Download Eye Evolution Eyes can provide information such as light direction

Eye Evolution
Eyes can provide information such as light direction and
contrast between light and dark, some eyes can form
focused images. Among multi-cellular animals there are
four main types of eyes, flat-sheet eyes, cup-shaped eyes,
vesicular eyes, and convex eyes. The structure and function
of eyes are to be able to detect the direction from which
light has entered the photoreceptive organ. Photoreception
is the ability to absorb light used as vision in animals.
Photoreceptors are cells that respond to light. All cells are
photosensitive. Plants absorb light and use it in
photosynthesis. Therefore to me this would give the
beginning to evidence photoreceptors have evolved. The
evolution of animal photoreceptor cells has been a
debatable question in science. With Charles Darwin
himself, the great English Naturalists, saying:
“To suppose that the eye with all its inimitable contrivances
for adjusting the focus to different distances, for admitting
different amounts of light, and for the correction of
spherical and chromatic aberration, could have been formed
by natural selection, seems, I freely confess, absurd in the
highest degree.”
Gave Creationist reason for their argument of Intelligent
Design being taught in schools. In this paper I will present
current evidence that photoreceptors have their origin in a
proto eye believed to have evolved some 540 million years
ago and the process only took a few million years thru
small mutations and natural selection.
Monophyly of all animal eyes is more widely accepted
based on shared anatomical and genetic features of all eyes.
At a molecular level the genes that control eye formation
are similar. The gene Pax6 that codes for a transcription
factor has been isolated from humans, mice, chickens, sea
urchins, and prosophila. Loss of function mutations in this
gene caused reduced absent eye structure in both vertebrate
and invertebrates. The involvement of Pax6, which
encoded transcription factors in the genetic control of eye
development in organisms ranging from planarians to
humans, argues strongly for a monophyletic origin of the
eye.
All light sensitive organs rely on photoreceptors systems.
Photoreceptive organs range in complexity from single
light sensitive cells to complex eyes that can form sharp
focused images. The structure of photoreceptor cells is
divided into two major types among the animals,
rhabdomic photoreceptors and ciliary photoreceptors. All
vertebrate photoreceptor cells are ciliary. Ciliary
photoreceptors have a single cilium protruding from a cell.
Ciliary photoreceptors are subdivided into two subclasses
rods and cones. Different mammalian have different rods
and cones based on different functionality. An example is
nocturnal mammals have a higher number of rods for better
vision in dim light.
Feature
Rods
Cones
Class of
Ciliary
Ciliary
photoreceptor
Shape
Sensitivity
Type of photo
Outer segment rod Outer segment
shaped
cone shaped
Sensitive to dim
Sensitive to bright
light
light
One type
Up to three types
pigment
A recent discovery I read in Principles of Animal
Physiology, Moyes & Schulte 2006, claimed that
rhabdomeric photoreceptors in some vertebrate pass
through a developmental stage in which they have cilia
suggest the possibility that all photoreceptor cells are
derived from an ancestral ciliated cell. Also the bilateral
ancestor of the Protostomes and Deuterostomes may have
already possessed two types of photoreceptors, one of
which may have been lost in some evolutionary linage.
W.J Gering (2005, Journal of Heredity) traced back the
evolution of the eye beyond bilaterians, he found highly
developed eyes in some box- jellies as well as in some
Hydrozoans. Hydrozoans have the same six genes required
for eye regeneration as in planarians, and in box-jellies,
Tripedalia.
Sight is obviously a huge survival advantage for a mobile
animal. Prey animals and competing predators alike would
be forced to rapidly match or exceed any such capabilities
to survive. Hence multiple eye types and subtypes
developed. Eye variations in animals show adaptation to
their requirements for survival, form to fit function. All
primates and many predators have binocular vision. This is
the ability to combine and compare information from each
eye to provide a three dimensional view of the world. An
example is convergent evolution of binocular vision in owls
and other binocular vision species. Owls, which have
excellent binocular vision, have eyes on the front of their
heads and a large binocular zone, but all of their optic
neurons cross at the optic chiasm. Owls use a different
mechanism to process the visual signal and provide
binocular vision. The two sides of the brain communicate
with the other in other parts of the visual pathway, allowing
both sides of the brain images from both eyes and
providing for the necessary condition for binocular vision.
In conclusion, tracing back the evolution of eyes leads
scientist to believe that the eye is a homologous organ
present in a wide variety of species. The structural
sequence of its’ evolution is evident on a molecular level
and thru small mutations because of natural selection.
Resources:
Arendt, Detlev. “ Evolution of Eyes and Photoreceptor Cell
Types.” Journal of Developmental Biology, 2003 47:563571.
Gerhrig, W.J. “New Perspective on Eye Development and
Evolution of Eyes and Photoreceptors.” Journal of
Heredity, 2005 96(3): 171-184.
Klanderf, Sherwood. Animal Physiology From Genes to
Organisms, 2005 Brooks/Cole division Thomson Learning.
Moyes, C & Schutte, P. Principles of Animal Physiology,
2006 Pearson Education, San Fran.