Evolution of Eye

THE EVOLUTION OF THE EYE

Eye, one of the five senses. It is regarded as the most vital sense organ. Eyes help organisms to detect the food and be mindful of any predators in the surroundings. In the tree of evolution, the eye was not seen initially in the single celled organisms like the bacteria, many single celled protozoans. One might assume that such a complex organ must have taken a lot of time to evolve. The video presents us a possible scientific explanation for the evolution of the eye mainly focusing on the generational development and the physics which were handled in the development of the eye. The scientist offering the explanation is Dr. Richard Dawkins, a renowned evolutionary biologist from Britain.

   According to Professor Robert MacLaren, an eye is similar to the “camera” where the light rays fall in the eye and will be focused by the lens (analogous to the lens of a camera) and then pass through a jelly chamber and are focused on to the retina which consists of the photoreceptor cells (similar to the film of the camera) and these signals from the photoreceptors are conveyed by the “optical nerve” to the brain where these light signals are converted into the image we actually observe. All these steps are very complex and developed over a period of time.

   Dr. Richard Dawkins tells us that the development of eye took place in many gradual stages. The first organism to show something analogous to an eye is “Euglena”, a unicellular flagellate protist. Euglena has something called an “eyespot” at the head end of the cell which helps the organism to detect light. It is just sensitive to light and hence cannot form any images and just can sense if it is light or dark. This could possibly be the first step for evolution of the eye. It was more like a flat film for light detection. It didn’t provide the organism with any directional sense of the light. It just provided the organism with photosensitivity. This was demonstrated using a flat Polaroid film which is sensitive to UV light, showed us blurred fluorescence when exposed to the UV light. Then the flat film for light detection started curving a little and this curving provided a little sense of direction to the light detection. Planarians showed such curved eye-like structures and hence have a huge evolutionary advantage compared to that in the organisms with flat eyespots where they could actually tell which direction the predators are coming from or moving towards. Dr. Richard Dawkins tells us in the video that the curving of this structure deepened further and led to the appearance of a structure similar to a “pin-hole camera”, which doesn’t form clear images but shows some blurred and inverted images.  A mollusc named “Nautilus” which has a pair of eyes on each side of the head which can actually form blurred images. In the next step of evolution to provide more clear images, there is a requirement for any curved transparent substance which acts similar to lens in focusing the light rays. This was explained by Dr. Dawkins using a polythene bag with some water, which is naturally curved and when placed in between the object and the frame of the pin-hole camera, and when properly adjusted, sharpened the image a little. This jelly or gunge like substance when hardened becomes a lens which can transmit a brighter clearer image. This was seen in sea snails which have a blob of jelly like lens, can distinguish predators and food much better than the Nautilus. The characteristics of this jelly-like lens, the curvature and the transparency, were improved generation after generation and evolved finally to a fully functional “eye” in the Octopus which is very similar to the vertebrate eye except that it is from a different evolutionary line.