The central finding of this work is that the social transmission of Oldowan technology is enhanced by teaching and, in particular, by language. This is in line with a gene-culture co-evolutionary account of human evolution and supports the hypothesis that Oldowan stone tool manufacture generated selection favouring increasingly complex teaching and language13,24,40. Although the learning period in this experiment (at 5 min long) is clearly unrealistically short compared with the length of time that Oldowan hominins likely had available to learn, particularly given available data showing that precise control of conchoidal fracture can take decades to acquire41 and anthropological data showing that knapping skills are acquired across an apprenticeship lasting several years42, a short learning period is sufficient to examine the relative rates of transmission, which is the focus of this work. As such, we cannot rule out the possibility that with a longer learning period, performance across conditions would have converged. However, given that knapping skills are known to take years to develop fully6,41, we suspect that increasing the time spent learning would initially only increase the differences in performance across conditions, with any convergence only occurring after extensive learning. Given their magnitude, the observed differences in performance between conditions would likely translate into significant fitness differences in the shorter term. Key to our findings’ support of a gene-culture co-evolutionary account of human technology and cognition is the continuous improvement in the rate of transmission observed with increasingly complex forms of communication. For example, if verbal teaching provided transmission benefits, but simpler forms of teaching did not, then the co-evolutionary process would not be able to account for the evolution of these simpler forms of teaching. Likewise, if the transmission of tool making benefitted from simple teaching, but gained no further benefit from verbal teaching, then the co-evolutionary process would stop with simpler forms of teaching and could not explain the evolution of verbal teaching.

Accordingly, our data imply that Oldowan tool making would have created a continuous selective gradient leading from observational learning to much more complex verbal teaching. This process need not have taken place entirely within the Oldowan, but was probably already underway during the Oldowan and likely continued well after, as Oldowan tools continued to be made for hundreds of thousands of years beyond the Oldowan time period. Furthermore, assuming that the transmission of more complex technologies also benefits from more complex means of communication, later technologies would have reinforced the gene-culture co-evolutionary dynamic. Such a process could have lasted for millions of years (and may be ongoing29), with more complex communication allowing the stable and rapid transmission of increasingly complex technologies, which in turn generate selection for even more complex communication and cognition, and so forth. Although this places little necessary constraint on when teaching and language may have evolved, our central contribution is to provide evidence that Oldowan tools, produced by hominins since at least 2.5 mya, were involved in this dynamic.

A second significant finding of this work is that the rate of transmission of Oldowan tool making is, at best, minimally enhanced by the addition of imitation/emulation relative to reverse engineering. That the low level of performance with imitation/emulation and reverse engineering is stable along chains (and that performance with teaching and language collapses to this level) suggests a baseline level of performance reliant on little transmitted knowledge, and which could well be achieved through intuition and individual trial-and-error learning. We suggest that the rapid decline of performance with teaching and language to this baseline merely reflects the short learning time employed in this study. Previous transmission chain studies have established that periods of individual practice can bolster the stability of socially transmitted knowledge43. This suggests that with more time to learn, with bouts of teaching and language integrated with periods of individual practice, the benefits of teaching and language would likely have been preserved for longer. Likewise, a benefit of observational learning relative to reverse engineering may well appear over a longer learning period. However, our data suggest that any such benefit is likely to be less than the benefit that would be derived through teaching across a similar timespan because of the improved rate of transmission with teaching. Accordingly, although we do not suggest that imitation is insufficient to transmit the technology per se, our findings support other recent work in implying that observation alone is an inefficient means to acquire stone tool-making skills23,44,45.

Limited information concerning tool manufacture can, no doubt, be rapidly acquired through imitation or emulation; for instance, the basics of core, hammerstone or flake selection36, the requirement to strike the core with the hammerstone and some idea of the force are required. However, it seems plausible that the rapid striking action associated with tool manufacture hinders the transmission of more subtle information crucial to knapping, such as details of the point of percussion or the platform edge and angle, through observation alone. It is here that teaching (for example, slowing down the striking action, pointing to appropriate targets, demonstrating core rotation, manual shaping of pupil’s grasp) and verbal instruction likely provide immediate benefits to the pupil. Indeed, transcripts from the verbal teaching condition show that abstract knapping concepts, such as the platform angle, were transmitted between individuals in the verbal teaching condition (see Supplementary Fig. 3). It may well be the capacity for arbitrary labels such as ‘platform angle’ that facilitates transmission with verbal teaching; such labels break the task into constituent parts, can be used to identify the important elements and provide a clear framework with which pupils can go on to teach others. Language not only allows transmission of the skill itself, but also the ability to transmit the skill to others effectively.

Third, our findings have implications for one of the most enduring puzzles of human evolution: the apparent stasis of the Oldowan technocomplex, which lasted 700,000 years8,11,19,45. Our experiment suggests that Oldowan technological change could have been restricted by low-fidelity forms of social transmission that prevented the spread of innovations. This suggestion is supported by the slow spread of Oldowan technology across Africa, which indicates that this technology was difficult for Oldowan hominins to transmit3. Furthermore, the acquisition of Oldowan knapping skills is not trivial even for modern humans, as shown by our finding that the benefits of teaching and language were rapidly lost in transmission. Although we cannot conclusively identify what form Oldowan transmission might have taken, our data indicate imitation or emulation as likely candidates. In naturalistic contexts, the relatively poor transmission that we observed with imitation and emulation could well be too slow and imprecise for innovations to be transmitted reliably, leaving the technology unable to increase in complexity until more effective communication had evolved.

