Humans' seemingly unique communicative system was likely the result of selection pressures arising from a unique environment. If so, then any selective scenario for the emergence of human communication must explain why it evolved in humans and not in any other animal species (Hurford, Reference Hurford, Dunbar, Knight and Power1998). Heintz & Scott-Phillips (H&S-P) propose a feasible and detailed answer to the understudied question of what ecology promoted the emergence of ostensive communication in human evolution. While we agree that H&S-P's proposal, according to which ostensive communication evolved in a partner choice ecology, is a plausible account, we believe that there are alternative scenarios that also deserve serious consideration.

If the explanandum is the evolutionary relation between two seemingly human-specific behavioral traits, ostensive communication and hyper-cooperativity, then another possibility is that ostensive communication, rather than emerging out of a highly cooperative ecology (as H&S-P suggest), was itself the chief driver of distinctive forms of cooperation among humans. Human cooperation is uniquely flexible in at least two related senses. First, our cooperation often requires planning on the basis of displaced or detached representations, because it pursues goals that are not immediately realized (Brinck & Gärdenfors, Reference Brinck and Gärdenfors2003). Second, our cooperative goals are open-ended, that is, the scope of possible goals is not innately prespecified. Paleoarcheological evidence suggests that the foraging behavior of the genus Homo involved a relatively wide range of flora and fauna (e.g., Roche, Blumenschine, and Shea, Reference Roche, Blumenschine, Shea, Grine, Fleagle and Leakey2009), often requiring the invention of novel means for obtaining and processing food. Whether in the form of hunting or confrontational scavenging (Toth & Schick, Reference Toth, Schick, Henke and Tattersall2015), this type of foraging necessitated sophisticated levels of collaboration.

We suggest that such flexibility of human cooperation would not be possible without a similarly flexible communicative system. Symbolic communication allows humans to transmit open-endedly propositional content about displaced entities (Hockett, Reference Hockett1960). This in turn could facilitate collaborative foraging, coalition building, negotiating benefit sharing, reaching agreements, and many other cooperative activities. Therefore, contrary to H&S-P's proposal that a cooperative, partner choice ecology explains the reliability and flexibility of ostensive communication, it is possible that it was ostensive symbolic communication that promoted new levels of cooperation and partner choice among hominins. This account, of course, raises the question of how ostensive communication evolved in the first place – just like H&S-P's proposal would demand an explanation for the original emergence of partner choice ecology. While H&S-P characterize their account as co-evolutionary in nature, their description of the incremental emergence of Gricean communication (target article, sect. 6) presupposes highly structured win-win cooperation among humans from the very beginning of this process.

A second alternative possibility regarding the evolution of cooperation and ostensive communication is that a third, and more ancient, factor drove the evolution of both of these human-specific traits. We propose that hominins' extensive reliance on technology, and especially on “recursive” technology that effectively separated means from ultimate ends, could be one such factor. Much of our knowledge about the behavior of hominins comes from the study of Paleolithic tools. While this might simply be because of a preservation bias, it provides us with relatively direct and verifiable evidence about some aspects of cognitive evolution of early humans. The Oldowan lithic industry from about 2.7 Ma already produced stone tools that are not seen elsewhere in the animal kingdom (Roche et al., Reference Roche, Blumenschine, Shea, Grine, Fleagle and Leakey2009). Some of these tools, like the hammerstone, were used to make other types of tools, like flaked artifacts, which were in turn used for animal butchery and possibly woodworking (Toth & Schick, Reference Toth, Schick, Henke and Tattersall2015). Such recursive toolmaking, which also involved transporting and storing of tools (Shick, Reference Shick1987), could have contributed to the co-evolution of communication and cooperation in various ways.

It has been suggested that the Oldowan technology relied on communicative demonstration for the transmission of the knapping skill from experts to naïve learners (Gärdenfors & Högberg, Reference Gärdenfors and Högberg2017). Moreover, the Acheulean technology, from approximately 1.76 Ma, involved even more cognitively demanding skills to make symmetrical handaxes and cleavers, and likely depended on more complex forms of communication (Gärdenfors & Högberg, Reference Gärdenfors and Högberg2017; Morgan et al., Reference Morgan, Uomini, Rendell, Chouinard-Thuly, Street, Lewis and Laland2015). Thus, the advent of new technologies, with increasingly long sequences of necessary steps that made human actions and tools “opaque” to naïve observers, might have led to the evolution of novel communicative means to convey skills and generic knowledge to novices (Csibra & Gergely, Reference Csibra and Gergely2011). Such an action-based communication system could have emerged gradually, and without the need of recruiting arbitrary symbols, because its contents derive from the natural meaning of the demonstrated actions, and (unlike communicating via established channels, such as speech) it demands making the communicative nature of the demonstration manifest, which is the very point of ostensive communication. Besides, the function of teaching technology fulfills all criteria that an account for the evolution of symbolic communication would have to meet (Laland, Reference Laland2017). Thus, increasing dependence on recursive tool use and toolmaking could have promoted the evolution of ostensive communication.

Reliance on technology would also create an ecology that fosters collaboration and novel social structures at many levels. As soon as tools are seen as valuable items for themselves and are stored for later exploitation, the rights for their use (i.e., their ownership) becomes a social issue, creating entitlements and obligations, as well as novel opportunities for sharing. The reliance on special technological expertise necessitates division of labor and fosters mutualistic and reciprocal cooperation. (E.g., building shelters and the control of fire in hearths might demand collaboration, possibly involving complementary roles.) These types of pressure together can create an ecology in which, as H&S-P rightly suggest, participating in joint enterprises is adaptively beneficial. Importantly, however, producing such an ecology already requires flexible communication, not just for transferring technological knowledge to others but also to maintain the social ecology that in turn maintains the technology that is needed by humans to survive. This perspective, unlike that of H&S-P's, is truly co-evolutionary and may also be supported by tangible paleoarcheological evidence.