagnetic spectrum—the visible spectrum—is because the earliest species to develop photosensitivity were aquatic, and only two specific ranges of electromagnetic radiation can travel through water, the most significant of which[clarify] is visible light. This same light-filtering property of water also influenced the photosensitivity of plants.[24][25][26]

[edit] Lens formation and diversification

Light from a distant object and a near object being focused by changing the curvature of the lens.
Light from a distant object and a near object being focused by changing the curvature of the lens.

The transparent cells over the pinhole eye's aperture split into two layers, with liquid in between. The liquid originally served as a circulatory fluid for oxygen, nutrients, wastes, and immune functions, allowing greater total thickness and higher mechanical protection. In addition, multiple interfaces between solids and liquids increase optical power, allowing wider viewing angles and greater imaging resolution. Again, the division of layers may have originated with the shedding of skin; intracellular fluid may infill naturally depending on layer depth.

Note that this optical layout has not been found, nor is it expected to be found. Fossilization rarely preserves soft tissues, and even if it did, the new humour would almost certainly close as the remains desiccated, or as sediment overburden forced the layers together, making the fossilized eye resemble the previous layout.

Compound eye of Antarctic krill.
Compound eye of Antarctic krill.

Vertebrate lenses are composed of adapted epithelial cells which have high concentrations of the protein crystallin. The refractive index gradient which makes the lens useful is caused by the radial shift in crystallin concentration in different parts of the lens, rather than by the specific type of protein: it is not the presence of crystallin, but the relative distribution of it, that renders the lens useful.[27]

It is biologically difficult to maintain a transparent layer of cells. Deposition of transparent, nonliving, material eased the need for nutrient supply and waste removal. In trilobites, the material was calcite; in humans, the material is crystallin. A gap between tissue layers naturally forms a biconvex shape, which is optically and mechanically ideal for substances of normal refractive index. A biconvex lens confers not only optical resolution, but aperture and low-light ability, as resolution is now decoupled from hole size—which slowly increases again, free from the circulatory constraints.

Independently, a transparent layer and a nontransparent layer may split forward from the lens: a separate cornea and iris. (These may happen before or after crystal deposition, or not at all.) Separation of the forward layer again forms a humour, the aqueous humour. This increases refractive power and again eases circulatory problems. Formation of a nontransparent ring allows more blood vessels, more circulation, and larger eye sizes. This flap around the perimeter of the lens also masks optical imperfections, which are more common at lens edges. The need to mask lens imperfections gradually increases with lens curvature and power, overall lens and eye size, and the resolution and aperture needs of the organism, driven by hunting or survival requirements. This type is now functionally identical to the eye of most vertebrates, including humans.

[edit] Other developments

The differences and similarities between human (left) and octopus (right) eyes demonstrate both convergent evolution and a distant (pre-Cambrian) shared ancestry.
The differences and similarities between human (left) and octopus (right) eyes demonstrate both convergent evolution and a distant (pre-Cambrian) shared ancestry.
"Backward" Illumination of Retina

The retina may revert on itself, forming a double layer. The nerves and blood vessels can migrate to the middle, where they do not block light, or form a blind spot on the retina. This type is seen in squids, which live in the dim oceans. In cats, which hunt at night, the retina does not revert. Instead a second, reflective layer (the tapetum) forms behind the retina. Light which is not absorbed by the retina on the first pass may bounce back and be detected. As a predator, the cat simply accommodates blind spots with head and eye motion.

Color vision

The ability to see colors presents distinct selective advantages for species, such as being better able to recognize predators, food and mates. As opsin molecules were subtly fine-tuned to detect different wavelengths of light, at some point, color vision developed when photoreceptor cells developed multiple pigments.[20] As a chemical instead of mechanical adaptation, this may have occurred at any of the early stages of the eye's evolution, and the capability may have disappeared and reappeared, as organisms became predator or prey. Similarly, night and day vision emerged when receptors differentiated into rods and cones, respectively.

Focusing mechanism

Some species move the lens back and forth, some stretch the lens flatter. Another mechanism regulates focusing chemically and independently of these two, by controlling growth of the eye and maintaining focal length. Note that a focusing method is not a requirement. As photographers know, focal errors increase as f-number decreases. Thus, countless organisms with small eyes are active in direct sunlight and survive with no focus mechanism at all. As a species grows larger, or transitions to dimmer environments, a means of focusing need only appear gradually.

[edit] In creationism and intelligent design

Some creationists have claimed that only intelligent design could create a structure as complex as the eye, claiming that it is irreducibly complex.[28] It has even been described as evolutionary biologists' "greatest challenge as an example of superb 'irreducible complexity' in God's creation".[29] Richard Dawkins gives examples for the eye itself not being irreducible in complexity [23]; however, intelligent design advocate Michael Behe has contended that Dawkins' focus - which is primarily the reducibility of the eye's gross morphology - does not sufficiently (nor experimentally) explain the eye's governing biochemical complexity. [30]

[edit] External links

  • Myers, PZ (2007-12-21). Evolution of vertebrate eyes. Pharyngula. ScienceBlogs. Retrieved on 2007-12-23. “a review of Lamb TD, Collin SP, Pugh EN Jr. (2007) Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup. Nat Rev Neurosci 8(12):960-76.”

