The evolution of the eye is attractive to study, because the eye
distinctively exemplifies an analogous organ found in many animal
forms. Complex, image-forming eyes have evolved independently between
50 to 100 times.
Complex eyes appeared first within the few million years of the
Cambrian explosion. From before the Cambrian, no evidence of eyes has
survived, but diverse eyes are known from the
Burgess shale of the
Middle Cambrian, and from the slightly older Emu Bay Shale. Eyes
are adapted to the various requirements of their owners. They vary in
their visual acuity, the range of wavelengths they can detect, their
sensitivity in low light, their ability to detect motion or to resolve
objects, and whether they can discriminate colours.
1 History of research
2 Rate of evolution
3 One origin or many?
4 Stages of eye evolution
4.1 Early eyes
4.2 Lens formation and diversification
4.3 Other developments
4.3.1 Color vision
4.3.2 Polarization vision
4.3.3 Focusing mechanism
5 Evolutionary baggage
7 See also
9 Further reading
10 External links
History of research
The human eye, showing the iris
In 1802, philosopher
William Paley called it a miracle of "design".
Charles Darwin himself wrote in his Origin of Species, that the
evolution of the eye by natural selection seemed at first glance
"absurd in the highest possible degree". However, he went on that
despite the difficulty in imagining it, its evolution was perfectly
...if numerous gradations from a simple and imperfect eye to one
complex and perfect can be shown to exist, each grade being useful to
its possessor, as is certainly the case; if further, the eye ever
varies and the variations be inherited, as is likewise certainly the
case and if such variations should be useful to any 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, should not be considered as subversive
of the theory.
He suggested a stepwise evolution from "an optic nerve merely coated
with pigment, and without any other mechanism" to "a moderately high
stage of perfection", and gave examples of existing intermediate
steps. Darwin's suggestions were soon shown to be correct, and
current research is investigating the genetic mechanisms underlying
eye development and evolution.
Biologist D.E. Nilsson has independently theorized about four general
stages in the evolution of a vertebrate eye from a patch of
photoreceptors. Nilsson and S. Pelger estimated in a classical
paper how many generations are needed to evolve a complex eye in
vertebrates. Another researcher, G.C. Young, has used the fossil
record to infer evolutionary conclusions, based on the structure of
eye orbits and openings in fossilized skulls for blood vessels and
nerves to go through. All this adds to the growing amount of
evidence that supports Darwin's theory.
Rate of evolution
The first fossils of eyes found to date are from the lower Cambrian
period (about 540 million years ago). The lower
Cambrian had a
burst of apparently rapid evolution, called the "
One of the many hypotheses for "causes" of the
Cambrian explosion is
the "Light Switch" theory of Andrew Parker: It holds that the
evolution of eyes started an arms race that accelerated evolution.
Cambrian explosion, animals may have sensed light, but did
not use it for fast locomotion or navigation by vision.
The rate of eye evolution is difficult to estimate, because the fossil
record, particularly of the lower Cambrian, is poor. How fast a
circular patch of photoreceptor cells evolve into a fully functional
vertebrate eye has been estimated based on rates of mutation, relative
advantage to the organism, and natural selection. However, the time
needed for each state was consistently overestimated and the
generation time was set to one year, which is common in small animals.
Even with these pessimistic values, the vertebrate eye would still
evolve from a patch of photoreceptor cells in less than 364,000
One origin or many?
Whether the eye evolved once or many times depends on the definition
of an eye. All eyed animals share much of the genetic machinery for
eye development. This suggests that the ancestor of eyed animals had
some form of light-sensitive machinery – even if it was not a
dedicated optical organ. However, even photoreceptor cells may have
evolved more than once from molecularly similar chemoreceptor cells.
Probably, photoreceptor cells existed long before the Cambrian
explosion. Higher-level similarities – such as the use of the
protein crystallin in the independently derived cephalopod and
vertebrate lenses – reflect the co-option of a more fundamental
protein to a new function within the eye.
