Evolution
of human eye as a device for communication
Hiromi
Kobayashi
Kyoto
University, Japan
In
humans, the widely exposed white sclera (white of the eye) surrounding the
darker colored iris makes it easy for others to discern the direction of gaze
and has been said to be a characteristic of humans not found in other primate
species. However, this has not been examined in detail. In order to clarify the
morphological uniqueness of the human eye and to obtain cues to understand its
adaptive significance, we compared the external morphology of the primate eye by
measuring nearly half of all extent primate species. The results clearly showed
exceptional features of the human eye: (1) the exposed white sclera is void of
any pigmentation, (2) humans possess the largest ratio of exposed sclera in the
eye-outline, and (3) the eye-outline is extraordinarily elongated in the
horizontal direction. The close correlation of the parameters reflecting (2) and
(3) with habitat type or body size of the species examined suggested that these
two features are adaptations for extending the visual field by eyeball movement,
especially in the horizontal direction. Comparison of eye coloration and facial
coloration around eye suggested that the dark coloration of exposed sclera of
non-human primates is adaptation to camouflage the gaze direction against other
individuals and/or predators, and that the white sclera of human is adaptation
to enhance the gaze signal. The uniqueness of human eye morphology among
primates illustrates the remarkable difference between human and other primates
in the ability to communicate using gaze signals.
Kobayashi,
Hiromi
KOBAYASHI,Hiromi
Japan Society for the Promotion of Science and
Section of Systematics and Phylogeny, Primate Research Institute, Kyoto University
Inuyama, Aichi 484-8506, Japan
hiromi@innocent.com