Savage et al. follow Dunbar and colleagues in tracing the selection pressure behind human music to a “bonding gap” in human sociality (Dunbar Reference Dunbar and Bannan2012a, Reference Dunbar2012b; Launay, Tarr, & Dunbar, Reference Launay, Tarr and Dunbar2016; Tarr, Launay, & Dunbar, Reference Tarr, Launay and Dunbar2014). Following Podlipniak (Reference Podlipniak2017) and Patel (Reference Patel and Honing2018), they further propose that Baldwinian gene-culture coevolution, or protracted cognitive niche construction, supplied the mode of selection for its evolution.

The relationship between grooming time and group size on which the bonding gap is premised is not linear, as Dunbar originally assumed, but starts leveling off for group sizes exceeding a few dozen individuals (Lehmann, Korstjens, & Dunbar, Reference Lehmann, Korstjens and Dunbar2007, Fig. 1). This allows some nonhuman primates to sustain groups far larger than the roughly 150 individuals for which a hypothetical human bonding deficit has been invoked (Dunbar, Reference Dunbar2012b; Lehmann et al., Reference Lehmann, Korstjens and Dunbar2007, Fig. 3b; Snyder-Mackler, Beehner, & Bergman, Reference Snyder-Mackler, Beehner and Bergman2012). The bonding gap construct itself is problematic, in other words.

A principal predictor of group size in primates is the ratio of neocortex to the rest of the brain (Dunbar, Reference Dunbar1992, Reference Dunbar1998). Neocortex capacity is not needed for grooming, but is essential for tracking social contingencies over time, for the acquisition of familiarity with individuals, and for budgeting the investment of time and effort in fruitful relationships. These are the principal determinants and the substance of personal bonds, whereas their expression (secondarily reinforcing them, in part hormonally, as in grooming) may vary, as the human versus nonhuman primate case shows (Dunbar, Reference Dunbar1993). For large-scale human sociality, moreover, personal bonds are joined by major factors such as human language as well as political and religious authority with power to administer, reward and punish (Turchin et al., Reference Turchin, Currie, Whitehouse, François, Feeney, Mullins and Spencer2018).

Mode of selection: Savage et al. explicitly treat music as a componential phenomenon based on a set of diverse behavioral capacities, with complex, protracted genesis. Not one of these, we are told, evolved by ordinary natural or sexual selection or the shaping agency of iterated learning by pure cultural transmission. Yet, at least one critical component of musicality cannot evolve by Baldwinian or niche construction mechanisms. Every song we sing became ours through a specialized cerebral mechanism for learning to reproduce vocally, using feedback from one's own voice, novel sound patterns received by ear. We alone among primates have this capacity for vocal production learning (Janik & Slater, Reference Janik and Slater1997), not to be confused (as some primatologists do) with learned modification of innate calls (Janik & Slater, Reference Janik and Slater2000). Without the capacity for vocal learning it is impossible to produce the behavior it enables, leaving Baldwinian or niche construction modes of selection without a starting point, by “invention,” happenstance, or otherwise. The stakes for explaining the origin of this human autapomorphy are high, indeed (Nottebohm, Reference Nottebohm, Harnad, Steklis and Lancaster1976).

Whence the esthetic extravagance of music? In behavioral biology terms, music belongs to the class of esthetic displays that forced Darwin to add sexual selection to his theory (Darwin, Reference Darwin1871; Fisher, Reference Fisher1930; Zahavi, Reference Zahavi1975). Neither a “solo” nor a “male” performance bias is integral to this mode of selection, as Savage et al. suppose. The key, instead, is manipulative signaling driven by divergence of interests between evolutionary players (Enquist, Arak, Ghirlanda, & Wachtmeister, Reference Enquist, Arak, Ghirlanda and Wachtmeister2002), something that cannot be assessed apart from social and mating system particulars. Thus, in monogamy sexual selection operates bi-directionally (Miller, Reference Miller, Crawford and Krebs1998, sects. 3.3 and 4; Wachtmeister, Reference Wachtmeister2001; Wachtmeister & Enquist, Reference Wachtmeister and Enquist2000). The esthetic extravagance of music, which Savage et al. do not attempt to explain, suggests a role for sexual selection in its genesis (Merker, Reference Merker, Thaut and Hodges2019). But, what of the many group-based formats and functions of music, emphasized by Savage et al.?

A unique communal display: On irregular occasions, typically when a foraging subgroup discovers a ripe fruit tree or when two subgroups of the same territory meet after a period of separation, chimpanzees launch an excited bout of loud calls, running, branch shaking, and slapping of tree buttresses. These so-called carnival displays, which feature no coordination between individuals of a musical nature, may last for hours, even a whole night, and induce others on the territory, both males and females, to join the fray (Ghiglieri, Reference Ghiglieri1984; Reynolds & Reynolds, Reference Reynolds, Reynolds and Devore1965; Sugiyama, Reference Sugiyama1969, Reference Sugiyama and Poirer1972; Wrangham, Reference Wrangham1975).

Note the social significance of the occasions for a carnival display: a lucky subgroup invites the rest of its territory mates to share in its windfall, and “celebration” by subgroups reunited after separation. Everyone participate in the commotion, which we may think of as a communal expression of belonging to a territorial group, a communal bonding display. The setting for this unique behavior is male communal territoriality combined with female exogamy, a rare combination of traits which on cladistic grounds is attributed to the common ancestor of humans and chimpanzees (Ember, Reference Ember1978; Ghiglieri, Reference Ghiglieri1987; Lovejoy, Reference Lovejoy2009; Wrangham, Reference Wrangham1979).

It is likely, therefore, that the common ancestor of humans and chimpanzees on occasion engaged in something like the communal bonding display of present-day chimpanzees. A common ancestor population in which display participants – unlike present-day chimpanzees – managed to synchronize their voices to a common, isochronous pulse (most readily provided by in-place locomotor activity), would achieve superposition of their voice amplitudes, resulting in a joint signal with far greater geographical reach than unsynchronized voices. This “supervoice” would carry far across neighboring territories to reach exogamously migrating females, attracting them to the synchronizers at the reproductive expense of non-synchronizing groups.

The just described “inter-communal sexual selection” pressure for synchrony would, once migrating females were “on territory,” be joined by an “intra-communal sexual selection” pressure for elaboration of the structural content of the display in the direction of added complexity and sophistication, in both pitch and rhythm domains, in keeping with the esthetic logic of sexual selection. The result: an ancestral group display featuring chanting (eventually singing) in synchrony to rhythmic movement “in place,” amounting to a form of dancing. Thus arose, I suggest, a major cross-cultural universal of music: our tendency to gather from time to time to sing and dance together in a group, that is, music was born (Merker, Reference Merker1999, Reference Merker, Wallin, Merker and Brown2000; Merker, Madison, & Eckerdal, Reference Merker, Madison and Eckerdal2009; Merker, Morley, & Zuidema, Reference Merker, Morley, Zuidema and Honing2018). Crucially, the bonding function and its motivational underpinnings were already present in the ancestral communal display prior to its elaboration into music by inter- and intra-communal sexual selection.

This account of the origin and function of music is compatible with the broad outlines Mehr et al. proposed in their target article. The origin of human vocal production learning in the ancestral setting just sketched is the topic of a forthcoming publication of mine.