A shared trait common to all light-sensitive organs are opsins. Opsins
belong to a family of photo-sensitive proteins and fall into nine
groups, which already existed in the urbilaterian, the last common
ancestor of all bilateral symmetrical animals. Additionally, the
genetic toolkit for positioning eyes is shared by all animals: The
PAX6 gene controls where eyes develop in animals ranging from
octopuses to mice and fruit flies. Such high-level
genes are, by implication, much older than many of the structures that
they control today; they must originally have served a different
purpose, before they were co-opted for eye development.
Eyes and other sensory organs probably evolved before the brain: There
is no need for an information-processing organ (brain) before there is
information to process.
Stages of eye evolution
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 cannot
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, and to predict day and night, the primary function
of circadian rhythms. Visual pigments are located in the brains of
more complex organisms, and are thought to have a role in
synchronising spawning with lunar cycles. By detecting the subtle
changes in night-time illumination, organisms could synchronise the
release of sperm and eggs to maximise the probability of
Vision itself relies on a basic biochemistry which is common to all
eyes. However, how this biochemical toolkit is used to interpret an
organism's environment varies widely: eyes have a wide range of
structures and forms, all of which have evolved quite late relative to
the underlying proteins and molecules.
At a cellular level, there appear to be two main "designs" of eyes,
one possessed by the protostomes (molluscs, annelid worms and
arthropods), the other by the deuterostomes (chordates and
The functional unit of the eye is the photoreceptor cell, which
contains the opsin proteins and responds to light by initiating a
nerve impulse. The light sensitive opsins are borne on a hairy layer,
to maximise the surface area. The nature of these "hairs" differs,
with two basic forms underlying photoreceptor structure: microvilli
and cilia. In the eyes of protostomes, they are microvilli:
extensions or protrusions of the cellular membrane. But in the eyes of
deuterostomes, they are derived from cilia, which are separate
structures. However, outside the eyes an organism may use the
other type of photoreceptor cells, for instance the clamworm
Platynereis dumerilii uses microvilliar cells in the eyes but has
additionally deep brain ciliary photoreceptor cells. The actual
derivation may be more complicated, as some microvilli contain traces
of cilia — but other observations appear to support a fundamental
difference between protostomes and deuterostomes. These
considerations centre on the response of the cells to light – some
use sodium to cause the electric signal that will form a nerve
impulse, and others use potassium; further, protostomes on the whole
construct a signal by allowing more sodium to pass through their cell
walls, whereas deuterostomes allow less through.
This suggests that when the two lineages diverged in the Precambrian,
they had only very primitive light receptors, which developed into
more complex eyes independently.
The basic light-processing unit of eyes is the photoreceptor cell, a
specialized cell containing two types of molecules in a membrane: the
opsin, a light-sensitive protein, surrounding the chromophore, a
pigment that distinguishes colors. Groups of such cells are termed
"eyespots", and have evolved independently somewhere between 40 and 65
times. These eyespots permit animals to gain only a very basic sense
of the direction and intensity of light, but not enough to
discriminate an object from its surroundings.
Developing an optical system that can discriminate the direction of
light to within a few degrees is apparently much more difficult, and
only six of the thirty-some phyla[note 2] possess such a system.
However, these phyla account for 96% of living species.
The planarian has "cup" eyespots that can slightly distinguish light
These complex optical systems started out as 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 exactly the 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 the light's angle. Pit eyes, which had arisen
Cambrian period, were seen in ancient snails,[clarification
needed] 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.
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, including the light and day-length 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.
Cambrian explosion, the development of the eye accelerated
rapidly, with radical improvements in image-processing and detection
of light direction.
The primitive nautilus eye functions similarly to a pinhole camera.
After the photosensitive cell region invaginated, there came a point
when reducing the width of the light opening became more efficient at
increasing visual resolution than continued deepening of the cup.
By reducing the size of the opening, organisms 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
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. An example of this can be observed in
Onychophorans where the
cuticula of the shell continues to the cornea. The cornea is composed
of either one or two cuticular layers depending on how recently the
animal has moulted. Along with the lens and two humors, the cornea
is responsible for converging light and aiding the focusing of it on
the back of the retina. The cornea protects the eyeball while at the
same time accounting for approximately 2/3 of the eye’s total
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
wavelength ranges of electromagnetic radiation, blue and green visible
light, can travel through water. This same light-filtering property of
water also influenced the photosensitivity of plants.
Lens formation and diversification
Light from a distant object and a near object being focused by
changing the curvature of the lens
In a lensless eye, the light emanating from a distant point hits the
back of the eye with about the same size as the eye's aperture. With
the addition of a lens this incoming light is concentrated on a
smaller surface area, without reducing the overall intensity of the
stimulus. The focal length of an early lobopod with lens-containing
simple eyes focused the image behind the retina, so while no part of
the image could be brought into focus, the intensity of light allowed
the organism to see in deeper (and therefore darker) waters. A
subsequent increase of the lens's refractive index probably resulted
in an in-focus image being formed.
The development of the lens in camera-type eyes probably followed a
different trajectory. The transparent cells over a pinhole eye's
aperture split into two layers, with liquid in between.[citation
needed] 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
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
Vertebrate lenses are composed of adapted epithelial cells which have
high concentrations of the protein crystallin. These crystallins
belong to two major families, the α-crystallins and the
βγ-crystallins. Both were categories of proteins originally used for
other functions in organisms, but eventually were adapted for the sole
purpose of vision in animal eyes. In the embryo, the lens is
living tissue, but the cellular machinery is not transparent so must
be removed before the organism can see. Removing the machinery means
the lens is composed of dead cells, packed with crystallins. These
crystallins are special because they have the unique characteristics
required for transparency and function in the lens such as tight
packing, resistance to crystallization, and extreme longevity, as they
must survive for the entirety of the organism’s life. 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.
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.
Trilobites used calcite, a mineral
which today is known to be used for vision only in a single species of
brittle star. In other compound eyes[verification needed] and
camera eyes, the material is crystallin. A gap between tissue layers
naturally forms a biconvex shape, which is optically and mechanically
ideal for substances of normal[clarification needed] 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. Indeed, "the basic pattern
of all vertebrate eyes is similar."
This section needs additional citations for verification. Please help
improve this article by adding citations to reliable sources.
Unsourced material may be challenged and removed. (October 2016)
(Learn how and when to remove this template message)
Five classes of visual pigmentation are found in vertebrates. All but
one of these developed prior to the divergence of cyclometers and
fish. Various adaptations within these five classes give rise to
suitable eyes depending on the spectrum encountered. As light travels
through water, longer wavelengths, such as reds and yellows, are
absorbed more quickly than the shorter wavelengths of the greens and
blues. This can create a gradient of light types as the depth of water
increases. The visual receptors in fish are more sensitive to the
range of light present in their habitat level. However, this
phenomenon does not occur in land environments, creating little
variation in pigment sensitivities among terrestrial vertebrates. The
homogeneous nature of the pigment sensitivities directly contributes
to the significant presence of communication colors. This presents
distinct selective advantages, such as better recognition of
predators, food, and mates. Indeed, it is thought[by whom?] that
simple sensory-neural mechanisms may selectively control general
behavior patterns, such as escape, foraging, and hiding. Many examples
of wavelength-specific behavior patterns have been identified, in two
primary groups: less than 450 nm, associated with natural light
sources, and greater than 450 nm, associated with reflected light
sources. As opsin molecules were subtly fine-tuned to detect
different wavelengths of light, at some point color vision developed
when photo-receptor cells developed multiple pigments. As a
chemical adaptation rather than a mechanical one, 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.[citation
Evolution of color vision
As discussed earlier, the properties of light under water differ from
those in air. One example of this is the polarization of light.
Polarization is the organization of originally disordered light, from
the sun, into linear arrangements. This occurs when light passes
through slit like filters, as well as when passing into a new medium.
