The Eye
A
The eye is one of the most complex organs in the human body, with a
structure and design that is uniquely suited to its sole purpose of
facilitating sight. Both the complexity and the functionality of the eye are a testament
to the wonder of evolution, through which this organ developed from the basic
sensory capabilities of our evolutionary ancestors. Although we often take our
eyesight for granted—until something goes wrong with it—it is important to
consider how this organ works, how it evolved, and what distinguishes the human
eye from the multitude of animal varieties that exist.
B
The eye works through a process which is very similar to a camera,
in that it uses the light rays which bounce off objects to perceive those
objects. These light rays enter the eyes through its transparent outer covering,
the cornea, before passing through the pupil, the round hole at the centre of
the eye. The
size of the pupil is controlled by the iris, which makes it bigger or smaller
to let more or less light in, so that it has the optimum amount to process
vision. The iris, which also determines eye colour, is very important in
protecting the retina from too much light. The lens then focuses the light rays
onto a point on the inner surface of the retina at the back of the eye. The
lens changes shape depending on where in the field of vision the eye is
focusing; if it’s an object in the far distance the lens will be stretched
thin, and if it’s a closer object the lens will be squished into a fatter
shape.
C
The retina itself acts like the film in a
camera, using the light to capture the image. It is covered in millions of
highly sensitive receptors known as rods and cones. The rod cells are far more numerous than
the cone cells, and are used in peripheral vision. They
can also function in much less intense light than the cone cells, which are
responsible for colour vision and work best in brighter light. Both the rods and cones contain what are known as pigment molecules,
which change shape when light hits them. When this happens a signal is sent to
the brain via the optic nerves.
D
This incredibly complex process thus
requires the coordination of various elements to work, with each playing an
essential part in facilitating sight. The elegance of its design would seem to
suggest that the eye emerged fully formed, or was planned by an intelligent
designer, but the process by which it reached this point was convoluted. It is thought
that the eye originated in simple photoreceptor cells which were sensitive to
light and could therefore guide creatures towards brightly lit areas where food
was likely to be found. These
could be found on small worm-like creatures in the Ediacaran period, around 550
million years ago, which according to the fossil record were able to move
towards food and away from prey.
E
It was not until the Cambrian explosion,
between 545 million and 530 million years ago, that evolution started progressing
beyond that stage. This
period saw an exponential increase in the rate of development, and many
scientists believe that this is because the first real eyes evolved around that
time, giving certain species the immense advantage of sight. Once the eye emerged, initially on trilobites with insect-like
compound eyes, the rate of development was staggering, prompted by what some have
called an arms race between predator and prey. The key development during this period was
the reduction of the light slit, which allowed for more accurate directional insight. This structure of eye can still be found
on the nautilus, a marine mollusk considered a ‘living fossil’ due to these
traits. The cornea then emerged,
which allowed the eye to develop the lens, along with a number of other advances
including photoreceptors. Around 388 million years ago the first evidence of
colour vision can be found in fish eyes with rods, cones and the modern day
camera-like eye of all vertebrates.
F
Although this evolutionary trajectory can appear in hindsight to be
the only one available, variations within the animal world show how evolution
can diverge. The compound eye of many
insects, for example, contains hundreds of light-sensing units, each of which
has its own lens and set of light receptor cells, although they are unable to
detect colours to the degree humans can. These eyes can detect very fast
movement and have a larger viewing angle than a human, but require a much bigger
proportion of space. If humans had compound
eyes they would have to have a radius of over 30 feet to match the resolution
of our eyes. On the other hand, the eyes of cephalopods such as the octopus,
squid and cuttlefish are remarkably similar to our camera-type eyes, although
they have several unique adjustments. One of these is strangely shaped pupils,
most notably the cuttlefish’s W-shaped pupils and the octopus’s dumbbell-shaped
pupils. These have developed as means of detecting colour and thus make up for
the cephalopods’ lack of light receptors. These creatures reveal how our eyes
might have developed under different conditions or pressures, and are therefore
a further testament to the power of evolution in striving for the continual
perfection of the eye.
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