A kinematic convergence in ape and monkey vertical climbing informs debates on early hominin arborealism

Recent debates on early hominin arborealism center around interpretations of fossilized foot bones as being either "African ape"- or "monkey"- like in their morphology. These dueling interpretations fuel competing locomotor reconstructions of the last common ancestor of chimpanzees and humans (LCA), including disagreements about the prevalence of vertical climbing behaviors. Yet it remains unclear if possessing an ape- or monkey-like foot impacts the safety and efficiency of climbing, in part because few kinematic data exist on cercopithecoid monkeys moving in their natural habitats. Here, we present detailed kinematic data on the feet of a prolific vertically-climbing cercopithecoid monkey (Cercocebus atys; the sooty mangabey) in the wild. We show that sooty mangabeys exhibit an extraordinary degree of midfoot dorsiflexion (~46°) during vertical climbs, a value nearly indistinguishable in magnitude to the ankle dorsiflexion of chimpanzees and slightly larger than the ankle dorsiflexion of climbing rainforest hunter-gatherers. This result suggests that, regardless of overall pedal morphology, the fitness challenges posed by vertical movements produced a kinematic convergence in primate foot biomechanics to improve climbing safety. Even if the LCA retained a more monkey-like foot compared to later hominins, our findings suggest that vertical climbing would still be a plausible component of its locomotor repertoire.