Sclera Pigmentation

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True   Likely   Speculative
Human Uniqueness Compared to "Great Apes": 
Relative Difference
Human Universality: 
Individual Universal (All Individuals Everywhere)
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In most primates, the outer fibrous covering of the eyeball, or sclera, is pigmented and appears as a uniform or dark brown color. Pigment deposited in the epithelium cornea, conjunctiva and sclera all contribute to the brown coloration of the eyeball in these primates. Brown coloration of the sclera provides little contrast with adjacent tissues, including the facial skin surrounding the eyeball and the pupils (brown pupils are the norm for all primates, save for humans and some lemurs), making it difficult to detect the position of the iris and thus the direction of gaze in nonhuman primates. Humans have a transparent conjunctiva and lack pigmentation in the sclera, giving us the distinctive feature of “whites of the eyes.” The white sclera in humans contrasts markedly with the pigmented skin surrounding the eye, and with the pupil (even in individuals with pale blue eyes), facilitating the detection of the orientation of the iris and the direction of gaze. In addition, humans have more of the sclera exposed (relative to face and body size) than do other primates, making the pupil (and direction of gaze) all the more conspicuous (relative to orangutans, the amount of visible sclera is two to three times larger in humans). Since humans are heavily dependent on cooperative social interactions, and frequently engage in joint attentional interactions with others (in which gaze following may play an important role in coordinating the attention of cooperating individuals), selection may have favored a loss of pigmentation in the sclera as a means of facilitating social communication, an idea known as the “cooperative eye hypothesis.”

 

 

 

Timing

Timing of appearance of the difference in the Hominin Lineage as a defined date or a lineage separation event. The point in time associated with lineage separation events may change in the future as the scientific community agrees upon better time estimates. Lineage separation events are defined in 2017 as:

  • the Last Common Ancestor (LCA) of humans and old world monkeys was 25,000 - 30,000 thousand (25 - 30 million) years ago
  • the Last Common Ancestor (LCA) of humans and chimpanzees was 6,000 - 8,000 thousand (6 - 8 million) years ago
  • the emergence of the genus Homo was 2,000 thousand (2 million) years ago
  • the Last Common Ancestor (LCA) of humans and neanderthals was 500 thousand years ago
  • the common ancestor of modern humans was 100 - 300 thousand years ago

Possible Appearance: 
2,000 thousand years ago
Definite Appearance: 
100 thousand years ago
Background Information: 

Tomasello et al., 2007. Reliance on head versus eyes in the gaze following of great apes and human infants: the cooperative eye hypothesis. J Hum Evol 52:314-320.

Occurrence in Other Animals: 

Interestingly, visible whites of the eyes are a feature of many domesticated dogs.

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References

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