[edit] References

  1. ^ Halder, G., Callaerts, P. and Gehring, W.J. (1995). "New perspectives on eye evolution." Curr. Opin. Genet. Dev. 5 (pp. 602–609).
  2. ^ Halder, G., Callaerts, P. and Gehring, W.J. (1995). "Induction of ectopic eyes by targeted expression of the eyeless gene in Drosophila". Science 267 (pp. 1788–1792).
  3. ^ Tomarev, S.I., Callaerts, P., Kos, L., Zinovieva, R., Halder, G., Gehring, W., and Piatigorsky, J. (1997). "Squid Pax-6 and eye development." Proc. Natl. Acad. Sci. USA, 94 (pp. 2421–2426).
  4. ^ Kozmik, Z., Daube, M., Frei, E. et al. (2003). "Role of Pax Genes in Eye Evolution A Cnidarian PaxB Gene Uniting Pax2 and Pax6 Functions." Developmental Cell. Volume 5, issue 5 (pp. 773–785).
  5. ^ Land, M.F. and Nilsson, D.-E., Animal Eyes, Oxford University Press, Oxford (2002).
  6. ^ "Seeing The Serpent: Ability To Spot Venomous Snakes May Have Played Major Role In Primate Evolution" (July 2006). Science Daily. “Some primate groups less threatened by snakes show fewer signs of evolutionary pressure to evolve better vision. For example, the lemurs of Madagascar do not have any venomous snakes in their environment, and in evolutionary terms "have stayed where they are," Isbell said. In South America, monkeys arrived millions of years before venomous snakes, and show less specialization in their visual system compared with Old World monkeys and apes, which all have good vision, including color.” 
  7. ^ In 1802, William Paley claimed that the eye was a miracle of design.
  8. ^ a b Darwin, Charles (1859). On the Origin of Species. London: John Murray.
  9. ^ Nilsson, D.E. (1996). "Eye ancestry: old genes for new eyes". Curr. Biol 6: 39–42. doi:10.1016/S0960-9822(02)00417-7. 
  10. ^ Halder, G., Callaerts, P. and Gehring, W.J. (1995). "New perspectives on eye evolution." Curr. Opin. Genet. Dev. 5 (pp. 602 –609).
  11. ^ Halder, G., Callaerts, P. and Gehring, W.J. (1995). "Induction of ectopic eyes by targeted expression of the eyeless gene in Drosophila". Science 267 (pp. 1788–1792).
  12. ^ Tomarev, S.I., Callaerts, P., Kos, L., Zinovieva, R., Halder, G., Gehring, W., and Piatigorsky, J. (1997). "Squid PAX-6 and eye development." Proc. Natl. Acad. Sci. USA, 94 (pp. 2421–2426).
  13. ^ a b Gehring, W. J. (13 January 2005). "New Perspectives on Eye Development and the Evolution of Eyes and Photoreceptors" (Full text). Journal of Heredity 96 (3): 171–184. Oxford Journals. doi:10.1093/jhered/esi027. 
  14. ^ Nilsson D-E, Pelger S (1994). "A pessimistic estimate of the time required for an eye to evolve". Proc R Soc Lond B 256: 53-58. 
  15. ^ Berlinski, David (April 2001). "Commentary magazine". 
  16. ^ Nilsson, Dan-E.. "Beware of Pseudo-science: a response to David Berlinski's attack on my calculation of how long it takes for an eye to evolve.". 
  17. ^ "Evolution of the Eye" on PBS
  18. ^ a b Eye-Evolution?
  19. ^ Land, M.F. and Fernald, Russell D. (1992). "The evolution of eyes." Annu Rev Neurosci 15 (pp. 1–29).
  20. ^ a b Fernald, Russell D. (2001). The Evolution of Eyes: How Do Eyes Capture Photons? Karger Gazette 64: "The Eye in Focus".
  21. ^ Conway-Morris, S. (1998). The Crucible of Creation. Oxford: Oxford University Press.
  22. ^ Korthof, Gert (2003) In the Blink of an Eye review
  23. ^ a b Dawkins, Richard (1986). The Blind Watchmaker.
  24. ^ Fernald, Russell D. (2001). The Evolution of Eyes: Why Do We See What We See? Karger Gazette 64: "The Eye in Focus".
  25. ^ Fernald, Russell D. (1998). Aquatic Adaptations in Fish Eyes. New York, Springer.
  26. ^ Fernald, Russell D. (1997). " The evolution of eyes." Brain Behav Evol. 50 (pp. 253–259).
  27. ^ Fernald, Russell D. (2001). The Evolution of Eyes: Where Do Lenses Come From? Karger Gazette 64: "The Eye in Focus".
  28. ^ Michael Behe. Darwin's Black Box. 
  29. ^ Sarfati, Jonathan (2000). Argument: 'Irreducible complexity', from Refuting Evolution (Answers in Genesis).
  30. ^ Michael Behe. Darwin's Black Box. , p.36-39
  • Lamb, T. D., Collin, S. P., & Pugh, E. N., "Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup", Nature Reviews: Neuroscience, v. 8 no. 12 (December 2007) pp. 960-976 doi:10.1038/nrn2283
  • Land, M. F., & Nilsson, D-E, Animal Eyes, Oxford: Oxford University Press, 2002 ISBN 0-19-8509685 "The origin of vision", Chapter 1, pages 1-15
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