Hominin reliance on Oldowan stone tools – which appear from 2.5mya and are believed to have been socially transmitted – has been hypothesised to have led to the evolution of teaching and language. Here we present an experiment investigating the efficacy of transmission of Oldowan tool-making skills along chains of adult human participants (N=184) using 5 different transmission mechanisms. Across six measures, transmission improves with teaching, and particularly with language, but not with imitation or emulation. Our results support the hypothesis that hominin reliance on stone tool-making generated selection for teaching and language and imply that (i) low-fidelity social transmission, such as imitation/emulation, may have contributed to the ~700,000 year stasis of the Oldowan technocomplex, and (ii) teaching or proto-language may have been pre-requisites for the appearance of Acheulean technology. This work supports a gradual evolution of language, with simple symbolic communication preceding behavioural modernity by hundreds of thousands of years.

All modern humans use tools to overcome limitations of our anatomy and to make difficult tasks easier. However, if tool use is such an advantage, we may ask why it is not evolved to the same degree in other species. To answer this question, we need to bring a long-term perspective to the material record of other members of our own order, the Primates.

* These authors contributed equally to this work. Paleoanthropologists use the distinctive characteristics of flaked stone tools both to distinguish them from naturally broken stones, and to interpret the behaviour of the hominins that produced them 4 . Suggested hallmarks of the earliest stone tool technology include (i) controlled, conchoidal flaking 5 , (ii) production of sharp cutting edges 6 , (iii) repeated removal of multiple flakes from a single core, (iv) clear targeting of core edges, and (v) adoption of specific flaking patterns 7 . These characteristics underlie the identification of intentional stone flaking at all early archaeological sites 3,5,[7][8][9][10][11][12] , as they do not co-occur under natural geological conditions. To date, comparisons between hominin intentional stone flaking and wild primate stone tool use have focused on West African chimpanzees (Pan troglodytes verus) [13][14][15][16] . Nevertheless, stone breakage during chimpanzee tool use is accidental 15 , a result of missed hits or indirect force application during activities such as nut-cracking. The resulting stone fragments lack most of the diagnostic criteria listed above for hominin flakes 10,17 . Even when the manufacture of sharp edges was taught to captive bonobos (Pan paniscus), the resulting flaked assemblage did not replicate the early hominin archaeological record 18 .The capuchins of Serra da Capivara National Park (SCNP) in Brazil use stone tools in more varied activities than any other known non-human primate, including for pounding foods, digging, and in sexual displays [19][20][21] . Bearded capuchins and some Japanese macaques (Macaca fuscata) are known to pound stones directly against each other 22 , however, the SCNP capuchins are the only wild primates that do so for the purpose of damaging those stones 19 . This activity, which we term stone on stone (SoS) percussion, typically involves an individual selecting rounded quartzite cobbles from a conglomerate bed (active hammers), and with one or two hands striking the hammerstone forcefully and repeatedly on quartzite In addition to deliberately crushing the surface of both the active and passive hammers, the capuchins regularly unintentionally break the stones during use (Extended Data 1). In addition, we observed a capuchin purposefully place a newly fractured stone flake on top of another stone, and then strike it with a hammer in a manner resembling chimpanzee nutcracking or human bipolar reduction (Extended Data 1). Nevertheless, while the monkeys were seen to re-use broken hammerstone parts as fresh hammers, they were not observed using the sharp edges of fractured tools to cut or scrape other objects.We collected fragmented stones immediately after capuchins were observed using them at the Oitenta site in SCNP (8º 52.394 S, 42º 37.971 W) (Figure 1), as well as from surface surveys and archaeological excavation in the same area (Extended Data 2). The assemblage consists of 111 capuchin modified stone artefacts, including complete and broken hammerst...

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