New York Times
January 13, 2007
For Human Eyes Only
By MICHAEL TOMASELLO
COL. WILLIAM PRESCOTT is said to have prepared
his troops for a charge from the British Army at
the Battle of Bunker Hill by telling his men,
ďDonít one of you fire until you see the whites of their eyes.Ē
If the opposing army had not been British men but
rather a horde of charging chimpanzees, the
American troops would have been summarily
overrun. Why? Because neither chimpanzees nor any
of the other 220 species of nonhuman primates
have whites of the eyes, at least not that can be
easily seen. This means that if their eyes are
looking in a direction other than the one in
which their heads are pointing, we can easily be
fooled about what they are looking at.
Why should humans be so different? And yet we
are. We canít fool anyone. The whites of our eyes
are several times larger than those of other
primates, which makes it much easier to see where
the eyes, as opposed to the head, are pointed.
Trying to explain this trait leads us into one of
the deepest and most controversial topics in the
modern study of human evolution: the evolution of cooperation.
The idea is simple. Knowing what another person
is looking at provides valuable information about
what she is thinking and feeling, and what she
might do next. Even young children know that when
a person is looking at one toy and not another,
she most likely prefers that toy and may reach
for it. Professional poker players are often so
worried about others reading their minds by
reading their eyes that they wear sunglasses.
Evolutionarily, it is easy to see why it is to
your advantage to be able to tell with maximum
certainty where I am looking. You may use this
information to detect food you wouldnít otherwise
have seen, or to detect the dominant male approaching in a fighting mood.
But evolution cannot select the color of my eyes
based on advantages to you. Evolutionary theory
tells us that, in general, the only individuals
who are around today are those whose ancestors
did things that were beneficial to their own
survival and reproduction. If I have eyes whose
direction is especially easy to follow, it must be of some advantage to me.
If I am, in effect, advertising the direction of
my eyes, I must be in a social environment full
of others who are not often inclined to take
advantage of this to my detriment ≠ by, say,
beating me to the food or escaping aggression
before me. Indeed, I must be in a cooperative
social environment in which others following the
direction of my eyes somehow benefits me.
Of course, itís possible that having large whites
of the eyes serves some other purpose, like
enabling me to advertise my good health to
potential mates. But such an advantage would
apply to other primates as well. Cooperation, on
the other hand, singles out humans, as humans
coordinate activities to do such things as
construct buildings, create social institutions
and even, paradoxically, organize armies for war.
In a recent experiment, our research team has
shown that even infants ≠ at around their first
birthdays, before language acquisition has begun
≠ tend to follow the direction of another
personís eyes, not their heads. Thus, when an
adult looked to the ceiling with her eyes only,
head remaining straight ahead, infants looked to
the ceiling in turn. However, when the adult
closed her eyes and pointed her head to the
ceiling, infants did not very often follow.
Our nearest primate relatives, the African great
apes (chimpanzees, bonobos and gorillas) showed
precisely the opposite pattern of gaze following.
When the human pointed her eyes only to the
ceiling (head remaining straight ahead), they
followed only rarely. But when she pointed her
head only (eyes closed) to the ceiling, they followed much more often.
It has been repeatedly demonstrated that all
great apes, including humans, follow the gaze
direction of others. But in previous studies the
head and eyes were always pointed in the same
direction. Only when we made the head and eyes
point in different directions did we find a
species difference: humans are sensitive to the
direction of the eyes specifically in a way that
our nearest primate relatives are not. This is
the first demonstration of an actual behavioral
function for humansí uniquely visible eyes.
Why might it have been advantageous for some
early humans to advertise their eye direction in
a way that enabled others to determine what they
were looking at more easily? One possible answer,
what we have called the cooperative eye
hypothesis, is that especially visible eyes made
it easier to coordinate close-range collaborative
activities in which discerning where the other
was looking and perhaps what she was planning, benefited both participants.
If we are gathering berries to share, with one of
us pulling down a branch and the other harvesting
the fruit, it would be useful ≠ especially before
language evolved ≠ for us to coordinate our
activities and communicate our plans, using our
eyes and perhaps other visually based gestures.
Infant research, too, suggests that coordinating
visual attention may have provided the foundation
for the evolution of human language. Babies begin
to acquire language through joint activities with
others, in which both parties are focused on the
same object or task. Thatís the best time for an
infant to learn the word for the object or activity in question.
We are still a long way from figuring out why
humans evolved to do so many complicated things
together ≠ from building houses to creating
universities to fighting wars. But the simple
fact that we have evolved highly visible eyes, to
which infants attune even before language,
supplies at least one small piece of the puzzle of how.
Michael Tomasello is the co-director of the Max
Planck Institute for Evolutionary Anthropology.
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