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Evolution Of The Human Eye Essay

5715 words - 23 pages

The human eye is an incredibly complex organ, it acts like a camera to collect and focus light and changes this light into an electrical signals that the brain then reads to form images. Instead of using film, the eye has specialized components such as the retina that detects light and process it using many types of neurons. The eye is so complex that its origin has long been a hot topic of debate and is often used to defend a creationist point of view, they say the eye is too complex to have evolved instead it must has been designed. The eye is a system that cannot function in the absence of any of its components. Even Charles Darwin remarked in the origin of species that ‘it might seem ...view middle of the document...

Compound vs camera

During the Cambrian explosion two radically different eyes arose. The first eye was the compound eye, in this eye, thousands of individual photoreceptor units work together to form a wide view image with moderate resolution or clarity. (s Nilsson. D) However, these eyes are not suited to large animals since it would be overly large, so as body size increased selective pressures began to favor a compact eye, the camera style eye. (Pitman. S.) This eye has photoreceptor cells which share a single lens, they are arranged as a sheet that lines the inside of the eye called the retina. (Lamb. T) There are two types of photoreceptors in jawed vertebrates, which include humans, cones, which enable daylight vision, and rods, which enable nighttime vision.

So where does this eye come from?

All key features of the eye are the same across all eyes of jawed vertebrates, of which humans are one. This suggests that this animal group inherited the features from a common ancestor and that the eye had already evolved to this stage 450 million years ago. Going back further the lamprey is one of the few surviving descendants of the jawless vertebrates. (Lamb. T) This fish also has a camera style eye complete with three layers and a lens, similar to our own eye. (Hardman. H) This indicates that an eye similar to our own must have been present in the common ancestor in both jawless and jawed vertebrates living about 500 million years ago. Clues to this an even earlier eye come from the hagfish another jawless fish. The hagfish eye lacks a lens and is buried deep beneath translucent skin. Importantly it has a 2-layered retina lacking the bipolar cells found in the human eye which process information from the photoreceptors. (Lamb. T) This means hagfish’s photoreceptor cells must connect directly to the sensory out-put neurons suggesting its eye may actually affect behavior and daily and seasonal rhythms, much like the human pineal gland, rather than playing a part in vision. According to Trevor D. Lamb ‘the ancestral eye of proto-vertebrates first served as a non visual organ and only later evolved the components needed for spatial vision’. Supporting this evolutionary idea, early in human embryological development the human eye is remarkable similar to that of the hagfish, then with further development takes on similarities to the lamprey eye before reaching full development into the sophisticated organ of the human.

The shape of the eye.

The shape of the eye took the classic C-shape in a remarkably short amount of time, around 500 million years. The eye began as a simple cluster of light sensory cells commonly know as an eye spot, these eye spots were only able to distinguish between light and dark and not the direction of light. (Pitman. S) Gradually selective pressures caused the spot to form cups, this slight curve allowed the direction of light to be known and therefore the direction of a predator/prey, an obvious...

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