Here's how eyes developed 500 million years ago

Eyes are so amazingly complicated. Charles Darwin himself said the concept of them having evolved appeared "ridiculous in the highest probable degree". But they did, and this TED-ed video below shows how.

Researchers have found the origin of the not-so-humble human eyeball to no less than 500 million years ago, when single-celled entities, such as the minuscule euglena, developed a simple, flat 'light spot' on the external of their bodies. This cluster of light-sensitive proteins was linked to the creatures' elongated tail-like flagellums, which were used to drive them through the water. This permitted them to answer to sources of light - and therefore food - and start dashing towards it.

A kind of flatworm called planaria developed somewhat more complicated form, which worked even better, since it was curved inwards so it could better perceive the direction of the incoming light. Not only could the worm use this organ to seek out light-reflecting food, but it could also figure out where bags of low light were situated and use these as safeguard from predators.

Then, says Joshua Harvey in the video below, over the progression of several millennia, these cup-shaped structures started shifting inside various creatures, such as the nautilus, until only a small opening was left exposed. What resulted was a 'pinhole effect', which happened to melodramatically upsurge resolution, and cut down on distortion, since only a thin beam of light could make it through the tiny opening to be processed.

But we can do better than a simple pinhole. The key to sight as we know it is the lens. The origin of the all-important lens has been found back to a layer of transparent cells that sheltered the pinhole opening for protection against the elements. Now being closed off, the internal structure of the eye became filled with fluid, and this actually helps to enhance light sensitivity and processing, says Harvey.

This development was followed by the evolution of a crystalline structure behind the lens that stuck around, since it was great at concentrating incoming light to a single point on the retina at the back. This simple structure ultimately became the human eye, as a coloured ring called the iris started appearing, so that the amount of light entering the internal bits and pieces could be adjusted according to the environment. The outer white area, known as the sclera, formed to keep the eye's structure in place, and we even developed tear ducts so our eyes would be safe by a continuous supply of clear, defensive film.

But there's no point having all of this cool stuff for processing light with if we don't know what to do with any of that information. So as the human eye developed to come more complicated and well-organized, so too did the human brain develop to complement it. Watch the TED-ed video above to find out how. 

And have you ever speculated how blue eyes got their colour? Turns out they don’t get their colour from pigment - it’s actually way more attractive than that.

Health and Medicine


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