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.
ABSTRACT
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.
INTRODUCTION
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).
CONCLUSION
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.
LITERATURE CITED
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.
