Sighted organisms employ a wide variety of different designs for their visual system.  But each system functions only if all its essential components are intact, without a trace of evolution.  There is also evidence of the fall of man.  Though neo-Darwinian evolution is not seen, evidence of “devolution” and degradation is abundant.  Some animals have degraded or absent vision whereas this was not true of their ancestors.  But the eyes we observe in healthy animals are clearly designed, each with certain specialty features for its environment.  This is what we expect of the handiwork of God.

We previously explored the differences between the inverted retina of the human eye, and the verted retina of cephalopods.  We found that each system was well-designed for the typical environment in which the creature lives.  This prompt us to ask what other types of eyes the Lord has created.  From a biblical creation perspective, we expect to find both similarities (due to having a common Creator who is a God of order) and differences (due to the Lord’s creativity) in the eyes of different varieties of animals.  Furthermore, we would expect each type of eye to be well-suited for the organism in its environment, and irreducibly complex.  And this is indeed exactly what we find.

Variations in the Fovea

In humans, we have a fovea in our retina at the center of our vision that has a high density of cones and no rods, supplying us with very sharp images of anything we look at directly.  The fovea is approximately circular.  However, a cheetah’s fovea is horizontally elongated, giving the animal an enhanced horizontal range of clarity, at the expense of vertical range.  It is the perfect arrangement for an animal that scans the wide African plains for hunting opportunities.

Many species of diurnal birds actually have two foveae in each eye!  Examples include hawks, eagles, falcons, and hummingbirds.  One fovea is located at roughly the center of the field of vision, much like ours, and is called the shallow fovea.  The other is located about 45 degrees to the side and is called the deep fovea.  It is deeper and has more cones per unit area than the shallow fovea.

Thus, an eagle has three spots of exceptionally high visual acuity in its field of view, in contrast to our single spot.  One is dead ahead, just like ours.  Since both eyes see this spot in high acuity, the image is 3D stereoscopic.  But the eagle also has high visual acuity at a spot 45 degrees to the left using its left eye, and 45 degrees to the right using its right eye.  These are not stereoscopic views, but have extremely high acuity.  The density of cones in the eagle’s deep fovea is about five times that of a human fovea.  Eagles therefore have greater visual acuity than humans.  They can see at 20 feet what a human can see at 5 feet.  Unlike humans, most diurnal birds have far more cones than rods.  And many birds are tetrachromats: having four types of cones and experiencing a 4-dimensional color space, in contrast to our 3-cone system.

Night Vision

The situation is different for nocturnal birds.  An owl has (at most) only one fovea per eye, located near the center of the retina much like ours.[1]  Unlike diurnal birds, the owl’s retina has a much larger number of rods than cones.  Even the fovea of the owl is dominated by rods which are completely absent in the human fovea.  This makes sense because owls are nocturnal creatures, and rods are much more sensitive to light than cones.  This is part of the reason why owls have such good night vision.

Other design features include the shape and size of the eyes.  Owls have eyes that are very large in proportion to their body, and are more cylindrical than round.  This maximizes the amount of light that enters the eye, but it also means that owls cannot rotate their eyes; they must turn their head to change the direction of their gaze.  Of course, the Lord equipped the owl with a neck that has an extraordinary turning range – up to 270 degrees in either direction!  This is another example of how irreducible complexity involves not only individual organs, but the relationships between them.  The extremely flexible neck of the owl would be unnecessary if it had “normal” rotating eyes; but its non-rotating eyes might be a disadvantage without the extraordinary neck.  Each part requires the other to have “evolved” first in order to be helpful for survival – unless of course the Lord created this animal with genetic information for all these co-dependent traits.

The forward-facing position of the eyes gives the owl a field of view of about 110 degrees, with 70 degrees of overlap between the two eyes, allowing for stereoscopic binocular vision.  Humans, on the other hand, have about a 180-degree field of view with about 140 degrees of stereoscopic binocular vision.  But then again, we can’t turn our neck as far as an owl.  God equipped each organism with traits useful for its survival.

Owls possess another design feature common to nocturnal animals: the tapetum lucidum.  This is a reflective surface that lies at the base of the retina.  It reflects back any light that missed the photoreceptors, giving the light a “second chance” to be detected.  This increases the retina’s sensitivity to light by nearly 50%, at the slight expense of visual acuity.  It has been estimated that an owl can detect light nearly one hundred times fainter than a human can detect.  Many other animals such as cats also have a tapetum.  This is why at night a cat’s eyes appear to shine when illuminated by a bright light.

Cats and owls are two very different animals.  Is it reasonable to suggest that they both evolved precisely the same mechanism to increase light sensitivity?  Many other nocturnal animals also possess a tapetum.  It makes perfect sense for the Lord to give these creatures the same mechanism for enhanced night vision since they are nocturnal.

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