Eye, an instrument of impeccable precision, a show piece of nature’s engineering skills and a great evolutionary advantage to those who have it. Have you ever wondered about its complexity which can even put world’s best camera to shame? Well if yes! then you are in the right place. People often argue against evolution trying to dismiss it merely as a theory with the example of eye saying that “How could something so complex, have developed through random mutations and natural selection, even over millions of years?”. We will try to answer that question with a fascinating evolutionary tale.
But first have a look at this model to revisit the intricacies of a Human Eye:
If you really want to understand and appreciate the Evolution few things you may want to keep in mind:
- It was not something that happened once and resulted into all of this [World around us]. It is an ongoing, continuous process and we all still evolving into future beings.
- Time Scale: The nature all around us had millions of years to produce all the existing complexities in different life forms. Everything in nature is not a result of one single event rather an overtime elaborate process of DNA transmission, DNA copying, genetic mutations, adaptations etc.
- Mutations are abrupt minute variations in DNA, they arise due to several factors which may be found within an organism or in the environment around it.
It all began when life was still in its primitive form, single celled bacteria were struggling for the continuation of their existence. It was indeed the “Survival of the fittest”. Due to error in copying DNA during reproduction some bacteria acquired light sensitive proteins. These proteins helped bacteria to flee the surface of the water body during daytime as it might damage the DNA present in their bodies which we all know is absolutely vital for the survival of any specie.
The Socket Eye
An unexpected, random event had given light-sensitive bacteria a decisive advantage over others and this feature was preserved and passed on to the next generation as they exchange their DNA with other bacteria. Take example of a multi-cellular flatworm Planaria, which developed rudimentary eye-like structures, sort of dimples embedded with light sensitive proteins. Later these dimple like structure changed into a socket like structure (like the ones we find in Euglena). Can you guess the reason? So that more surface area is exposed to sunlight thereby helping them to obtain the image (though blurred!) in dimly lit conditions, along with that it helped to judge the direction from where the light is coming. As the time progressed the creatures developed multi-functional eye forms which could be used not just to detect day or night but served as an instrument in alarming threats, locating prey and of course! for better movement through water.
The Pin-Hole Eye
But how much good a blurred image could be? Certainly not much! Those eyes needed improvement. Over next thousands of years the eye evolved into something very similar to what we call a Pin-Hole camera. This happened as the opening contracted into the size of a pin and a protected transparent membrane developed which will give shape to present day Cornea. This little change was massive development towards modern day eyes, the pin hole provided sharp focused image and the internal sensitive parts of eye were protected by the membrane filling up the internal volume with water. This type of eyes are still found in an organism called Mollusca Nautilus.
The next big thing happened in the evolutionary tail was the development of the lens. The internal watery fluid shrunk into a lens like object with little adjustment capabilities. This paved the way for near and far sights, abilities that we now take for granted, as the image could be refocused on the eye surface by the lens. The lens which became a crucial element in the entire optical setup slowly developed into today’s version made up of crystalline proteins.
The evolution of eye was not an insulated event however it was right in synchronization with the development of our brain and the part which process the signals from eye The Visual Cortex through Optic Nerve. But that will be an evolutionary tale for some other time.
I stumbled across a very interesting website mountimprobable.com which is based on the works of Richard Dawkins back in 80s. It allows the user to simulate the evolutionary process through Artificial Selection. I took a Mollusca (you can even customize it) and let it breed for four generation while I was the artificial agent to guide evolution. Just look at the image below and wonder how much organisms can differ in just four generation of breeding going through artificial selection.
- COSMOS: A Spacetime Odyssey
- The Evolution of the Eye (2010) – Richard Dawkins https://www.youtube.com/watch?v=mb9_x1wgm7E
- The evolution of the human eye – Joshua Harvey https://www.youtube.com/watch?v=qrKZBh8BL_U