The Ecology of Humans (15-26) Evolution of Sight continued 5


The last known ancestor shared between humans and octopi might have been a worm of some sort; but humans and octopi have eyes that bear more than passing resemblance. Both roughly resemble a camera, in having a single lens at front, with a light-sensitive sheet – the retina – at the rear. The only reasonable conclusion, based upon a myriad of data relating to development and structural details, is that octopus and human eyes evolved independently, but converged to similar solutions. Similar, but with significant differences. Octopus eyes are more straightforward. The light-sensitive receptors point out to light, with neural pathways running to the optic nerve.

Human eyes have the receptors at the back of the neural cluster, requiring light to pass through a neuronal mesh to reach rods and cones. This arrangement makes the optic nerve a blind spot.

It is difficult to account for the convoluted arrangement of the human eye. While retinal tissues are largely transparent, various nerve cells in front of the rods and cones obstruct incoming photons, changing the wave energy. There is no evidence that this is some sort of sublimation process, such as acting as a wave guide. There is no geometric precision in the arrangement of the cells in front of the rods and cones.

Though not a primary photoreceptor, ganglion cells are sensitive to light level. There may be some optimality in the ganglion layer getting an account of overall brightness before the residual photons terminally deposit themselves in rods and cones; but that is purely speculative.

The human retina is an integrated cellular construct. The light reception cells are embedded within a support structure that has an ample blood supply immediately available. This arrangement supports a continuous regeneration of photosensitive pigments, facilitating rapid adjustment to changing light conditions.

The human eye is the most energetic organ in the body, consuming, on a per-gram basis, more oxygen than the brain. Sustaining such a high metabolic rate is likely beyond the capability of an octopus eye. But then, octopi live underwater, at lower light intensity, and probably do not have to maintain a high photo-pigment turnover.

While depth perception is had by many cues, binocular vision improves depth perception. Many organisms, especially predators, have binocular vision.

Conversely, maximizing field of view is valuable for potential prey. Hence monocular vision, where each eye may provide separate viewing. The mind creates panoramic images, though without sharp focus. Mammals subject to predation, such as rabbits and horses, have monocular vision.

Most birds have monocular sight. But predatory birds, such as owls and hawks, have binocular vision.

Sight is more than mental images, which come in dreams, as well as waking daydreams, without external input. Is sight limited to vision: visual images from the eyes to the mind-brain? Are eyes necessary for sight? In other words, is sight necessarily form and function, or does image processing via different reception qualify as sight? This is not just a semantic issue.

Numerous species hide themselves in plain sight. Is generating camouflage a product of sight? Cephalopods create camouflage effects beyond their visual capabilities. A plant vigorously responds to light. Is it blind? Is echolocation sight, or is it hearing?