Tag Archives: animals

How Is It Possible to Evolve Eyes?


What’s the scoop?

We’ve all heard it before, and we’ve probably all even thought it: How can something as complex as an eye possibly evolve? It’s a classic argument against evolution known as irreducible complexity, which claims that biological products, such as the eye, are so complex with their many interacting parts that if one were removed then the whole system would stop working. In other words, it would be impossible for evolution (which offers no foresight) to gradually put together a complex system with so many pieces that have no use, and it would be extremely improbable for hundreds of random mutations to put together so many pieces in one shot. Many liken the chances of this occurring to a tornado passing through a junkyard and assembling a Boeing 747.

While those in favor of irreducible complexity are quite persuasive with their analogies, they are a bit, dare I say, short-sighted. The argument fails in its central prediction, that if one component is taken away, then the system becomes useless. It’s true that if we removed the lens from your eye, it would cease to work. But we must recognize that evolution does not work backwards. A gradual progression of less complex to more complex eye systems can not only be hypothesized but can also be observed in nature, and each of these “primitive” eyes offer their own benefits– to paraphrase Richard Dawkins, even 1% of an eye is better than no eye at all.

What’s in an eye?

When we think of an eye, we probably imagine a human eye– the colored iris with the dilating pupil surrounded by white, and a lens inside with the retina which sends signals to brain via the optic nerve. It is our window to the world, our camera which allows us to see and react to visual stimuli. Of course, in order to see something there must be light. Thus, in very simple terms we could describe an eye as an organ which senses light.


It probably won’t surprise you that as we look across species we see (pun intended) that there are many different types of eyes. In fact, it is by considering these many different species that we can form a gradual progression to see how our eyes may have evolved. Even though humans do not have the optimal eye structure, we can use our eyes as our “final” step that we are trying to reach. If we work logically toward the human eye, we see diverse examples in present-day nature of what our ancestors’ eyes may have looked like. In each step we must realize that not only do the simpler eye structures function, they are also quite helpful to survival.

Step 1: Photosensitive Epithelium

If an eye is an organ which detects light, the simplest eyes would just be a layer of cells that are sensitive to light. In fact, that’s exactly what we see in flatworms and many other annelids. They have pigmented (I think we all know that black absorbs more light than white) eyespots which connect to receptive nerves to send signals to their brain. A simple experiment done in introductory biology classes involves exposing flatworms to light and watching them shy away from light.

Flatworm Eye

So what’s the use of this? Most worm species dwell underground or in water where the sun don’t shine. It’s pretty obvious why this is– we’ve all seen earthworms cooked crispy on a sunny sidewalk. Additionally, many of their predators, such as birds, hangout above ground. Thus, any worm that can detect light and avoid it is more likely to survive than a worm that can’t detect light.

Step 2: The Eye Cup

The next logical step is an inward indentation of the layer of photosensitive cells. We see this in gastropods such as the limpets of Patella.


Why would this be helpful? Well, imagine you are moving a light from left to right across a bowl that you’ve turned vertical. As you move, what happens? As we move to the peripheries of the bowl’s edges, a shadow is cast inside the bowl. Thus, an eye cup allows the animal to see not just the presence of light but the movement and general direction of light. And the deeper the eye cup the better, because a larger shadow will be cast.

Richard Dawkins does a great job of showing this, doing science in style:


Step 3: The Pinhole

As the eye cup becomes deeper and deeper, approaching a spherical shape, it is also optimal that the point of entry to the eye cup become smaller and smaller. Light will be focused to one area, and the direction of light will be better received. Additionally, some shapes will be distinguished. We see this type in mollusks such as Nautilus.


(It seems that nature was way ahead of da Vinci’s invention of the camera obscura.)

Step 4: The Lens

Next, we can add a lens inside our pinhole. This lens is curved so as to refract light into a focused path. If you’ve ever had a childhood, you know that a magnifying glass can be used to focus sunlight to a concentrated beam to savagely kill helpless insects. The lens in your eye works about the same way. The more curvature, the better the focus, and the better to see the shapes of food to eat or of predators attacking. We see eyes like these in marine snails of Murex.


Step 5: The Cornea and Iris

Splitting from the lens, a transparent layer known as the cornea and a nontransparent layer known as the iris  finishes our quest to reach a “complex” eye found in humans. The cornea accounts for about 2/3 of the focusing power of the human eye, while the iris allows for more blood vessels and thus the possibility for larger eyes. Additionally, we could speculate that pigmented irises have played a special role in human evolution due to its stark contrast to the whites of our eyes. The iris is the part of our eye which conveys the direction we are looking, leading to the communication of motives or emotions.

Eye Stages

The Big Picture (yes, another pun)

Not only can we hypothesize what the eyes of our ancestors may have been like, we can also find them in nature where it wasn’t necessary for species to evolve complex eyes to survive. We also see that at each step, where “pieces” of the complex human eye are not present, the eye still works and is still beneficial. Thus, it is quite possible, arguably simple, to evolve complex eyes, and the argument for irreducible complexity breaks down.