Sensitivity to polarized light is especially useful for organisms
whose habitats are located more than a few meters under water. In this
environment, color vision is less dependable, and therefore a weaker
selective factor. While most photoreceptors have the ability to
distinguish partially polarized light, terrestrial vertebrates’
membranes are orientated perpendicularly, such that they are
insensitive to polarized light. However, some fish can discern
polarized light, demonstrating that they possess some linear
photoreceptors. Additionally, cuttlefish are capable of perceiving the
polarization of light with high visual fidelity, although they appear
to lack any significant capacity for color differentiation. Like
color vision, sensitivity to polarization can aid in an organism's
ability to differentiate surrounding objects and individuals. Because
of the marginal reflective interference of polarized light, it is
often used for orientation and navigation, as well as distinguishing
concealed objects, such as disguised prey.
By utilizing the iris sphincter muscle, 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. In
addition, the pupil shape can be used to predict the focal system
being utilized. A slit pupil can indicate the common multifocal
system, while a circular pupil usually specifies a monofocal system.
When using a circular form, the pupil will constrict under bright
light, increasing the focal length, and will dilate when dark in order
to decrease the depth of focus. Note that a focusing method is not
a requirement. As photographers know, focal errors increase as
aperture increases. 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.
Prey generally have eyes on the sides of their head so to have a
larger field of view, from which to avoid predators. Predators,
however, have eyes in front of their head in order to have better
Flatfish are predators which lie on their
side on the bottom, and have eyes placed asymmetrically on the same
side of the head. A transitional fossil from the common symmetric
position is Amphistium.
Vertebrates and octopodes developed the camera eye independently. In
the vertebrate version the nerve fibers pass in front of the retina,
and there is a blind spot where the nerves pass through the retina. In
the vertebrate example, 4 represents the blind spot, which is notably
absent from the octopus eye. In both vertebrates and octopodes, 1
represents the retina, 2 represents the nerve fibers, and 3 represents
the optic nerve.
Main article: Evolutionary baggage
The eyes of many animals record their evolutionary history in their
contemporary anatomy. The vertebrate eye, for instance, is built
"backwards and upside down", requiring "photons of light to travel
through the cornea, lens, aqueous fluid, blood vessels, ganglion
cells, amacrine cells, horizontal cells, and bipolar cells before they
reach the light-sensitive rods and cones that transduce the light
signal into neural impulses, which are then sent to the visual cortex
at the back of the brain for processing into meaningful patterns."
While such a construct has some drawbacks, it also allows the outer
retina of the vertebrates to sustain higher metabolic activities as
compared to the non-inverted design. It also allowed for the
evolution of the choroid layer, including the retinal pigment
epithelial (RPE) cells, which play an important role in protecting the
photoreceptive cells from photo-oxidative damage.
The camera eyes of cephalopods, in contrast, are constructed the
"right way out", with the nerves attached to the rear of the retina.
This means that they do not have a blind spot. This difference may be
accounted for by the origins of eyes; in cephalopods they develop as
an invagination of the head surface whereas in vertebrates they
originate as an extension of the brain.
^ David Berlinski, an intelligent design proponent, questioned the
basis of the calculations, and the author of the original paper
refuted Berlinski's criticism.
Berlinski, David (April 2001). "Commentary magazine".
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
Evolution of the Eye" on PBS
^ The precise number varies from author to author.
^ Land, M.F. and Nilsson, D.-E., Animal Eyes, Oxford University Press,
^ Lee, M. S. Y.; Jago, J. B.; Garcia-Bellido, D. C.; Edgecombe, G. E.;
Gehling, J. G; Paterson, J. R. (2011). "Modern optics in exceptionally
preserved eyes of Early
Cambrian arthropods from Australia". Nature.
474: 631–634. doi:10.1038/nature10097. PMID 21720369.
