Evolution Diary

Ian Musgrave wrote this article on the “bad design” of the vertebrate eyes versus the better invertebrates eyes (cephalopod such as octopus, squid etc).

Just to recap, vertebrates (like ourselves), and the invertebrates Squid and Octopi have “camera eyes“. They differ in how the photoreceptors in the retina, the part of the eye that receives the image, is wired up to the brain. The vertebrate wiring system is often cited as an example of “bad”, or at least quirky, design that is explainable by evolution.

The vertebrate retina is wired “backwards”. That is the photoreceptors point to back of the retina, away from incoming light, and the nerves and blood vessels are on the side of the incoming light, this means that any image formed on the vertebrate retina has to pass though layers of blood vessels and ganglion cells, absorbing and distorting the image.

To get decent visual acuity, vertebrates must focus light on a small patch of retina where the blood vessels and nerves have been pushed aside, the fovea. This patch must be small because of the nutrient requirements of the retina. Also, the construction of the vertebrate retina means that blood vessels and nerves must pass through the retina, creating a “blind spot”, where no image is formed. Finally, the “backwards” retina means that vertebrates have a high risk of retinal detachment. Altogether this shows that having the nerves and blood vessels in front of the photoreceptors is less than optimal design.

Click to read the article