The human eye has often been made a target by creationists as an organ that is supposedly “too complex to have evolved” and thus an example of evidence against evolution by natural selection. The argument typically goes something along the lines of that having “only half an eye” is useless and that because every part of the eye needs to be in place for it to work, the complexity of the organ seems to be too great to have evolved over time (Evolution of the eye, n.d.). Indeed, the eye is a deeply fascinating organ. However, as with all parts of the human body, its development can be explained based on external stimuli selecting for certain favorable traits within a population of our ancestors over time, in this case those traits being associated with improved visual detection capability.
Returning for a moment to the concept of having only “half an eye”. Certainly, having “half an eye”, meaning a human eye literally split in half wouldn’t be very useful. However, as famed evolutionary biologist Richard Dawkins pointed out, the idea of “half an eye” is a little bit misleading (Richard Dawkins, n.d.). Half an eye isn’t very useful, but what about an eye that is partially functional, providing some visual detection capability, albeit limited. This latter scenario is exactly how the story of the eye begins. In fact, the first forms of the eye weren’t an eye at all, but rather a sheet of light sensitive cells, only capable of determining light from dark. This still may not sound very useful to us with our highly evolved modern vision systems. However, in the primordial days of the Earth, this ability would have provided a significant evolutionary advantage, allowing those who possessed it to have greater awareness of their surroundings, including whether or not a predator might be there, and so forth (Evolution of the eye, n.d.).
After the sheet of light-sensitive cells, the next step in our evolutionary timeline is a small indentation with those same cells lining the walls on all sides. Now, as opposed to simply being able to determine whether or not a predator is there, the angle that lights strikes the cells lining the indentation provides a way for the organism to determine from which direction light is coming, and hence from which direction a predator may be coming. Over time, the opening at the top of the indentation becomes smaller and smaller, until it is only about the size of a pinhole. Along with this change in the size of the opening, the precision of the organism to detect exactly from where light is coming increases greatly, until the organism is able to detect the exact angle and height of the light source. By the time the opening becomes sufficiently small, the organism is even able to detect a small, blurry image of its surroundings (Evolution of the eye, n.d).
At this point in our evolutionary timeline, the organism has the ability to perceive an image, but not a very clear one. Furthermore, being that the pinhole at the top of the opening is so small, the ability of the organism to visualize its surroundings is isolated to only a very small area at a time. Lastly, the pinhole has no protection from the outside, meaning the cells inside the opening could be susceptible to injury or infection. In order to remedy each of these issues, in the next phase, a lens of skin begins to develop over the opening of the pinhole. This lens serves several purposes. Firstly, it provides a layer of protection from the outside. Secondly, the lens allows for light to enter the pinhole from a wider range of angles. Specifically, the lens focuses the light on the pinhole and, when combined with the development of a retina and other parts of the eye, eventually allows for the projection of a full-scale image. Over time, the lens and eye continue to develop to become more complex and well defined until we eventually, after millions of years of evolution, arrive at the fully developed human eye.
That is the story of the development of the human eye. But what about eyes in other living organisms? In studying this subject, other species can provide a great example of each stage of the timeline of the eye that we just discussed, as well as give us a better idea of how evolution by natural selection works in this context (Evolution of the eye, n.d). Mollusks, clams as well as squids and octopuses all help demonstrate the existence of more primitive forms of the eye, all the way up to something that is very similar to its human counterpart (See Appendix A). For example, nautilus, a type of abalone, provides a great example of the second stage of our timeline. In examining the case of nautilus, we can see that it has an indentation full of light sensitive cells. However, we can also see it never moved past this to develop a lens or complex cornea. Why is this the case? The reality is that evolutionary processes cannot predict the future, and that the only way evolution can move is forward, not backward. Therefore, in the case of the nautilus, it’s likely that there was a point in time, where, evolutionary speaking, continuing to specialize in the second stage of our evolutionary timeline was equally attractive evolutionary as moving towards developing a pinhole camera. Again, because evolutionary processes cannot predict the future, they had no way of knowing that eventually having a pinhole camera would become superior. Thus, they, in essence, became stuck in the second stage of our timeline and, because there is no such thing as going backward in evolution, were unable to change course (Richard Dawkins, n.d).
Other aspects of the development of the eye that are frequently brought up by creationists include whether or not enough time has elapsed for the eye to evolve. Specifically, the estimated evolutionary timeline for the eye is several hundred thousand million years, which some creationists believe is too short of a time for the organ to have fully developed. However, while this is indeed a relatively short time, it is important to note that, scientists believe this is still sufficient time for evolution of this kind to take place. Furthermore, for the majority of evolutionary history, early organisms had very short lifespans, only lasting about one year at a time. Because evolution takes place based on genetic differences between generations, these short lifespans allowed for the eye to develop faster than if the lifespans of early organisms had been longer (Eye evolution, n.d.).
In general, the controversy surrounding the eye helps demonstrate some of the misconceptions held by creationists about evolutionary processes. Therefore, addressing these misconceptions is also paramount to addressing the foundations of creationist’s skepticism regarding evolutionary theory. In general, we can see that, based on the eye’s evolution, evolutionary processes are far from omniscient; they can only react to external stimuli as they are presented to them. Furthermore, we can also see there are indeed instances where the result is not completely ideal, such as in the case of nautilus. However, each of these outcomes still seems reasonable if we understand how evolution by natural selection is applied in the real world. Conversely, if we instead adopt the idea of intelligent design, almost none of these phenomena make sense. Even the human eye itself, though highly efficient, is far from perfect. Why couldn’t an all-knowing super being have provided us with a better way to adapt our eyesight to extreme light or extreme dark, for example? The list goes on. Still, the human eye works well enough for us to observe the beauty all around us. From scenic vistas too bustling cities, the majesty of reality is manifest for all to see. However, as this article demonstrates, this has been made possible for to us through hundreds of thousands of years of evolutionary progress, not through the work of some floating grandpa in the sky.
References:
Richard Dawkins demonstrates the evolution of the eye. (n.d.-b). YouTube. https://www.youtube.com/watch?v=2X1iwLqM2t0&t=205s
Zhang Y, Mao F, Mu H, Huang M, Bao Y, Wang L, Wong NK, Xiao S, Dai H, Xiang Z, Ma M, Xiong Y, Zhang Z, Zhang L, Song X, Wang F, Mu X, Li J, Ma H, Zhang Y, Zheng H, Simakov O, Yu Z. The genome of Nautilus pompilius illuminates eye evolution and biomineralization. Nat Ecol Evol. 2021 Jul;5(7):927-938. doi: 10.1038/s41559-021-01448-6. Epub 2021 May 10. Erratum in: Nat Ecol Evol. 2021 Dec;5(12):1637. doi: 10.1038/s41559-021-01571-4. PMID: 33972735; PMCID: PMC8257504.
Evolution of the eye. New Scientist. (n.d.). https://www.newscientist.com/definition/evolution-of-the-eye/
Eye evolution. (n.d.). https://learn.genetics.utah.edu/content/senses/eye/
Appendix:
Appendix A:
