Wednesday, February 6, 2013

Evolution of the Human Eye

In honor of Mary Leakey's birthday I'm posting my final project for my Human Evolution class that I took Junior year of college. This class was taught by one of the Leakey's colleagues, Dr. Alan Walker. So here you have it a glimpse into some of what I worked on in my college career: The Evolution of the Human Eye.

            Scientists have worked hard to find more about the evolution of the human eye by comparing it to the eyes of other mammals, especially primates. During studies of the eyes and surrounding features of apes, scientists have discovered a trend in the evolution of the human eye. Many observations have been made during research projects about the evolution of the human eye. These observations include, but aren’t limited to: the whites of the eyes, convergence of the orbital bones, binocular vision, and color vision. These observations have lead scientists to believe that as evolution ran its course from primates to humans, it brought on the sophistication of the eye and its features.


White of the Eyes.
In modern humans the whites of the eyes help onlookers to tell where the person is looking. As humans, the whites of our eyes are much larger than those of other primates, such as chimpanzees who have a majority that don’t have any at all. This means that if someone were looking at a chimpanzee, they wouldn’t be able to tell what they were looking at. This is an example of the evolutionary process that allows humans to have the whites of the eyes, thus understanding those around us. It is a valuable skill to know where someone else is looking at so that one can survive in their environment (Tomasello, 2007).
The early humans used this technique frequently since the lines of communication were thin. They used this to detect food that another human has seen that would otherwise go unnoticed by him, allowing him to have the upper hand. It was also used to advertise good health to others, especially potential mates. The way that the whites helped advertise the health of the individual was it allowed the other parts of the eye to be visible, the color of the whites changed with illness. One could tell the size of the eyes, which was attractive to some. Another use of the white of the eyes was to tell what a potential threat was going to do next. The use of all these skills though required that the individual to be in a social environment with many others around, or it was useless (Tomasello, 2007).
To test this theory Michael Tomasello, the co-director of the Max Planck Institute for Evolutionary Anthropology, and his team of researchers conducted a study on infants and chimpanzees and their reaction to eye movement. They first took children around their first birthday, before they had any linguistic skills, and sat them in front of an adult. The adult would then look up at the ceilings with their eyes, and the child would also look up. Then the adult would look up to the ceiling this time moving their whole head, but this time the infants didn’t look up. Tomasello did the same experiment with chimpanzees. They sat the chimpanzee in front of an adult and they looked up with their eyes. The chimpanzee didn’t look up. Then the adult looked up with their head and the chimpanzee looked up. The research team then concluded that this ability to judge eye movement was an evolutionary trend that became necessary to survive in a social environment (Tomasello, 2007).
Out of this experiment Tomasello came up with the Cooperative Eye Hypothesis which states:  “Especially visible eyes made it easier to coordinate close-range collaborative activities in which discerning where the other was looking and perhaps what he/she was planning, benefited both participants” (Tomasello, 2007). So this skill was something that early humans had to have to survive in their environment, and if they didn’t have it they were quickly overtaken by those who did. This is an example of Darwin’s idea of the “survival of the fittest.”
Early humans used the whites of eyes as an alternative to verbal communications, so this question is then raised: Since modern humans can verbally communicate with each other, what is the need for the white of the eyes today?  The answer is there are many different ways that we use this skill today. In wars it is used to figure out what the opponent is going to do next, since they’re not going to come out and tell you. It is also used when doing activities together, such as construction, when verbal communication is not available. Perhaps the most useful is when children are born, and cannot yet speak, are very aware of the movement of the eye.
Orbital Convergence
            Just by looking at human skulls and primate skulls together, one can see a difference in the placement of the orbital bones. Scientists have many explanations for this occurrence, but there are two main theories. The first one is the correlation of orbital convergence and larger brain size, and the second is the need to adapt to the environment, especially to what they are looking at around them.
            As evolution took place among primates, the size of the brain started to increase as evolution occurred. With the increase of the size of the brain came the increase in the sophistication of the brain to analyze and interpret more and more. Along with this increase was the sophistication of sight and other senses. Silcox, Dalmyn, and Bloch at the National Academy of Science have conducted many tests on this theory. The most helpful was when they analyzed the number of neurons in the separate layers of the brain.
          The vertical axis shows the number of neurons in the brain, and the horizontal axis shows the orbital convergence. As the number of neurons increase, for most of the subjects the orbital convergence also increases. This shows that as the evolutionary cycle takes place, the neurons increase and along with does the orbital convergence (Silcox, Dalmyn, and Bloch, 2009).
            The second, which is a growing theory among scientists, is that the eyes got closer together because of the need to adapt to the environment. There became a need for stereoscopic and binocular vision when it came to foraging for food. For both of those things to happen the eyes needed to be closer together, thus orbital convergence came to be. Also the fact that the eyes were closer together opened the door for focus when participating in tool making since one could focus on what was in front of them. Also playing a big role in this environmental adaptation was the shift from nocturnal to diurnal living (Silcox et al, 2009).
            The figure below (Fig. 2) shows the normal range of vision of a primate. The 98° range on the diagram is what would be able to be seen directly in front of them. The right and left monocular fields are what would be able to be seen using peripheral vision.

