Evolution of eye

MPhil First years– Bhandary Shashank Kishore

– Rinu Thomas– Greeshma G

Biological evolution• 5 billion years ago• Evolution of vision – consequence of light on earth• Paleontological evidence

– Eyes evolved independently in different organisms– 40-60 times– Image forming eyes arose in

• Cnidarians• Molluscans• Arthropods• Chordates

• Pax6 – molecule (role in evolution)

Darwin’s theory of evolution

Evolution - “descent with modification”

Similarities among organisms is from a common ancestor Differences among

organisms is the result of natural selection, which adapted them to different environments

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Fossil Evidence of Evolution

Human eye

Types of eyes• Simple eye

– Camera eyes– Concave mirror eye– Pinhole eyes

• Compound eyes– Apposition Eye– Superposition Eyeo Ommatidia receptors

Compound eyes

How Could Eyes Evolve?• Eyes evolved from light sensitive neurons• Light sensitive spot to a fully developed lens eye

Process of evolution

Light sensitive proteins• Eye spots• Unicellular organisms

– Detection of light and dark

Eyespot – photosynthesis

Multicellular “Cup formation”• Shallow light depression where light sensitive cells.-Allowed detection of light direction• Light has to be angled into cup, stimulating only a

portion of cells.-As depression became deepened, the sense of direction

became finer-Possibility formation before appearance of the brain• No need for processing

Pit and Pinhole eyes• Formation of deeper depressions and

narrow openings.-Less ambient light giving finer sensitivity.

Cambrian explosion and light switch theory• Introduction of visual fields with simple brains.

-Needed the process of interaction of light and cells to form an image.

• First visual field was just shadows.

Separation form the environment• Transparent outgrowth of cells on the top of cup

depression.-Separation of light sensitive cells and external environment.

• Protection• Specialization

– Higher refractive index– Color filtering – Blocks UV

Separation form the environment

• Also allowed for the operation of the sense organ in Aquatic and Terrestrial environments.

-Major step in evolution.

Lens formation and diversification

• Evolved independently form multiple linages.-originally used for seeing in darker waters.

• Separation of double layer with aqueous middle- Allowed for waste removal and nutrient supply.- Increased protection, optical power, viewing angle, and resolution- Could not be found in fossil records.

Developments due to selective pressures

• Color vision -Advantages for finding food, mates and avoiding prey.

• Focusing-Environmentally dependent• Amount of light in environment

• Location-Non predatory animals typically have eyes on the side of the head.• Increased visual range for detection of predators.

Developments due to selective pressures.• Location

-Predators have eyes located on the front of the head.• Increased depth perception.

• Muscle attachments-Movement of the eye.

Structural Evolution of the Eye: Nilsson

• Spatial recognition-Light sensitive patch of cells resting on a dark pigmented back ground

• Centre patch – Depression• Edges started constricting and raising• Cupping occurred - Molluscans

Structural evolution of the Eye• Pinhole eye was formed

– No good resolution– Dim image

• Any change that improves clarity and illumination will be favored

• Secondary assumption– Development of a lens– Increase in the size of the eye

• Progress from a light sensitive disk to a camera-eye

The fact………….

What is the genetic factor behind ?????

Pax6 gene• It is the master control in the gene development• Controls position of the eyes on the body plain• This gene is universal in all bilateria• This gene has a critical affect on the eye development

is not solely to eyes• Also in formation of nervous system, brain and nose

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Precursor of Pax6 gene

• Cnidarians: Less classes of Pax genes• Duplication of Pax genes resulted in the formation of

Pax6 genes

Hox gene• Responsible for the bilateral symmetry• Paired eyes

Discovery of similarities

• Walter Gehring and Rebecca Quiring • Similar genes of PAX-6 gene in mice and Aniridia in humans• Mutations in these analogues can truncate the development of

eyes in mice and cause serious defects in the human eye.

Annelids eye

Annelid eye• A- transparent cornea• B, C- granular tissue enclosing globe of

eye• D- external surface of reticular pigment

epithelium• F- internal surface of reticular pigment

epithelium• E- iris• G- crystalline lens, optic nerve• K- vascular trunks around base of eyeball

Eye of Arthropods

Ommatidia are the functional units of insect eyes.       Ommatidia contain receptor cells that send axons to the CNS.

