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  • The human eye is an amazing mechanism,

  • able to detect anywhere from a few photons to direct sunlight,

  • or switch focus from the screen in front of you

  • to the distant horizon in a third of a second.

  • In fact, the structures required for such incredible flexibility

  • were once considered so complex

  • that Charles Darwin himself acknowledged that the idea of there having evolved

  • seemed absurd in the highest possible degree.

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

  • The story of the human eye begins with a simple light spot,

  • such as the one found in single-celled organisms,

  • like euglena.

  • This is a cluster of light-sensitive proteins

  • linked to the organism's flagellum,

  • activating when it finds light and, therefore, food.

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

  • Being cupped, rather than flat,

  • enables it to better sense the direction of the incoming light.

  • Among its other uses,

  • this ability allows an organism to seek out shade and hide from predators.

  • Over the millenia,

  • as such light cups grew deeper in some organisms,

  • the opening at the front grew smaller.

  • The result was a pinhole effect, which increased resolution dramatically,

  • reducing distortion by only allowing a thin beam of light into the eye.

  • The nautilus, an ancestor of the octopus,

  • uses this pinhole eye for improved resolution and directional sensing.

  • Although the pinhole eye allows for simple images,

  • the key step towards the eye as we know it is a lens.

  • This is thought to have evolved

  • through transparent cells covering the opening to prevent infection,

  • allowing the inside of the eye to fill with fluid

  • that optimizes light sensitivity and processing.

  • Crystalline proteins forming at the surface

  • created a structure that proved useful

  • in focusing light at a single point on the retina.

  • It is this lens that is the key to the eye's adaptability,

  • changing its curvature to adapt to near and far vision.

  • This structure of the pinhole camera with a lens

  • served as the basis for what would eventually evolve into the human eye.

  • Further refinements would include a colored ring, called the iris,

  • that controls the amount of light entering the eye,

  • a tough white outer layer, known as the sclera, to maintain its structure,

  • and tear glands that secrete a protective film.

  • But equally important was the accompanying evolution of the brain,

  • with its expansion of the visual cortex

  • to process the sharper and more colorful images it was receiving.

  • We now know that far from being an ideal masterpiece of design,

  • our eye bares traces of its step by step evolution.

  • For example, the human retina is inverted,

  • with light-detecting cells facing away from the eye opening.

  • This results in a blind spot,

  • where the optic nerve must pierce the retina

  • to reach the photosensitive layer in the back.

  • The similar looking eyes of cephalopods,

  • which evolved independently,

  • have a front-facing retina, allowing them to see without a blind spot.

  • Other creatures' eyes display different adaptations.

  • Anableps, the so called four-eyed fish,

  • have eyes divided in two sections for looking above and under water,

  • perfect for spotting both predators and prey.

  • Cats, classically nighttime hunters, have evolved with a reflective layer

  • maximizing the amount of light the eye can detect,

  • granting them excellent night vision, as well as their signature glow.

  • These are just a few examples of the huge diversity of eyes in the animal kingdom.

  • So if you could design an eye, would you do it any differently?

  • This question isn't as strange as it might sound.

  • Today, doctors and scientists are looking at different eye structures

  • to help design biomechanical implants for the vision impaired.

  • And in the not so distant future,

  • the machines built with the precision and flexibilty of the human eye

  • may even enable it to surpass its own evolution.

The human eye is an amazing mechanism,

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TED-Ed】人眼的進化--約書亞-哈維。 (【TED-Ed】The evolution of the human eye - Joshua Harvey)

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    稲葉白兎 發佈於 2021 年 01 月 14 日
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