For several decades, there has been a scientific consensus that the sclera is uniform across the human species. According to this consensus, the human eye lost pigmentation over the course of evolution to make it easier for others to follow one’s gaze, thanks to the contrast between the iris (typically blue or brown) and the sclera. This idea is known as the “cooperative eye hypothesis.”
Now, a study led by Juan Olvido Perea García, Distinguished Professor at the University Institute for Biomedical and Health Research (iUIBS) of the University of Las Palmas de Gran Canaria (ULPGC) and Ulam fellow at the Nicolaus Copernicus University (NCU), suggests that this consensus is, at least in part, the result of biased sampling.
According to the study, previous research has focused on individuals from Northern Europe and Asia, who generally have less eye pigmentation than people from equatorial regions or rural populations. In these other populations, the appearance of the sclera can be much more diverse, including irregular patches of pigment and generally higher levels of melanin.
“That people with more skin pigmentation have irregular pigmentation around the eyeball is not news – this is routinely reported in ophthalmological studies and any practitioner will agree with this observation,” says Perea García. “Ida Mann, the pioneering female ophthalmologist, already noted this regarding Australian indigenous people as early as 1966.”
As such, the team advocates for precise measurements of scleral brightness and pigmentation in a sufficiently diverse and representative population sample, in order to identify differences and explain their causes. In ophthalmology, the presence or variation of eye pigmentation can be an early indicator of different pathologies, so recalibrating reference values could potentially lead to more accurate diagnoses.
“For our team, being able to support our observations with anecdotal reports from ophthalmological studies was essential,” Perea García notes. The team also reinforced their point with the help of professional photographers. “This is a good example of the importance of practitioners being involved in academic research, as they have unique first-hand experiences that may not be the case for other researchers.”
But variability in scleral pigmentation across human populations is only part of the story. “Though this aspect may be most interesting to eye care specialists, our study also argues that other primate species have substantially depigmented tissues around the iris”, adds Kai R. Caspar, co-author of the study. “It was only after comparative psychologists claimed that what made human eyes unique was their uniform depigmentation, that human variability became relevant to discussing the cooperative eye hypothesis”.
