The evolution of the human eye – Joshua Harvey

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[Music]
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the human eye is an amazing mechanism
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able to detect anywhere from a few
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photons to direct sunlight or switch
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focus from the screen in front of you to
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the distant horizon in a third of a
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second in fact the structures required
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for such incredible flexibility were
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once considered so complex that Charles
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Darwin himself acknowledged that the
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idea of their having evolved seemed
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absurd in the highest possible degree
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and yet that is exactly what happened
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starting more than 500 million years ago
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the story of the human eye begins with a
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simple light spot such as the one found
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in single-celled organisms like you
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Glenna
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this is a cluster of light-sensitive
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proteins linked to the organisms
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flagellum activating when it finds light
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and therefore food a more complex
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version of this light spot can be found
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in the flatworm planaria being cupped
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rather than flat enables it to better
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sense the direction of incoming light
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among its other uses this ability allows
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an organism to seek out shade and hide
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from predators
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over the millennia as such light cuts
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groove deeper in some organisms the
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opening at the front grew smaller the
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result was a pinhole effect which
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increased resolution dramatically
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reducing distortion by only allowing a
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thin beam of light into the eye
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the Nautilus an ancestor of the octopus
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uses this pinhole eye for improved
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resolution and directional sensing
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although the pinhole eye allows for
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simple images the key step towards the
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eye as we know it is a lens this is
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thought to have evolved through
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transparent cells covering the opening
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to prevent infection allowing the inside
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of the eye to fill with fluid that
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optimizes light sensitivity and
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processing crystalline proteins forming
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at the surface created a structure that
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proved useful in focusing light at a
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single point on the retina
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it is this lens that is the key to the
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eyes adaptability changing its curvature
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to adapt to near and far vision this
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structure of the pinhole camera with a
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lens served as the basis for what would
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eventually evolve into the human eye
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further refinements would include a
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colored ring called the iris that
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controls the amount of light entering
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the eye a tough white outer layer known
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as the sclera to maintain its structure
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and tear glands that secrete a
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protective film but equally important
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was the accompanying evolution of the
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brain with its expansion of the visual
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cortex to process the sharper and more
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colorful images it was receiving we now
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know that far from being an ideal
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masterpiece of design our eye bares
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traces of its step by step evolution for
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example the human retina is inverted
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with light detecting cells facing away
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from the eye opening this results in a
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blind spot where the optic nerve must
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pierce the retina to reach the
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photosensitive layer in the back the
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similar looking eyes of cephalopods
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which evolved independently have a
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front-facing retina allowing them to see
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without a blind spot other creatures
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eyes display different adaptations anub
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lapse the so called four-eyed fish have
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eyes divided in two sections for looking
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above and underwater perfect for
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spotting both predators and prey cats
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classically nighttime hunters have
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evolved with a reflective layer
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maximizing the amount of light the eye
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can detect granting them excellent night
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vision as well as their signature glow
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these are just a few examples of the
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huge diversity of eyes in the animal
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kingdom so if you could design an eye
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would you do it any differently this
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question isn’t as strange as it might
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sound today doctors and scientists are
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looking at different eye structures to
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help design biomechanical implants for
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the vision impaired
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and in the not-so-distant future the
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machines built with the precision and
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flexibility of the human eye may even
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enable it to surpass its own evolution
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[Music]

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