Evolution and the Poorly Designed Human Eye

The human eye is marvelous but also very poorly designed. The poor design is evidence against intelligent design and in favor of the “unguided, unplanned, messy, quirky, and historically contingent” process of evolutionary design. A short piece from 2008, Suboptimal Optics: Vision Problems as Scars of Evolutionary History, does a nice job explaining.

Most well known is that the wiring is backwards.

The most obvious design flaw of the retina is that the cellular layers are backwards. Light has to travel through multiple layers in order to get to the rods and cones that act as the photoreceptors. There is no functional reason for this arrangement—it is purely quirky and contingent.

Even in a healthy and normally functioning eye, this arrangement causes problems. Because the nerve fibers coming from the rods and cones need to come together as the optic nerve, which then has to travel back to the brain, there needs to be a hole in the retina through which the optic nerve can travel. This hole creates a blind spot in each eye. Our brains compensate for this blind spot so that we normally do not perceive it—but it is there.

From a practical point of view, this is a minor compromise to visual function, but it is completely unnecessary. If the rods and cones were simply turned around so that their cell bodies and axons were behind them (oriented to the direction of light), then there would be no need for a blind spot at all.

Cephalopod’s like octopuses took a slightly different evolutionary path and have a better design:

But the reversal of the wiring isn’t the only design flaw.

The arrangement of the extraocular muscles—the muscles that move the eyes—is also difficult to explain without appealing to evolutionary contingency. There are more muscles than are minimally necessary and yet there is no functional redundancy. In order to move a sphere in any direction, only three muscles would be necessary, evenly spaced like the legs of a tripod. The human eye has six—the superior, inferior, lateral, and medial rectus, and the superior and inferior oblique. And yet, despite the extra three muscles, the loss of function of any one muscle causes an impairment of eye movement and results in double vision or displaced vision. A more frugal design with only three muscles would be more efficient and less prone to malfunction, as there are fewer components to break down.

If the eye were to be designed with more than the minimal three muscles, then it would make sense to arrange the muscles so that the loss of one or even more would not impair eye movement.

Read the whole thing.

Hat tip: Paul Kedrosky.


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