The legend of gesture : mapping humans’ use and understanding of hominid gestures

In the search for human uniqueness, language stands out due to its seemingly infinite flexibility that underpins our ability for complex social interactions and large-scale cooperation. Yet, if language is such an adaptive trait, why is it unique to our species? One prominent theory suggests that it is less unique than we may think, and that language evolved from a gestural communication system inherited from our great ape ancestors. Each ape species uses approximately 80 distinct gestures flexibly and intentionally in daily social interactions. Many of these gestures are used similarly across species, with a positive correlation between repertoire overlap and phylogenetic relatedness, suggesting a common ancestral origin. However, human access to this ‘ape-typical’ gestural repertoire remains largely under-studied. My PhD addresses this gap by examining how humans use and understand ape-typical gestures, employing comparative methods borrowed from primatologists. Through an observational study, I analysed the gestures used by 0.5–6.5-year-old children across eight countries using the GesturalOrigins coding system and found that children used the majority of the ape gestural repertoire (68 out of 85 actions; 80%). Children as young as six-months used 24 of these gestures, with an additional 24 emerging before 1.5 years—highlighting the importance of gestures before and during language acquisition. Importantly, they continued to use them into their 7th year, well after language was in place. I also conducted an online study where language-proficient adults interpreted ape-typical gestures exchanged between human adults. Adults agreed on the meaning of 9 out of 10 gestures, with 6 matching their original use in chimpanzees. These findings suggest that the ability to understand ape-typical gestures persists into adulthood. Together, my findings support a phylogenetic origin for an ape-typical gestural system that emerges early in human development and continues to provide an important cognitive scaffold for language use.