^ a b Darwin, Charles (1859). On the Origin of Species. London: John
^ Gehring WJ (2005). "New perspectives on eye development and the
evolution of eyes and photoreceptors". J. Hered. 96 (3): 171–84.
doi:10.1093/jhered/esi027. PMID 15653558.
^ Nilsson, D.-E. (2013). "
Eye evolution and its functional basis".
Visual Neuroscience. 30: 5–20. doi:10.1017/s0952523813000035.
^ a b Nilsson, D.-E.; Pelger, S. (1994). "A pessimistic estimate of
the time required for an eye to evolve". Proceedings of the Royal
Society B: Biological Sciences. 256: 53–58.
doi:10.1098/rspb.1994.0048. PMID 8008757.
^ Young, G. C. (2008). "Early evolution of the vertebrate eye –
fossil evidence". Evo Edu Outreach. 1: 427–438.
^ Parker, A. R. (2009). "On the origin of optics". Optics & Laser
Technology. 43 (2): 323–329. Bibcode:2011OptLT..43..323P.
^ Parker, Andrew (2003). In the Blink of an Eye: How Vision Sparked
the Big Bang of Evolution. Cambridge, MA: Perseus Pub.
^ a b Nilsson, D-E; Pelger S (1994). "A pessimistic estimate of the
time required for an eye to evolve". Proceedings of the Royal Society
B. 256 (1345): 53–58. doi:10.1098/rspb.1994.0048.
^ Nilsson, D. E. (1996). "
Eye ancestry: old genes for new eyes".
Current Biology. 6 (1): 39–42. doi:10.1016/S0960-9822(02)00417-7.
^ Zinovieva, R.; Piatigorsky, J.; Tomarev, S. I. (1999).
"O-Crystallin, arginine kinase and ferritin from the octopus lens".
Biochimica et Biophysica Acta (BBA) - Protein Structure and Molecular
Enzymology. 1431 (2): 512–517.
^ a b Scotland, R. W. (2010). "Deep homology: A view from
systematics". BioEssays. 32 (5): 438–449.
doi:10.1002/bies.200900175. PMID 20394064.
^ Ramirez, MD; Pairett, AN; Pankey, MS; Serb, JM; Speiser, DI;
Swafford, AJ; Oakley, TH (26 October 2016). "The last common ancestor
of most bilaterian animals possessed at least 9 opsins". Genome
Biology and Evolution: evw248. doi:10.1093/gbe/evw248.
^ Yoshida, Masa-aki; Yura, Kei; Ogura, Atsushi (5 March 2014).
Cephalopod eye evolution was modulated by the acquisition of Pax-6
splicing variants". Scientific Reports. nature.com. 4.
PMC 3942700 . PMID 24594543. Retrieved June 30,
^ Halder, G.; Callaerts, P.; Gehring, W. J. (1995). "New perspectives
on eye evolution". Current Opinion in Genetics & Development. 5
(5): 602–609. doi:10.1016/0959-437X(95)80029-8.
^ Halder, G.; Callaerts, P.; Gehring, W. (1995). "Induction of ectopic
eyes by targeted expression of the eyeless gene in Drosophila".
Science. 267 (5205): 1788–92. Bibcode:1995Sci...267.1788H.
doi:10.1126/science.7892602. PMID 7892602.
^ Tomarev, S. I.; Callaerts, P.; Kos, L.; Zinovieva, R.; Halder, G.;
Gehring, W.; Piatigorsky, J. (1997). "Squid Pax-6 and eye
development". Proceedings of the National Academy of Sciences of the
United States of America. 94 (6): 2421–2426.
PMC 20103 . PMID 9122210.
^ 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. Oxford Journals. 96 (3): 171–184.
doi:10.1093/jhered/esi027. PMID 15653558. Retrieved
^ a b c d e f g h M F Land; R D Fernald (1992). "The
Eyes". Annual Review of Neuroscience. 15: 1–29.
doi:10.1146/annurev.ne.15.030192.000245. PMID 1575438.
