Chapter 9: Vision

Shubin began this chapter by comparing the history of the eye to that of a car, where we can trace its history through the different parts present. He specified that the eyes capture light by using sensitive light-gathering cells. These cells allow the retina to absorb light, and, thus, allow us to see. I was intrigued that we had two types of these cells: one that is very sensitive (black and white vision), and one that is less sensitive (color vision). By finding the percentage of each type of light-sensing cell in an animal's eye, we can determine whether they are specialized for daylight or night...and 70 percent of the sensory cells in our body are these light-gathering cells!! That allowed me to see how much we rely on our vision for survival. Since we use these cells everyday, this made me want to know how they work...so I found it extremely intereseting how the molecules that collect light divide into two parts: one derived from Vitamin A and the other from a protein called opsin (we use different opsins to see in black and white or in color). For our vision, we need three light-gathering molecules for color vision, whereas, we only need one of these for black and white vision. These molecules change shape in light, which is why it takes awhile for us to see when we go from some place extremely bright to some place darker. Opsins use the same receptor path as seen in parts of certain molecules in bacteria! So, in a sense, we can see because of these specific molecules in bacteria. Color vision developed in a similar way as which our odor-receptor genes developed...that is, one of the genes in other mammals duplicated and copies specialized overtime for different light sources...so our sight and sense of smell have more similarities than I originally perceived. Scientists believe the switch to color vision correlates to the switch from one forest to another forest with more colorful food. Scientists also found that animal eyes have a change in their tissue formation (vertebrate vs. invertebrate). The eyeless gene, or Pax 6, was found to control the development of eyes. This gene, when mutated, can result in small eyes, or no eyes at all. And when this gene is turned on in different parts of the body, an eye is developed there! As a result, I learned that Pax 6 controls the development in everything that has eyes....and even if eyes are different, the genetic switches that make them are the same.