Like other primates, humans are visual animals;that is, like other apes we have stereoscopic vision (this gives us depth perception) and color vision. Both of these traits provide survival value for tree-dwelling animals. In the brain the incoming stream of messages coming infrom the eyes is initially processed in the occipital lobe in the back of the brain. The information is then sent through a series of brain regions where it is finally analyzed and given meaning in the frontal lobes.
Our eyes are sense organs. Biological anthropologist Desmond Morris, in his book The Naked Woman: A Study of the Female Body, writes:
“The eyes are the dominant sense organs of the human body. It has been estimated that 80 per cent of our information about the outside world enters through these remarkable structures. Despite all the talking and listening we do we remain essentially visual animals.”
Human eyes, however, are different from those of other animals: the whites of the eyes are visible. In his book Lone Survivors: How We Came to be the Only Humans on Earth, Chris Stringer writes:
“For example, in most of the primates—and probably our ancient African ancestors—the outer covering of the eyeball, the sclera, is dark brown. This means that the pupil and iris in the center of the eye, which move to focus the gaze, are difficult to differentiate from the surrounding tissue, especially where they are dark. But humans have an enlarged, unpigmented, and therefore white sclera, which means we can detect where other people are looking; equally, they can detect where we are looking. This must have evolved as part of the development of our social signaling, enabling us to ‘mind-read’ each other.”
In her book The Human Brain, Rita Carter reports:
“Unlike in most species the visible white of the human eye makes it easy to see in which direction a person is looking and thus where their attention is directed. People have a strong instinct to follow another person’s eye gaze, and this simple mechanism ensures that when someone is in sight of another person, they can manipulate each other’s attention and share information without ever having to communicate with words.”
Desmond Morris writes:
“It is this non-optical part of the human eye that is its most unusual feature. Uniquely, parts of the whites of our eyes are visible to onlookers.”
Thomas Suddendorf reports:
“The iris of our eyes is relatively small and surrounded by white rather than dark sclera, making it easy for us to identify the direction of another’s gaze.”
Thomas Suddendorf also reports:
“When you stare into the eyes of chimpanzees, one thing you will notice is that their eyes do not show any white.”
In his book A Troublesome Inheritance: Genes, Race and Human History, Nicholas Wade writes:
“One of the strangest features of human anatomy, when people are compared with the other 200 monkey and ape species in the primate family, is the sclera, or the white of the eye. In all our primate cousins, the sclera is barely visible. In humans it stands out like a beacon, signaling to any observer the direction of the person’s gaze and hence what thoughts may be on their mind.”
The whites of the eyes have enormous implications for human behavior. Thomas Suddendorf writes:
“Human eyes signal gaze direction. We advertise where we look, and we read where others are looking.”
This means that humans are able to read the intentions of other humans, a type of mind-reading without words. Nicholas Wade puts it this way:
“The whites of the eyes are the mark of a highly social, highly cooperative species whose success depends on the sharing of thoughts and intentions.”
Desmond Morris writes:
“The effect of this small evolutionary change is that during social encounters shifts in gaze direction are easily detected, even at a distance.”
The eyes of Homo sapiens are somewhat different from those of one of extinct relatives, Homo neandertalensis. Neanderthal fossils show two distinct differences: first, the eye sockets are about twenty percent larger than those of modern humans, and, second, they have a large occipital “bun” for processing visual information. The occipital “bun” suggests that they had a well-developed visual system. In his book Human Evolution, Robin Dunbar writes:
“Might it be that they developed an unusually large visual system to cope with living under low light regimes at high latitudes. And if so, was their large brain disproportionately devoted to vision, and hence less well endowed with those regions at the front of the brain that are so important for social cognition?”
The trade-off that is suggested here is that Neanderthals had reduced social cognition compared to modern humans, and thus lived in smaller communities. The evolutionary path that led to modern humans selected for social cognition in the form of larger frontal lobes and the importance of the whites of the eyes in interpersonal communication.
Human Origins
Human Origins: Making Spoken Language Possible
Human Origins: Teeth
Human Origins: Humans as naked apes
Human Origins: Sex
Human Origins: Menopause
Human Origins: Bipedalism
Human Origins: The Human Hand
Human Origins: The Large Brain