Lens

Receptor cells

Ommatidia

Axons

Insects appear to have two eyes also, but they are compound!Each ommatidium sends separate information to their brain.

Spider eye

40Eyes of vertebrates

Anatomical Evidence for Evolution

41Eyes of Mollusks

Anatomical Evidence for Evolution

Cornea Lens

Retina (photoreceptors are on the inside surface) Sensory

nerves to brain

The Cephalopod Eye

This “design” is “more intelligent” than that of mammals (humans) because it lacks the blind spot and maximizes light exposure to receptors

Molluscans

• Horseshoe crab eyes

Evolution of eye in vertebrates

Anatomy of fish eye

Fish• Fish eyes have spherical lens along with rods and

cones• Most species have colour vision• Complete fish eye could have evolved in less than

350,000 years• Fish can be sensitive to polarized and ultraviolet light• The first ancestor to conquer the world was the fish

before amphibians, reptiles, birds

• Most fishes have fixed pupil size either circular or slit-like.

• Lens of the fish works more than other optical parts of the eye.

• Free from spherical aberration.• Foveas are highly sensitive to light to dark waters in

the water bed.

Eye working in water

• Water absorbes light, light is decreased as it is deeper.• Fixed pupil size in most fishes• Sharks and rays with muscular iris• Photoreceptors are deeper inside the eye and are very sensitive in

absorbing wavelength• Layers of rods and cones arranged irregularly• Irregularity changes with development• Tapetum layer for absorbance of sensitive light

• Visual stabilization by vestibule-ocular reflex• Accommodation controlled by lens• Bony fishes have retractor lentis for near vision• Cartiliangous fishes have the protractor lentis for far

vision

Zones where different types of fish are present• surface waters or the sunlit zone.

Four eyes

•  Four eyed fish have raised above the top of the head and divided in two different parts, so that they can see below and above the water surface at the same time.

• Eye of a four-eyed fish1) Underwater retina 2) Lens 3) Air pupil4) Tissue band 5) Iris 6) Underwater pupil7) Air retina 8) Optic nerve

Mesopelagic fishes

• Deep water fish• Light does not support photosynthesis• Adapted for an active life under low light conditions• Big lenses and rods• Very Sensitive to light signals

The bathypelagic fishes

• Below 1000 meters• Ocean is pitch black• Very little food and no sunlight• Bioluminescence

Fishes inside caves• Heat shock protein 90 (HSP90) stress• Cavefish habitats, including pH, oxygen content and temperature.• Conductivity much lower in the caves•  Eye-size variation• Adaptive small-eye phenotype.

Barrel eyes

• They have large telescopic eyes.• No cone cells• Fish has property of avoiding stings from other fishes• Deepwater fishes, like this Antarctic toothfish, often have large,

upward looking eyes, adapted to detect prey • The telescope fish has large, forward-pointing telescoping eyes

with large lenses

Amphibians

• The eyes of amphibians also can function in two environments: in air and in water.

• Eye focusing performed in the same way as in a camera: the lens moves along the optic axis of the eyeball toward or from the retina.

• For protection of the eyes’ surface from drying, there are eyelids

• Some kinds of amphibians — mainly terrestrial ones — have also lachrymal glands.

• It is interesting that frogs perceive by eyes moving objects only. Shrubs, trees, sky they perceive as background.

Frog eye

• Choice of food is determined only by size and movement. • Can be fooled easily not only by a piece of dangled meat but by any

moving small object. • Large round lens of the frog• The frog is naturally nearsighted (myopic) to -6 diopters giving it a

focus of approximately 6 inches. • Frogs and toads can change their focus by moving the lens out

towards the cornea. • In frogs the focus range is a few diopters and in toads the focus range

is 5 diopters giving a best myopia of -1 diopters. • The advantage of nearsightedness is that it blurs the background

clutter making foreground object characterization much easier. • Only 75% of the light intensity entering the eye reaches the retina

Retinal anatomy

• The optic nerve of the frog consists of approximately 470,000 unmyelinated fibers and 15,000 myelinated fibers.

• In the toad it consists of 320,000 unmyelinated fibers and 10,000 myelinated fibers.