                                  Figure 2
Binocular Vision
            Binocular vision is a skill that humans have that some other primates don’t have. Most people don’t even think about it, but whenever they are looking at something with both their eyes open they are using binocular vision. A Dictionary of Biology defines binocular vision as: the ability, found only in animals with forward-facing eyes, to produce a focused image of the same object simultaneously on the retinas of both eyes. This permits three-dimensional vision and contributes to distance judgment.
Objects that are held near the eyes require considerable vergence for binocular fusion to occur, and binocular disparity signals may have an additional function in the control of such vergence eye movements. This evolutionary trend was made possible by the convergence of the orbital bones and the growth in brain development. As everything else got more sophisticated so did the way that we see things (Silcox et al, 2009).

Color Vision
            The last trend in the evolution of the human eye is the ability to see multiple colors. As humans, we have trichomatic vision just like apes and Old World monkey, which means we see shades of blue, red and green. On the other hand New World monkeys, and several other monkeys like squirrel monkeys, marmosets, etc can only see shades of blue and red. The trichromatic vision comes from a combination of different genes. We all have autosomal genes that encode the blue-light sensitive pigment. We also have at least two X-linked genes that encode red-sensitive pigments as well as green-sensitive pigments (Shyue, Hewett-Emmett, Sperling, and Hunt, 1995).

                        Figure 3
            Figure three shows the evolution of color vision. The ancestors of vertebrates originally had genes that could distinguish four different colors, known as the four-color vision system. Later, this color system devolved to two-color vision, then again evolved to three-color vision through duplication and mutation (Satta and Go, 2006).
            This evolution of color vision has many different explanations to why we need trichomatic vision. Dinosaurs went extinct some 65 mya and the nocturnal ancestors of mammals returned to daylight activity. The genes that these animals had for seeing red and blue shade duplicated and then mutated 30 mya. This gave rise to the green-type gene capable of perceiving green light. Slowly the three-color vision system developed. Along with that, the mammals with the ability to see color gained dominance in the jungle because the skill of being able to distinguish colors is important for diurnal animals. This was when our ancestors gained the ability to see color as we enjoy it today (Satta and Go, 2006).

          It is easy to see that humans have come a long way from their ancestors when it comes to the function of the eye. The sophistication of the eye has correlations with adaptation to the environment, brain size, and communication within social settings. These trends of the whites of the eyes, orbital convergence, binocular vision, and color vision that have been discussed in this paper are all essential in showing the evolution of the human eye. By doing many studies and comparing the eyes of humans to those of primates scientists have pinned down these trends and have figured out how essential they are for both early and modern human survival.

            Olshausen BA and, Field DJ. Vision and the coding of natural images. Am Sci 88(3):238 – 246.
Satta Y, Go Y. 2006. The ability to see and fall in love – the history of biological evolution and devolution. The Story of Light and People. Harvard.
Shyue, S, Hewett-Emmett, D, Sperling, HG, Hunt, DM, et al. 1995 Adaptive evolution of color vision genes in higher primates. Science 269(5228): 1265-1268.
 Silcox MT, Dalmyn CK, and Bloch JI. 2009. Virtual endocast of Ignacius graybullianus (Paromomyidae, Primates) and brain evolution in early primates. Proc Natl Acad Sci USA  106:10987-10992.
            Tomasello M. 2007. For human eyes only; [Op-Ed]. New York Times A15.


Post a Comment