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The human eye is an amazing mechanism,

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

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or switch focus from 
the screen in front of you

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to the distant horizon 
in a third of a second.

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In fact, the structures required
for such incredible flexibility

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were once considered so complex

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that Charles Darwin himself acknowledged
that the idea of their having evolved

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seemed absurd in the highest
possible degree.

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

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

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such as the one found 
in single-celled organisms,

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like euglena.

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This is a cluster 
of light-sensitive proteins

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linked to the organism's flagellum,

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activating when it finds light,
and therefore food.

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A more complex version of this light spot
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
the direction of the incoming light.

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Among its other uses,

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this ability allows an organism
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 in some organisms,

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the opening at the front grew smaller.

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

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reducing distortion by only allowing
a 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 
resolution and directional sensing.

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Although the pinhole eye allows
for simple images,

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the key step towards the eye 
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 
the opening to prevent infection,

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allowing the inside of the eye
to fill with fluid

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that optimizes light sensitivity
and processing.

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Crystalline proteins 
forming at the surface

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created a structure that proved useful

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in focusing light 
at a single point on the retina.

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

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changing its curvature to adapt
to near and far vision.

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This structure of the pinhole 
camera with a lens

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

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

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that controls the amount 
of light entering the eye,

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

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and tear glands that secrete 
a protective film.

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But equally important was the accompanying
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 it was receiving.

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

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our eye bares traces
of its step by step evolution.

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For example,
the human retina is inverted,

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with light detecting cells facing away
from the eye opening.

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This results in a blind spot,

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where the optic nerve
must pierce the retina

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to reach the photosensitive 
layer in the back.

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The similar looking eyes 
of cephalopods,

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which evolved independently,

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

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Other creatures eyes display
different adaptations.

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Anableps, the so called four-eyed fish,

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

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perfect for spotting 
both predators and prey.

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

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maximizing the amount of light
the eye can detect,

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

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

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

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

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Today, doctors and scientists are looking
at different eye structures

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to help design biomechanical implants
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
and flexibilty of the human eye

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may even enable it to surpass
its own evolution.