The stabilized gaze signalling hypothesis and the evolution of human eye appearance

Human eyes are commonly described as having a 'white sclera': they typically combine scleral exposure with depigmentation, making eye direction readily visible. However, variation in scleral pigmentation among other primates has challenged claims of human uniqueness and renewed debate about its role in gaze communication. Here we develop the stabilized gaze-signalling hypothesis (SGSH), which makes four predictions: (1) depigmented, exposed sclera supports more robust eye-gaze direction discrimination than pigmented sclera across variable viewing conditions by preserving joint visibility of the iris and eye outline; (2) coordination benefits increase when viewing conditions vary or monitoring more group members increases attentional demands, relative to sustained close-range dyadic interaction; (3) humans show greater receiver sensitivity to eye-gaze cues than other primates across development; and (4) heritable morphological variation was available for selection, with signalling benefits potentially exceeding costs of revealing information. Existing evidence supports perceptual benefits under some conditions; the other predictions require further testing. We also consider selective pressures, alternative origins, and complementary functions. The SGSH provides a revised framework for testing whether social benefits from maintaining readable eye direction during group activities could explain the spread and maintenance of depigmentation of exposed sclera in humans.