The mammalian eye, a refractive cornea non-compound eye, to be precise, is a wonderful example of the bizarre quirks of evolution that we see in nature. These are only really bizarre, of course, from the view-point of intelligent design.

Our visual system is made up of many tiny light receptors on our retina known as rods and cones. Each receptor picks up a small portion of light and relays the message to our brain via nerve connections in the optic nerve. Information from thousands of light receptors is pieced together by the brain to form an image of the world around us. The rest of the eye is designed to focus light onto the retina in the most efficient way possible. Logically, you would expect, therefore, that the eye would be designedso that nothing blocked light from reaching the retina. And yet, in the mammalian eye, the nerves and blood vessels connecting to the rods and cones protrude outwards in front of them. In order to connect back to the brain, the nerve fibres must then break through the wall of light receptors, creating a blind spot where they join the optic nerve.

Evolution has found several ways to prevent this design flaw from hindering our vision too greatly. Although there is no way around the blind spot, the nerve fibres and blood vessels have been carefully placed so as to skirt around the areas of the eye that are most crucial. One area on the retina, known as the fovea, is particularly crucial for vision. This area has the highest density of cone cells (the cells responsible for colour vision) and light is focussed on the fovea to achieve high visual acuity. The nerve cells and blood vessels in the eye avoid crossing the fovea, ensuring that their disruption of our vision is minimised. This is an evolutionary fudge, however; a reasonable solution given imperfect design.

To even the untrained observer, the layout of the mammalian eye looks awkward and inefficient. Why wouldn’t you place light receptors the other way round, so that the nerve connections protrude out behind the receptors, preventing any disruption of the signal entering the eye? It isn’t simply that this layout is the only one achievable. Octopuses have a camera eye just like ours, with the notable exception that they lack a blind spot. The camera eye evolved separately in the mammalian and cephalopod (octopus) lineages, and in octopuses the light receptors are positioned in a more logical way, with the nerve endings behind the retina

So why are our eyes backwards? Unfortunately, the exact evolutionary pathway that lead to the inverted mammalian eye is not fully understood yet, since it occurred so deep in evolutionary time (around 600 million years ago). Furthermore, as eyes are formed of soft tissue, they do not leave many hints in the fossil record for us to base our theories upon. Nevertheless, the mere fact that the mammalian eye is inverted at all is strong evidence that it was evolved rather than designed.

In the kingdom of the blind, the one-eyed man is king

The eye is often cited by creationists as an example of an organ far to complex to have ever evolved.  In 1802 Christian philosopher William Paley called it a miracle of design.  The argument generally goes that there is no logical series of small steps leading from no eye to an eye – what use would half an eye be? This argument has little basis on science, however. It is easy to imagine that in a world with no vision at all, any mutant individual possessing a single, simple, light sensitive cell, would have a significant advantage. This mutation would quickly spread. Further step-by-step increases in the number or sensitivity of these cells would improve vision and be selected for. Groups of photoreceptor cells containing a light sensitive protein called opsin, are known as eye spots, and have evolved independently at least 40 times. Clearly vision itself is not difficult to evolve.

However, a flat eyespot does not allow for the direction of light to be discerned to any high degree of accuracy. In a world populated by organisms with simple eyespots, a mutation causing a slight inward curvature of the eyespot would enable a slightly higher degree of accuracy in detecting directionality, and thus confer an advantage. Further increases in curvature would improve on this and at each stage a new mutant with a slightly more curved eyespot would have a selective advantage and thus the mutation would be passed on to the next generation. Pit eyes such as this are found in some species of snail and first appeared around 500 million years ago. Further increases in the curvature of the eye would eventually lead to the development of a pin-hole camera type eye, enabling shapes to be distinguished. These pin-hole camera eyes were then elaborated upon with lenses which allowed light to be focused onto the light sensitive cells in the retina.

Not only can a logical series of small steps leading from a single light-sensitive cell to the modern camera eye, each conferring a selective advantage of the previous, be easily postulated, but there are examples of each stage in extant organisms today. Far from being a hindrance to evolution, the eye actually provides evidence for evolution.

Articles in this Series:

1. Intro: Reasons Why Evolution is True
2. Part One: The Panda’s Thumb
3. Part Two: Parasitoid Wasps
4. Part Three: Ring Species
5. Part Four: Galapagos Finches
6. Part Five: The Quirky Human Eye
7. Part Six: Homology
8. Part Seven: Coevolution
9. Part Eight: PreCambrian Rabbits
10. Part Nine: DIY Evolution
11. Part Ten: Convergent Evolution

Want to Know More?

• Richard Dawkins demonstrates the evolution of the eye (Video)
• Lamb, Collin and Pugh (2007) The Evolution of the Vertebrate Eye: opsins, photoreceptors, retina and eye cup Nature Reviews Neuroscience (8) 960 – 976
• Evolution of the eye
• Darwin on the evolution of the eye
• Gehring (1996) The master control gene for morphogenesis and evolution of the eye Genes to Cells(1) 11 – 15
• The evolution of eyes Annual Review of Neuroscience(15) 1 – 29