HOW THE EYE EVOLVED By Adrea R. Benkoff, M.D.

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1 HOW THE EYE EVOLVED By Adrea R. Benkoff, M.D.

2 HOW THE EYE EVOLVED BY ADREA R. BENKOFF, M.D.

3 CREATIONISM vs. NATURAL SELECTION The complex structure of the eye has been used as evidence to support the theory that they have been designed by a creator. The eye is a system that cannot function in the absence of any of its components and therefore cannot have evolved naturally from a more primitive form. Charles Darwin himself wrote in the Origin of Species that the evolution of the eye by natural selection as first glance seemed absurd in the highest possible degree but perfectly feasible.

4 RATE OF EVOLUTION 4 Billion years ago life began on earth 1 Billion years ago multicellular animals diverged into two groups. Radially symmetrical body plan (a top and bottom side but no front or back) Bilaterally symmetrical body plan (left and right sides that are mirror images and a head end) 600 Million years ago this diverged into 2 groups

5 RATE OF EVOLUTION INVERTEBRATES Spineless creatures VERTEBRATES Cambrian Explosion ( Million years ago) created a diversity of animal body plans and laid the groundwork for the emergence of our complex eye

6 CAMERA STYLE EYE, the photoreceptors all share a single light-focusing lens and are arranged as a sheet (the retina) that lines the inner surface of the wall of the eye. RATE OF EVOLUTION 2 STYLES OF EYES AROSE: COMPOUND EYE, found in invertebrate arthropods, i.e. insects, spiders and crustaceans Compound eyes are an array of identical imaging units, each of which constitutes a lens or reflector that beams light to a handful of light-sensitive elements called photoreceptors. These eyes are very effective for small animals offering a wideangle view and moderate spatial resolution in a small volume. They are not practical for large animals.

7 COMPOUND EYES

8 RATE OF EVOLUTION The first fossils of eyes are from the lower Cambrian period With the evolution of the eye there was a burst of rapid evolution dubbed the Cambrian Explosion Eyes common to vertebrates, took shape in less than 100 million years, evolving from a simple light sensor for circadian (daily) and seasonal rhythms around 600 million years ago to an optically and neurologically sophisticated organ by 500 million years ago.

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10 STAGES OF EYE EVOLUTION Predecessors of the eye were photoreceptor proteins that sense light opsins In the unicellular organism, euglena, the eyespot allows the organism to move in response to light to assist in photosynthesis and to predict day and night, the primary function of circadian rhythms. The basic light-processing unit of eyes is the photoreceptor cell This cell contains 2 types of molecules in a membrane: the opsin and pigment that distinguishes colors.

11 PREDECESSOR OF THE EYE

12 STAGES OF EYE EVOLUTION Groups of photoreceptor cells form an Eyepatch on the animal s surface and permit it to gain a basic sense of the direction and intensity of light. Not enough to discriminate an object from its surroundings. Shallow cup formation Allows for direction of light based on shadows created by the shallow cup. The direction a predator was coming from could be determined. Pit Eyes arose in the Cambrian period and are seen today in Planaria

13 PIT EYESPOTS

14 STAGES OF EYE EVOLUTION Pinhole 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. The Nautilus has this eye but because the opening is small the image is dim.

15 PINHOLE CAMERA EYE

16 STAGES OF EYE EVOLUTION Camera-Type Eyes Overgrowth of transparent cells prevented contamination and parasitic infestation through the aperture of the pinhole eye. These split into 2 layers with liquid in between. Increase liquid increases the convexity of the lens A sharpened, refined, bright, upside down image is formed.

17 STAGES OF EYE EVOLUTION A transparent layer and a nontransparent layer may split forward from the lens: forming a separate cornea and iris Separation of the forward layer again forms the aqueous humor This increases refractive power and eases circulatory problems Formation of a nontransparent ring allows more blood vessels, more circulation, and larger eye sizes

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19 THE 3rd EYE Pineal Gland A small, hormone-secreting body in the vertebrate brain modulates circadian rhythms and seasonal activities such as feeding and breeding. In nonmammalian vertebrates it contains photoreceptor cells In mammalian vertebrate, the cells have lost their ability to detect light

20 COLOR VISION Ability to see colors presents selective advantages for species Recognition of predators, food and mates Opsin molecules were fine-tuned to detect different wavelengths of light Wavelength specific behavior patterns associated with natural vs. reflected light sources Escape, foraging for food, hiding

21 COLOR VISION Color vision developed when photoreceptor cells developed multiple pigments Night and day vision emerged when receptors differentiated into rods and cones Depended on whether animal was predator or prey Trichromatic color vision in primates occurred when their ancestors switched to daytime activity and began eating fruits and leaves from flowering plants Color vision with UV discrimination Birds, turtles, lizards, many fish and some rodents

22 FOCUSING MECHANISM Some species move the lens back and forth, some stretch the lens flatter Another mechanism regulates focusing chemically by controlling growth of the eye and maintaining focal length A focusing mechanism is not a requirement in organisms with small eyes that are active in direct sunlight due to small aperture. As a species grows larger or exists in a dimmer environment, it needs a means to focus

23 EYE LOCATION Prey generally have eyes on the sides of their head to have a larger field of view from which to avoid predators. Predators, have eyes in front of their head in order to have better depth perception. Flatfish are predators which lie on their side on the ocean s bottom and have eyes placed asymmetrically of the same side of the head

24 EVOLUTIONARY BAGGAGE Vertebrate eye is built backwards and upside down Light travels through the cornea, aqueous fluid, lens, vitreous, blood vessels, ganglion cells, amacrine cells, horizontal cells, and bipolar cells before it reaches the light-sensitive rods and cones that transform 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.

25 EVOLUTIONARY BAGGAGE Defects in the design of the vertebrate eye degrade image quality Blood vessels that cross the retina s inner surface cast shadows on the retina Nerve fibers that gather together to push through a single opening in the retina to become the optic nerve, create a blind spot Light must pass through nerve fibers and cell bodies before hitting the photoreceptors

26 EVOLUTIONARY BAGGAGE Camera eyes of cephalopods (octopus) are constructed the right way out. The nerve is attached to the rear of the retina and therefore they do not have a blind spot The difference may be accounted for by the origins of the eyes: in cephalopods they develop as an invagination of the head surface whereas in vertebrates, they originate as an extension of the brain.

27 VERTEBRATES AND OCTOPUSES DEVELOPED THE CAMERA EYE INDEPENDENTLY VERTEBRATE OCTOPUS

28 HOW LONG TO CREATE AN EYE? Swedish Zoologist, Dan Erik Nilsson, developed a computer program to determine how long it would take for evolution, through small variations in natural selection, to create an eye Answer: 1/2 million years 1/2 million years may sound like a long time but in evolutionary terms IT S THE BLINK OF AN EYE.