Revisiting our primate roots in infants grooming.

Grooming is a common behavior across many animal species, including non-human primates (NHP), and serves a variety of functions, such as cleaning hair, alleviating stress, and enhancing social bonds. This study aims to explore whether grooming-like behaviors are present in humans during early developmental stages, before the emergence of gestural and language communication. Through the observations of 67 preverbal infants, we identified frequent manual behaviors, including grasping, holding, and behaviors resembling grooming, particularly directed toward caregivers’ hairy skin. These behaviors were analyzed and validated by twelve independent primatologists, who confirmed that behavioral sequences and their kinematics closely resembled grooming behaviors seen in NHPs, while also distinguishing them from other types of manual actions such as holding or grasping. Longitudinal analyses demonstrated a significant reduction in infants’ grooming behaviors beginning at 8 months, with these behaviors no longer observed by 15 months, a developmental shift that coincided with the emergence of more sophisticated gestural communication. Interestingly, grooming frequency, but not other actions, peaked during a specific time window, between 2pm and 4pm, which corresponds with a well-documented circadian dip in beta-endorphins, a neurochemical associated with stress regulation and social bonding. This alignment points toward a potential physiological underpinning for the timing of these behaviors. These findings suggest that infant grooming behavior represents a vestigial motor pattern, likely reflecting conserved evolutionary mechanisms shared with non-human primates. This behavior may represent a primitive form of early social interaction, highlighting the role of ancient motor programs in shaping prelinguistic communication.