Our flawed retinas

Mastiff looking up

In one of the later chapters of Richard Dawkins’ The Greatest Show on Earth, he discusses a couple of curious characteristics of the sort of spherical lensed eyes that humans and other animals possess. Specifically, he describes how the human retina is essentially ‘backwards,’ with the light sensing components at the back and the nerves conveying the information in front of them. Because the nerves are in front, they need to assemble somehow and get through the retina, which they do by means of the blind spot (mentioned before). It would surely be more sensible to have the light sensors at the front, with an unimpeded view and the data-carrying nerves behind. The problems associated with the reversed arrangement are largely compensated for by the ways in which our brains process the information from our eyes; among other things, you need to use a test like the one in the post linked above to be able to see that you even have them.

Dawkins is pretty convincing in arguing that this is not the sort of ‘intelligent design’ we might expect if the spherical lensed eye was something consciously created by an intelligent being. Rather, the roundabout organization is the product of sexual selection and chance mutations. Like the corrections made to the flawed mirror on the Hubble Space Telescope, the ‘software’ processing of human vision allows for some of the problems associated with the physiology of our eyes to be mitigated.

When it comes to blind spots, cephalopods like squid, octopodes, and cuttlefish are one up on us. Their eyes have no blind spots (since the photo receptors are in the front), and are also sensitive to the polarization of light. Perhaps one day people will splice squid genes into their children, in exchange for improved vision.

Author: Milan

In the spring of 2005, I graduated from the University of British Columbia with a degree in International Relations and a general focus in the area of environmental politics. In the fall of 2005, I began reading for an M.Phil in IR at Wadham College, Oxford. Outside school, I am very interested in photography, writing, and the outdoors. I am writing this blog to keep in touch with friends and family around the world, provide a more personal view of graduate student life in Oxford, and pass on some lessons I've learned here.

9 thoughts on “Our flawed retinas”

  1. Perhaps one day people will splice squid genes into their children, in exchange for improved vision.

    Sounds Lovecraftian!

  2. Proof that goats cannot be trusted

    “Goat Eye Syndrome is characterized by eyes afflicted with horrific horizontal pupils similar to those of cephalopods such as octopi, squid, or cuttlefish. The pupils of these beasts are approximately the shape of a kidney bean, but instead of dividing the eye vertically, in the manner of noble, trustworthy beasts such as tigers, bobcats, and snakes, the GES pupils transfix the eye horizontally. This is disgusting. The only other type of animal to display such disgusting, vomit-inducing eyes are the previously mentioned cephalopods (which have a long association with death from the murky depths and Cthulhu) and Kermit the Frog, who is a felt puppet created by Jim Henson, and should not be considered an example of an accurate representation of frog physiology.”

  3. “Squid, along with the rest of the family Cephalopoda, haven’t shared a common ancestor with us vertebrates in some 500 million years—long before the evolution of our camera-like eyes. And yet, there the cephalopods are, flagrantly swimming about with eyes that use a lens to project an image onto a retina. Call it Squid Eye for the Vertebrate Guy. So, how’s it work?

    Convergent evolution, my friends. Convergent evolution. We happened to hit on similar solutions to the same problem of sight, even though the eyes of vertebrates and cephalopods evolved separately, in very different ways, at different times. Today, we can see that legacy in cephalopod and vertebrate fetal development. With vertebrates, the eyes grow on stalks, reaching out from the brain. In cephalopods, the eyes start as a clumping of cells on the surface of the skin and reach backwards, into the head, to make brain contact. Similar destinations. Very different road maps.”

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