The cooperative eye hypothesis is a proposed explanation for the appearance of the

human eye The human eye is a sensory organ in the visual system that reacts to light, visible light allowing eyesight. Other functions include maintaining the circadian rhythm, and Balance (ability), keeping balance. The eye can be considered as a living ...

. It suggests that the eye's distinctive visible characteristics

evolved Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

to make it easier for

human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

s to follow another's

gaze In critical theory, philosophy, sociology, and psychoanalysis, the gaze (French: ''le regard''), in the figurative sense, is an individual's (or a group's) awareness and perception of other individuals, other groups, or oneself. Since the 20th ...

while communicating or while working together on tasks.Michael Tomasello, Brian Hare, Hagen Lehmann and Josep Call (2007). Reliance on head versus eyes in the gaze following of great apes and human infants: the cooperative eye hypothesis. ''Journal of Human Evolution'' 52: 314-320

Differences in primate eyes

Unlike other

primate Primates is an order (biology), order of mammals, which is further divided into the Strepsirrhini, strepsirrhines, which include lemurs, galagos, and Lorisidae, lorisids; and the Haplorhini, haplorhines, which include Tarsiiformes, tarsiers a ...

s, all human beings have

eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

s with a distinct colour contrast between the white

sclera The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective outer layer of the eye containing mainly collagen and some crucial elastic fiber. In the development of t ...

, the coloured iris, and the black

pupil The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black becau ...

. This is due to a lack of

pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...

in the sclera. Other primates mostly have pigmented sclerae that are brown or dark in colour. There is also a higher contrast between

human skin The human skin is the outer covering of the body and is the largest organ of the integumentary system. The skin has up to seven layers of ectodermal tissue (biology), tissue guarding Skeletal muscle, muscles, bones, ligaments and organ (anato ...

, sclera, and irises.

Human eye The human eye is a sensory organ in the visual system that reacts to light, visible light allowing eyesight. Other functions include maintaining the circadian rhythm, and Balance (ability), keeping balance. The eye can be considered as a living ...

s are also larger in proportion to body size, and are longer horizontally. Among primates, humans are the only species where the outline of the eye and the position of the iris can be clearly seen in each individual.

Studies

The cooperative eye hypothesis was first proposed Michael Tomasello and colleagues in 2007, on the basis of a study by H. Kobayashi and S. Kohshima in 2001. The main tenets of the hypothesis are laid out in an earlier article, but this is not cited in Tomasello and colleagues' 2007 study. It is therefore unclear whether the cooperative eye hypothesis builds directly on Kobayashi & Kohshima's data, or whether it is based on Emery's 2000 article. The cooperative eye hypothesis was tested by

Michael Tomasello Michael Tomasello (born January 18, 1950) is an American developmental and comparative psychologist, as well as a linguist. He is professor of psychology at Duke University. Earning many prizes and awards from the end of the 1990s onward, he is ...

and others at the

Max Planck Institute for Evolutionary Anthropology The Max Planck Institute for Evolutionary Anthropology (, shortened to MPI EVA) is a research institute based in Leipzig, Germany, that was founded in 1997. It is part of the Max Planck Society network. Well-known scientists currently based at ...

Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

. Researchers examined the effect of head and eye movement on changing gaze direction in

s and other great apes. A human experimenter, observed by either a human

infant In common terminology, a baby is the very young offspring of adult human beings, while infant (from the Latin word ''infans'', meaning 'baby' or 'child') is a formal or specialised synonym. The terms may also be used to refer to juveniles of ...

, a

gorilla Gorillas are primarily herbivorous, terrestrial great apes that inhabit the tropical forests of equatorial Africa. The genus ''Gorilla'' is divided into two species: the eastern gorilla and the western gorilla, and either four or five su ...

, a

bonobo The bonobo (; ''Pan paniscus''), also historically called the pygmy chimpanzee (less often the dwarf chimpanzee or gracile chimpanzee), is an endangered great ape and one of the two species making up the genus ''Pan (genus), Pan'' (the other bei ...

, or a

chimpanzee The chimpanzee (; ''Pan troglodytes''), also simply known as the chimp, is a species of Hominidae, great ape native to the forests and savannahs of tropical Africa. It has four confirmed subspecies and a fifth proposed one. When its close rel ...

, did one of four actions: *Tilted his head up while closing his eyes *Looked at the ceiling with his eyes while keeping his head stationary *Looked at the ceiling with his head and his eyes *Looked straight ahead without moving his head or his eyes The apes were most likely to follow the gaze of the experimenter when only his head moved. The infants followed the gaze more often when only the eyes moved. The results suggest that humans depend more on eye movements than head movements when trying to follow the gaze of another. Anthropologists not involved in the study have called the hypothesis plausible, noting that "human infants and children both infer cooperative intentions in others and display cooperative intentions themselves."

Evolutionary cause

Studies of great ape behavior show that they are good at cooperating in situations where there is no potential of deception, but behave egotistically in situations where there are motives for deception, suggesting that their "lack of cooperativeness" is not a lack of a cognitive ability at all, but rather a necessary adaptation to a society full of deception. This suggests that human cooperativeness began when proto-humans began to successfully avoid competition, which is also supported by the fact that the oldest evidence of care for the long-term sick and disabled are from shortly after the first emigration of hominins out of Africa about 1.8 million years ago.

Other hypotheses

The cooperative eye hypothesis is not the only one that has been proposed to explain the appearance of the human eye. Other hypotheses include the proposal that white sclerae are a sign of good health, useful in

mate selection Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior.Bateson, Paul Patrick Gordon. "Mate Choice." Mate Choi ...

, or that eye visibility promotes

altruistic Altruism is the concern for the well-being of others, independently of personal benefit or reciprocity. The word ''altruism'' was popularised (and possibly coined) by the French philosopher Auguste Comte in French, as , for an antonym of egoi ...

behaviour by letting people know they are being watched. A study by the Max Planck Institute for Evolutionary Anthropology noted that "these hypotheses are not mutually exclusive, and highly visible eyes may serve all of these functions." The cooperative eye hypothesis has been questioned based on examinations of the correspondence between known eye-gaze following behaviors and patterns of (de)pigmentation leading to conspicuous eye appearances in hominoids.

Differences in primate eyes

Studies

Evolutionary cause

Other hypotheses

See also

References

Further reading