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The human eye is an amazing mechanism,

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able to detect anywhere [br]from a few photons to direct sunlight,

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or switch focus from [br]the screen in front of you

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to the distant horizon [br]in a third of a second.

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In fact, the structures required[br]for such incredible flexibility

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were once considered so complex

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that Charles Darwin himself acknowledged[br]that the idea of there having evolved

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seemed absurd in the [br]highest possible degree.

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And yet, that is exactly what happened,[br]starting more than 500 million years ago.

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The story of the human eye begins[br]with a simple light spot,

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such as the one found [br]in single-celled organisms,

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like euglena.

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This is a cluster [br]of light-sensitive proteins

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linked to the organism's flagellum,

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activating when it finds light[br]and, therefore, food.

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A more complex version of this light spot[br]can be found in the flat worm, planaria.

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Being cupped, rather than flat,

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enables it to better sense[br]the direction of the incoming light.

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Among its other uses,

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this ability allows an organism[br]to seek out shade and hide from predators.

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Over the millenia,

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as such light cups grew deeper[br]in some organisms,

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the opening at the front grew smaller.

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The result was a pinhole effect,[br]which increased resolution dramatically,

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reducing distortion by only allowing[br]a thin beam of light into the eye.

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The nautilus, [br]an ancestor of the octopus,

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uses this pinhole eye for improved [br]resolution and directional sensing.

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Although the pinhole eye allows[br]for simple images,

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the key step towards the eye [br]as we know it is a lens.

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This is thought to have evolved

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through transparent cells covering [br]the opening to prevent infection,

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allowing the inside of the eye[br]to fill with fluid

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that optimizes light sensitivity[br]and processing.

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Crystalline proteins [br]forming at the surface

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created a structure that proved useful

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in focusing light [br]at a single point on the retina.

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It is this lens that is the key[br]to the eye's adaptability,

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changing its curvature to adapt[br]to near and far vision.

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This structure of the pinhole camera[br]with a lens

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served as the basis for what would [br]eventually evolve into the human eye.

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Further refinements would include[br]a colored ring, called the iris,

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that controls the amount [br]of light entering the eye,

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a tough white outer layer,[br]known as the sclera, to maintain its structure,

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and tear glands that secrete [br]a protective film.

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But equally important was the accompanying[br]evolution of the brain,

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with its expansion of the visual cortex

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to process the sharper and more colorful images[br]it was receiving.

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We now know that far from being[br]an ideal masterpiece of design,

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our eye bares traces[br]of its step by step evolution.

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For example,[br]the human retina is inverted,

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with light-detecting cells facing away[br]from the eye opening.

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This results in a blind spot,

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where the optic nerve[br]must pierce the retina

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to reach the photosensitive [br]layer in the back.

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The similar looking eyes [br]of cephalopods,

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which evolved independently,

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have a front-facing retina,[br]allowing them to see without a blind spot.

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Other creatures' eyes display[br]different adaptations.

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Anableps, the so called four-eyed fish,

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have eyes divided in two sections[br]for looking above and under water,

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perfect for spotting [br]both predators and prey.

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Cats, classically nighttime hunters,[br]have evolved with a reflective layer

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maximizing the amount of light[br]the eye can detect,

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granting them excellent night vision,[br]as well as their signature glow.

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These are just a few examples of the huge [br]diversity of eyes in the animal kingdom.

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So if you could design an eye, [br]would you do it any differently?

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This question isn't as strange [br]as it might sound.

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Today, doctors and scientists are looking[br]at different eye structures

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to help design biomechanical implants[br]for the vision impaired.

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And in the not so distant future,

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the machines built with the precision[br]and flexibilty of the human eye

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may even enable it to surpass[br]its own evolution.