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Evolution of the Eye - Nature's Visual Miracle

The visual pathway is truly one of nature’s wonders. The fact that we can see near, far, black and white and colour all while covering such a vast field of vision is certainly an evolutionary benefit that is nothing short of a miracle. But how well do we really understand this miracle of human life? 

We are now going to break down the awesomeness that is sight into numerous parts and try to understand it from the bottom up. This particular article will discuss the evolution of sight as we know it. 

In order to understand this story of sight, we are going to go back in time and travel forward so that we understand the theories of evolution of the eye and we also understand, step by step, how the eye evolved. 

Lets start with the present. Today, there are three major types of eyes that we know : the camera type, the compound type and the mirror type. The camera type of eye is the eye that we are familiar with and is present in vertebrates and humans. The compound type of eye is, as the name suggests, a fusion of many eyes transmitting information simultaneously. This is seen in flies, for example. The mirror type of eye is a more primitive set up and is famously seen in scallops but is much less common than the other two types of eyes. 

 
types of eyes.jpg
 

Any discussion about evolution is incomplete without talking about the rockstar of evolution Charles Darwin. Now, Darwin had a very interesting observation about the development of eyes. Initially, Darwin noted that the fact that such a structure could have evolved was “absurd in the highest degree.” But he then reconsidered and claimed that slow gradations of development of the eye from simple to imperfect to complex and finally to the perfect eye could be possible. 

But this was just a speculation made on observation. There was only a hypothesis but no proof. So scientists decided to look into it and, as with most major scientific discoveries, the scientists were polarised into two groups. One group of scientists believed that eyes had a polyphylogenetic origin, which meant that eye evolved multiple times independently in various species. The other group of scientists believed in a monophylogenetic theory, which stated that all eyes originate from a single ancestor and that each type of eye evolved separately based on the needs of the organism. 

So which theory was right? As it turns out, eyes have a monophylogenetic origin. Genetic studies have shown that the coding for the light sensing ability called the pax6 gene is common to all species which have sight. In addition all three types of eyes are seen in molluscs which inherently points to a monophylogentic origin of eyes.

Now we know that eyes developed from a single ancestor species. But how did they develop to become the complex organs of vision we know them to be today? To understand this, let us try to work our way froward from the most primitive eyes to the eyes of today. 

No one knows how long it took for these developments to take effect. However, two biologists named D.E. Nillson and S. Pelger theorised that it would take about 364,000 years for the eyes to develop in the fashion that they did. They also divided the development of the eye into four stages:

  1. Efficient photopigments 
  2. Directionality
  3. Photoreceptor membrane folding
  4. Focusing optics

Let us understand these four stages.

The most primitive eye is a single photoreceptor which was made up of two chemicals - a photoreceptor chemical and a pigment. This photoreceptor was able to perform a single function - detect the presence or absence of light. So it was a lot like an electrical switch. It was either on or it was off. 

 
 Single Primitive Photoreceptor

Single Primitive Photoreceptor

 

The use of such a structure was simple. It was not used for movement or location the way we use our eyes today. Rather, it was used for a process called photoperiodicity which was the modern equivalent of the circadian rhythm. Essentially, in the presence of light, certain reactions took place while in the absence of light, they did not happen. 

The next step in the evolution of the eye was the coalescing of these photoreceptors to form an eyespot. The advantage that the eyespot allowed was that it allowed the organism to detect the intensity of light. Simply put, if multiple photoreceptors were activated in an eye spot, there was a lot of light and vice versa. 

 
 Eye spot - Combination of multiple Photoreceptors

Eye spot - Combination of multiple Photoreceptors

 

Along with the eyespot, organisms developed mechanisms of locomotion, like flagella and cilia. This necessitated the need for location of the source of light. In order to accomplish this function, multiple eyespots came together and formed and eye patch. No, not like a pirate. But more like a patchwork quilt. Now these eye patches allowed the organism to vaguely localise the light source and move towards it. 

 
 Eye patch - Combination of multiple eye spots

Eye patch - Combination of multiple eye spots

 

In order to improve the localisation, the eye patch had to get bigger and bigger, but space was an evolutionary constraint. And so natures answer was the eye cup, or the optic cup which was an eyepatch folded into a dome. This modification allowed the organism to localise the exact angle of incident ray of light depending on the photoreceptor that was activated. 

At this point in time the primitive eye was ready and all the species that had the primitive eye came into massive competition for resources. And this competition resulted in an evolutionary war which is known today as the Cambrian Explosion which took place 540 million years ago. Andrew Parker, a zoologist, proposed the Light Switch Theory, which stated that there was a rapid acceleration in the evolution of the eye in this Cambrian Explosion because accurate detection of prey and light gave the organism a survival benefit. So the better the eye, the easier it was to survive. 

The period post the Cambrian explosion allowed for the development of two important structures of the eye -  the iris and then the crystalline lens. The first step was to separate the eye from the external environment which was done by the development of a transparent epithelial layer which we know today as the Cornea. 

Once the photo detection system was separated, the next need that offered an evolutionary benefit was focusing power. The higher the focusing power, the better the localisation of the object. The first structure to develop that improved focusing power was the aperture or the iris. This allowed all the light coming in to be funnelled into a single beam based on the direction of the light rays which drastically improved the localisation of the source. 

The next major development was the crystalline lens. The lens allowed light rays which traveled from a great distance to be focused directly on the retina rather than behind it. This allowed the organism to not only localise the source of light but also detect the shape and depth of the object and other such properties. 

 
 D.E. Nillson and S. Pelger Model of the Evolution of the Eye

D.E. Nillson and S. Pelger Model of the Evolution of the Eye

 

 

There you have it. The eye as we know it. At different branching points, each species developed its own peculiarities. For example jellyfish have eyes but no brain- the eyes transmit information directly to the muscles. The nautilus has an aperture but no crystalline lens. Flies have many eyes and a larger field of vision. Predators have eyes that are trained forward so that they have greater depth perception, where as prey have eyes on the sides which allows a larger field of vision. Human beings have developed the ability to artificially enhance their vision by using appendages called spectacles. The list in endless.

The question that remains is, what next?

Author: Narendran Sairam (Facebook)

Sources and citations

Gehring, W. J. “New Perspectives on Eye Development and the Evolution of Eyes and Photoreceptors .” OUP Academic, Oxford University Press, 13 Jan. 2005, academic.oup.com/jhered/article/96/3/171/2187545).

Nillson, Dan Eric. “The Evolution of Eyes and Visually Guided Behaviour.” Www.ncbi.com, US National Library of Medicine, 12 Oct. 2009, www.ncbi.nlm.nih.gov/pmc/articles/PMC2781862/.