• The frog Rana pipiens has approximately 440,000 small ganglion cells (7 to 10 microns in diameter) and 12,000 of the larger ganglion cells 14 to 20 microns in diameter.

• The frog brain consists of only 16 million neurons one can see that the retina makes up a significant portion of its brain's total neuron number.

• The frog retina has three cell layers as shown in figure 3: the outer, middle, and inner granular layers, and two fiber layers: the inner and outer plexiform layers.

• The frog has a limited color discrimination as indicated by the red and green rods.

• Only the ganglion cells from the inner granule layer send axons to the brain

• Some of the thick arbors tend to separate into two vertical layers

Reptiles

The pupil is elliptical or round and may give an indication of habitat and lifestyle.Round pupils and nocturnal species have vertical pupils. Coral snakes and all New World non-venomous snakes, except the boa constrictor have round pupils, while pit vipers have vertical slit pupils. • Sclera - The eye has no ossicles (unlike other reptiles) and the

sclera is composed entirely of tendinous connective tissue. • Posterior segment - The retina is usually grey mottled with white

or red spots and appears with semi-opaque nerve fibres radiating uniformly outwards from the optic disc that is obscured in families having a conus

Birds • Birds have a total of six pigments: four cone pigments plus pinopsin (a pineal

photoreceptive molecule) and rhodopsin for black and white vision.• Chickens, humans and mice all have the rhodopsin pigment; mice in addition have blue

and green; humans have blue, green, and red; and birds have these three pigments plus violet and pinopsin.

• For every colour that humans perceive, birds can see very distinct multiple colours, including ultraviolet light. Birds use infrared light (which we sense as heat) for night vision, allowing them to rapidly visualize their young in a dense, dark tree.

• The four pigments in estrildid finches' cones extend the range of colour vision into the ultraviolet

• There are two sorts of light receptors in a bird’s eye, rods and cones.

• Most birds are tetrachromatic, possessing four types of cone cells each with a distinctive maximal absorption peak.

• Pigeons probably have an additional pigment and therefore might be pentachromatic

Perception

• Birds can resolve rapid movements better than humans, for whom flickering at a rate greater than 50 Hz appears as continuous movement.

• Humans cannot therefore distinguish individual flashes of a fluorescent light bulb oscillating at 60 Hz, but budgerigars and chickens have flicker thresholds of more than 100 Hz.

• A Cooper's hawk can pursue agile prey through woodland and avoid branches and other objects at high speed; to humans such a chase would appear as a blur.

MAMMALIAN EYES

Till now

Few drops from plenty• Mice • Cat • Horse• Monkey

Ocular Dimensions

Mice

Holangiotic Retina:3-4 major venules and arteries radiating to optic disc.

Mouse eyesight is as good as what human see in far off peripheral vision

Cat• Cat eye has Tapetum Lucidum -a reflective layer behind the

retina• Ability to see in dark• Decrease visual acuity

• Pupil size minimizes to streak or round under bright illumination

• Visual field 2000 -eyes face forward• Cats have central band known as visual streak instead of

fovea.• Eye colour gold,green,orange• They don’t blink frequently squint their eyes to

communicate with other cats• Third eyelid (nictating membrane) thin cover closes from

sides.• Partially closes when cat is sick.

Sight at night.• Visual streak: A region with few and smaller retinal

vessels recognized as horizontal band dorsal to the disc representing area of higher ganglion cell and cone density for improved visual acuity.

• They have more number of cones and rods

• Only 2 cones cells were evolved till then • They distinguish among blue and violet colours rather than

colours near red spectrum

Horse• Lateral placement of eyes, horizontally elongated,

rectangular pupil-able to see broad wide range landscape.• Almost 360 degrees each eye view separately • In addition to these photo-receptors and other mechanisms

for adjusting to light, the horse has one of the largest eyes among land mammals, which allows more light to enter the eye.

• Less steropsis,Very sensitive to motion

How do they see

• Horses have amazing field of vision and they see everything except those in their centre hence they look down as they walk

Monkey

• Apes and old monkeys were trichromats and the new world monkeys are colour blind,more common in males than females

Vertebrate Retina

cone

rod

light

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