Object manipulation scaffolds the development of tool use in children.

Our species' extensive reliance on tool use is a key aspect of what makes us human, yet naturalistic data on the social and environmental drivers of tool-use development in children remain scarce. We examined the development of tool use in 31 children in Swiss daycares (age: 0.5-4.5 years, 17 females, n = 467 video focal follows). All children frequently engaged with objects. Simple object-manipulation bouts became shorter with age, and the number of complex tool-use bouts increased. Social learning opportunities increased the frequency of material engagement, whereas the mere presence of others reduced it. Higher object availability predicted both higher frequency and complexity of material engagement. In sum, children were intrinsically motivated to manipulate objects, and environmental and social factors further influenced tool-use development. Our naturalistic study paves the way for comparisons with non-human species-crucial to understand the evolution of material culture and cognition.