Re: How or why did the human eye evolve?

How did the human eye evolve?

There are two ways to approach this question, and so I’ll try to give you 
a brief outline of both. A more detailed response would require a more 
directed question.

Because the eye is soft tissue it does not fossilize. Therefore, we can 
only make statements about eye evolution through inference from 
comparative anatomy. Comparative anatomy that the eye does not show any 
major differences across the vertebrate phylum. Human eyes are basically 
the same as dog eyes, fish eyes and cow eyes (we use cow eyes in my human 
anatomy class in lieu of human eyes for dissection precisely because there 
are no important differences between them except that the cow eye is 
bigger). Therefore, in terms of human evolution, we would conclude that 
there really has been no important evolution of the eye. In this case, you 
would want to explore the evolution of the eye itself. The “seeing organ” 
has evolved at least 11 different times in the history of life on earth. 
The evolution of the eye is basically a transition from photosensitive 
cells to an organ that specializes in focusing light rays and interpreting 
their images. More information on this concept can be found in textbooks 
of Zoology and Comparative Anatomy. Two that I like are:

Hickman, CP, Roberts, LS and Hickman, FM (1984) Integrated Principles of 
Zoology. Times Mirror: St. Louis

Romer, AS and Parsons, TS (1986) The Vertebrate Body. Saunders Publishing: 
Philadelphia

A discussion of the evolution of the eye specific to human evolution must 
first start with an examination of how the human eye differs from that of 
other primates. This examination will show that humans differ primarily in 
the fact that they show more variation in eye color (pigmentation of the 
iris). You could therefore conclude that that evolution of the human eye 
is an investigation of eye color variation. Brown is the most common human 
eye color, and that is the only eye color you see in most primates. So, 
blue, green and hazel eye colors are human innovations. You will also note 
that these lighter eye colors are more common in fair skinned people (but 
not East Asian people). What is going on here? It appears that light eye 
color is less sensitive to glare, which could be an advantage to people 
living in northern climates. East Asian people have an epicanthic eye fold 
that seems to accomplish much the same result. Therefore, it makes sense 
that East Asian people would not have light colored eyes  More information 
about this concept can be found in textbooks of human variation and human 
genetics. Two that I like are:

Underwood, JH (1979) Human Variation and Human Micro-Evolution. Prentice-
Hall: Englewood Cliffs, NJ

Hartl, DL (1983) Human Genetics. Harper & Row: New York.