^ Autrum, H (1979). "Introduction". In H. Autrum. Comparative
Evolution of Vision in Invertebrates- A: Invertebrate
Photoreceptors. Handbook of Sensory Physiology. VII/6A. New York:
Springer-Verlag. pp. 6–9. ISBN 3-540-08837-7.
^ Arendt, D.; Tessmar-Raible, K.; Snyman, H.; Dorresteijn, A.W.;
Wittbrodt, J. (29 October 2004). "Ciliary Photoreceptors with a
Opsin in an Invertebrate Brain". Science. 306 (5697):
869–871. doi:10.1126/science.1099955. PMID 15514158.
^ Eye-Evolution? Archived 15 September 2012 at the Wayback Machine.
^ a b Fernald, Russell D. (2001). The
Evolution of Eyes: How Do Eyes
Capture Photons? Karger Gazette 64: "The
Eye in Focus".
^ Conway-Morris, S. (1998). The Crucible of Creation. Oxford: Oxford
^ Dawkins, Richard (1986). The Blind Watchmaker.
^ a b c Schoenemann, B.; Liu, J. N.; Shu, D. G.; Han, J.; Zhang, Z. F.
(2008). "A miniscule optimized visual system in the Lower Cambrian".
Lethaia. 42 (3): 265–273.
^ Ali, M.A. and M. A. Klyne. 1985. Vision in vertebrates. New York:
^ Fernald, Russell D. (2001). The
Evolution of Eyes: Why Do We See
What We See? Karger Gazette 64: "The
Eye in Focus".
^ Fernald, Russell D. (1998). Aquatic Adaptations in Fish Eyes. New
^ Fernald RD (1997). "The evolution of eyes". Brain Behav. Evol. 50
(4): 253–9. doi:10.1159/000113339. PMID 9310200.
^ a b Slingsby, C.; Wistow, G. J.; Clark, A. R. (2013). "
crystallins for a role in the vertebrate eye lens". Protein Science.
22: 367–380. doi:10.1002/pro.2229.
^ Fernald, Russell D. (2001). The
Evolution of Eyes: Where Do Lenses
Come From? Karger Gazette 64: "The
Eye in Focus".
^ Burgess, Daniel S. (2001). Brittle Star Features
^ Ali, Mohamed Ather; Klyne, M. A. (1985). Vision in Vertebrates. New
York: Plenum Press. p. 1. ISBN 0-306-42065-1.
^ a b Osorio, D; Vorobyev, M (2005). "Photo-receptor spectral
sensitivities in terrestrial animals: adaptations for luminescence and
color vision". Proc. R. Soc. B.
^ Menzel, Randolf (1979). "Spectral Sensitivity and Color Vision in
Invertebrates". In H. Autrum. Comparative Physiology and
Vision in Invertebrates- A: Invertebrate Photo-receptors. Handbook of
Sensory Physiology. VII/6A. New York: Springer-Verlag.
pp. 504–506; 551–558. ISBN 3-540-08837-7.
^ a b Cronin, T; Shashar, N; Caldwell, R; Marshall, J; Cheroske, A;
Chiou, T (2003). "Polarization vision and its role in biological
signaling". Integr. Comp. Biol.
^ Mäthger, Lydia M.; Barbosa, Alexandra; Miner, Simon; Hanlon, Roger
T. (2006-05-01). "Color blindness and contrast perception in
cuttlefish (Sepia officinalis) determined by a visual sensorimotor
assay". Vision Research. 46 (11): 1746–1753.
doi:10.1016/j.visres.2005.09.035. PMID 16376404.
^ Malstrom, T; Kroger, R (2006). "pupil shape and lens optics in the
eyes of terrestrial vertebrates". The Journal of Experimental
^ "Carnivores". U.S. Department of the Interior, Bureau of Land
Management. 2009-12-14. Retrieved 2011-03-28.
^ Boroditsky, Lera (1999-06-24). "Light & Eyes: Lecture Notes".
Lecture Notes. Stanford. Retrieved 11 May 2010.
