Fumihiro Kano – What is unique about the human eye and how communicative is it? Comparative perspective on the evolution of human white sclera

Humans have the whites of the eye, the near-complete loss of pigmentation in the exposed sclera, which has been hypothesized to be unique to this species among primates and related to eye-gaze signaling. However, recent morphological studies have questioned this hypothesis based on new quantitative analyses. Here, I access the existing morphological and experimental evidence and propose that the key assumptions of the hypothesis are supported, although substantial updates are necessary. The critical updates are that 1) the eyes of many primate species are conspicuous in the face and thus unlikely to be camouflaged, and that 2) it is only under visually challenging conditions (e.g., distances and shades) that the human eyes have greater efficiency to signal the eye-gaze compared to the other primate eyes. These updates indicate that the human eyes may have a signaling advantage over the nonhuman eyes under a very specific communicative situation; when an observer human needs to know both eye and head directions of the signaler (not only the head directions) even across various visual conditions. The human eye may have evolved to achieve effective communication even during relatively large-scale group interactions where group members are distributed over space.