The suggestion that low-fidelity social transmission is a limiting factor on technological development might contribute to an understanding of why human culture is so complex compared with the behavioural traditions of non-human animals46,47. Although human social transmission has allowed the cumulative elaboration of a vast number of technologies and behaviours, non-human animal social transmission has not. It seems possible that this is because non-human animal social transmission, which appears to be largely limited to forms of observational learning less sophisticated than those of humans43, lacks the fidelity required to transmit more complex innovations, thus constraining cumulative cultural evolution34,35,48. Even the modest knapping ability of extensively trained bonobos49,50 may rely on their prior training in symbolic communication51. Although it is plausible that a similar co-evolutionary process has operated to a lesser degree in some other species, such as other apes52, it remains an open question as to why their tool use did not generate selection for the higher-fidelity social transmission (teaching and language) observed in humans. One possibility is that the technologies of other apes are either sufficiently simple that they can be acquired through more basic mechanisms or so hard to acquire that they can only rarely be transmitted successfully, removing the benefit to teaching9. Task difficulty might also explain a previous experimental finding that simple transmission mechanisms were sufficient for cumulative cultural evolution in the context of human paper-plane design53; this task may be sufficiently simple that teaching is of little benefit. Alternatively, ape reliance on tool use could be insufficient for the benefits of tool-use to outweigh the costs of complex social transmission, thus preventing teaching from increasing fitness9. Any of these constraints would undermine selection for higher-fidelity social transmission, hindering the co-evolutionary process.

Given that our findings support a co-evolution of Oldowan tool use and complex communication, it might seem puzzling that the Oldowan stasis should last so long. If the selective advantage was present, why did more complex communication not evolve for 700,000 years? A likely explanation is that more complex communication may well have evolved during the Oldowan, but that this alone was insufficient for the evolution of stone tool technology. The appearance of Acheulean tools may have additionally been contingent on the evolution of other aspects of cognition, such as technical comprehension or the hierarchical planning of actions54,55,56, as well as demographic and socio-ecological factors57,58. Accordingly, the extraordinary length of the Oldowan stasis could indicate that a large number of limiting factors needed to be overcome before innovations could appear and spread.

Given this, our findings imply that the appearance of Acheulean tools 1.7 mya (refs 17, 18) reflects, in part, the evolution of mechanisms of transmission that facilitated the more effective transmission of Oldowan tool-making, but also enabled the reliable transmission of the sub-goals and techniques required to make the distinctive and regularly shaped Acheulean tools59. We cannot specify the form of this transmission with precision. However, given the observation that chimpanzees are capable of some form of observational learning, yet cannot produce stone tools approaching the quality of the earliest known Oldowan examples13, combined with the complexity of Acheulean technology36, we suggest that teaching in the form of facilitated observation (similar to our basic teaching condition) is the minimal plausible form of social transmission for Acheulean hominins and that rudimentary forms of language are a possibility. However, although our findings suggest that Oldowan hominins would have benefitted from modern language, the suggestion that modern language evolved during the Oldowan seems unlikely given how slowly technology evolved thereafter. This leaves open the possibility that the transmission of Acheulean technology was reliant on a form of (gestural or verbal) proto-language12,60,61. This need not imply that Acheulean hominins were capable of manipulating a large number of symbols or generating complex grammars. Our findings imply that simple forms of positive or negative reinforcement, or directing the attention of a learner to specific points (as was common in the gestural teaching condition), are considerably more successful in transmitting stone knapping than observation alone. This is supported by existing theoretical work that suggests positive and negative feedback greatly enhances the rate of transmission33. Whether or not simple symbolic communication was present during the Acheulean, we anticipate that the gene-culture co-evolutionary dynamic between tools and communication was, and that it would continue beyond the Acheulean, generating selection favouring the use of symbols for increasingly subtle and abstract concepts, and contributing to the eventual evolution of modern language capabilities.

In sum, our data support the hypothesis that a gene-culture co-evolutionary dynamic between tool use and social transmission was on-going in human evolution, starting at least 2.5 mya and potentially continuing to the present. The simplicity and stasis of Oldowan technology are indicative of a limited form of social transmission, such as observational learning, that only allowed the transmission of the broadest concepts of stone knapping technology. Whatever its nature, this was sufficient to support limited transmission among individuals with prolonged contact, but insufficient to propagate innovations more rapidly than they were lost, and would have contributed to the stasis in the Oldowan technocomplex. However, hominin reliance on stone technology would have generated selection for increasingly complex communication that allowed the more effective spread of stone-tools. Under this continued selection, teaching, symbolic communication and eventually verbal language may have been favoured, allowing the ready transmission of abstract flaking concepts, such as the role of the exterior platform angle in choosing where to strike38, which our findings shown are effectively transmitted by language. Given the increased complexity of the later Acheulean and Mousterian lithic technologies, with their reliance on ‘long sequences of hierarchically organized actions’36,38 and other abstract concepts, our results imply that hominins possessed a capacity for teaching—and potentially simple proto-language—as early as 1.7 mya.