^ Dr. Michael Shermer, as quoted by Christopher Hitchens in his book
"God is Not Great" (pg.82)
^ Reichenbach A, Bringmann A. (2010). Müller cells in the healthy and
diseased retina. New York: Springer. pp 15 - 20.
^ Diagrammatic representation of disc shedding and phagosome retrieval
into the pigment epithelial cell.
^ Serb, Jeanne M.; Eernisse, Douglas J. (2008-09-25). "Charting
Evolution's Trajectory: Using Molluscan
Eye Diversity to Understand
Parallel and Convergent Evolution". Evolution: Education and Outreach.
1 (4): 439–447. doi:10.1007/s12052-008-0084-1.
Lamb TD, Collin SP, Pugh EN (December 2007). "
Evolution of the
vertebrate eye: opsins, photoreceptors, retina and eye cup". Nat. Rev.
Neurosci. 8 (12): 960–76. doi:10.1038/nrn2283. PMC 3143066 .
PMID 18026166. Illustration. Review
Lamb, TD (2011). "
Evolution of the Eye" (PDF). Scientific American.
305: 64–69. doi:10.1038/scientificamerican0711-64.
Land, Michael F.; Nilsson, Dan-Eric (2012). "The origin of vision".
Animal Eyes (2 ed.). Oxford: Oxford University Press. pp. 1–22.
Journal Evolution: Education and Outreach[dead link] Volume 1, Number
4 / October 2008.
Special Issue: The
Evolution of Eyes. 26 articles,
Ivan R. Schwab (2012). Evolution's Witness: How Eyes Evolved. New
York: Oxford University Press. ISBN 9780195369748.
Hayakawa S, Takaku Y, Hwang JS, Horiguchi T, Suga H, Gehring W, et al.
(2015). "Function and evolutionary origin of unicellular camera-type
eye structure". PLoS ONE. 10 (3): e0118415.
PMC 4348419 . PMID 25734540.
Greuet, C (1968). "Organisation ultrastructurale de l'ocelle de deux
Peridiniens Warnowiidae, Erythropsis pavillardi Kofoid et Swezy et
Warnowia pulchra Schiller". Protistologica. 4: 209–230.
Gregory S. Gavelis, Shiho Hayakawa, Richard A. White III, Takashi
Gojobori, Curtis A. Suttle, Patrick J. Keeling, Brian S. Leander
(2015). "Eye-like ocelloids are built from different endosymbiotically
acquired components". Nature. 523: 204–7.
PMID 26131935. CS1 maint: Uses authors parameter (link)
Oakley, Todd H.; Speiser, Daniel I. (2015). "How Complexity
Evolution of Animal Eyes". Annual Review of Ecology,
Evolution, and Systematics. 46: 237–260.
Ed Young; photographs by David Liittschwager (February 2016). "Inside
the Eye: Nature's Most Exquisite Creation". National Geographic. 229
Evolution of the Eye". WGBH Educational Foundation and Clear Blue Sky
Productions. PBS. 2001.
Creationism Disproved? Video from the National Center for Science
Education on the evolution of the eye
Evolution: Education and Outreach
Evolution and Eyes
volume 1, number 4, October 2008, pages 351-559. ISSN 1936-6426
(Print) 1936-6434 (Online)
Evolutionary history of life
Index of evolutionary biology articles
Outline of evolution
Timeline of evolution
Earliest known life forms
Evidence of common descent
Last universal common ancestor
Origin of life
Evolutionary developmental biology
dolphins and whales
Programmed cell death
Life cycles/nuclear phases
Tempo and modes
Renaissance and Enlightenment
Transmutation of species
On the Origin of Species
History of paleontology
The eclipse of Darwinism
History of molecular evolution
Extended evolutionary synthesis
Teleology in biology
Vision in animals
Simple eye in invertebrates
Evolution of the eye
Evolution of color vision
Evolution of color vision
Evolution of color vision in primates
Albinism in biology
Blindness in animals
Infrared sensing in snakes