Evolution of The Eye - Evolutionary Baggage

Evolutionary Baggage

Vertebrate Octopus Vertebrates and octopuses developed the camera eye independently. In the vertebrate version the nerve fibers pass in front of the retina, and there is a blind spot where the nerves pass through the retina. In the vertebrate example, 4 represents the blind spot, which is notably absent from the octopus eye. In vertebrates, 1 represents the retina and 2 is the nerve fibers, including the optic nerve (3), whereas in the octopus eye, 1 and 2 represent the nerve fibers and retina respectively.

The eyes of many taxa record their evolutionary history in their imperfect design. The vertebrate eye, for instance, is built "backwards and upside down", requiring "photons of light to travel through the cornea, lens, aqueous fluid, blood vessels, ganglion cells, amacrine cells, horizontal cells, and bipolar cells before they reach the light-sensitive rods and cones that transduce the light signal into neural impulses, which are then sent to the visual cortex at the back of the brain for processing into meaningful patterns." While such construct seems to have drawbacks, it allows the outer retina of the vertebrates to sustain higher metabolic activities as compared to the non-inverted design. It also allowed for the evolution of the choroid layer, including the retinal pigment epithelial (RPE) cells, which play an important role in protecting the photoreceptive cells from photo-oxidative damage.

The camera eyes of cephalopods, in contrast, are constructed the "right way out", with the nerves attached to the rear of the retina. This means that they do not have a blind spot. This difference may be accounted for by the origins of eyes; in cephalopods they develop as an invagination of the head surface whereas in vertebrates they originate as an extension of the brain.