The human eye is imperfect. Blood vessels run over our retina (which light needs to reach for us to see). Those vessels can lead to problems. Our retina can also detach, our eye ball grows imperfectly and needs glasses to see properly. So how did this complex machine evolve?
The journey started off billions of years ago in simple organisms made up on just one cell that had ‘eye spots’. Eye spots have evolved independently around 40 to 65 times. They let an organism know whether it was in a bright area or dark, like what happens when you pass your hand over a fly and the eye spots on the back of its head tell it to fly away.
Groups of eye spots turned into a cuped shape to form a pit. This can be swivelled to find out where the bright light is coming from. These cup eyespots are still found in the flatworms planaria – stunningly these can regenerate body parts.
In the Cambrian explosion around 500 million years ago larger life forms evolved. These were free moving and hunting each other. Eyes are key to spotting your prey. During this time eyes developed rapidly one change at a time improving image-procession and light detection.
Afterwards, an eye developed similar to a pinhole camera. A small hole in front of light receptors allows the amount of light entering the eye to change, improving image quality. The still living mollusc Nautilus has such an eye.
Transparent cells soon developed over the small hole. This prevents parasitic infection and protect the eye. Early examples are found in the velvet worms. After this eye evolution became a bit tricky with diverse paths taken. Many animals found their own way to build a complex retina, lens, colour vision, light polarisation, focusing and other features that make our flawed eye a marvellous biological machine.