1. Introduction
Thirty years ago, Steven Pinker and Paul Bloom made the “incredibly boring” (Reference Pinker and Bloom1990, p. 708) argument that language is the product of natural selection, resulting from adaptations for communication. This was, in fact, controversial: despite the facts that language is universally used to communicate information essential to survival and reproduction; that all people typically acquire language easily in infancy; that languages have deep computational structure unrelated to technological or societal progress; that neural injuries cause specific language impairments; and that specialized neuroanatomy enables speech production – many believed that language arose from byproducts of adaptations for cognition, not communication (e.g., Chomsky, Reference Chomsky1968). The question of how language evolved is far from settled but it continues to generate testable hypotheses and productive results (e.g., Atkinson, Meade, Venditti, Greenhill, & Pagel, Reference Atkinson, Meade, Venditti, Greenhill and Pagel2008; Christiansen & Chater, Reference Christiansen and Chater2008; Fitch, Reference Fitch2017; Searcy, Reference Searcy2019).
Music shares many of the above facts with language but its contributions to survival and reproduction, if any, are less evident than those of language. As such, there is no consensus surrounding why humans make and listen to music; why music has its particular features and not others; or how music evolved. Three views on the evolution of music are prominent: a byproduct view, where music developed as a result of non-musical adaptations; an adaptationist view, where music evolved to create and maintain social bonds; and a second adaptationist view, where music evolved to signal mate quality.
We will argue that these views are incomplete or incorrect, proposing instead that the human psychology of music is built on adaptations for at least two categories of vocal signals common across species: territorial advertisements and contact calls. In these contexts, music can communicate overt information about covert properties of the human mind, functioning as a credible signal. This account explains some basic musical phenomena and the limited scope of music's proper domain, laying a foundation for cultural-evolutionary processes that shape the diversity of music worldwide.
2. What constitutes evidence for adaptation by natural selection?
Since antiquity, it has been recognized that unlike abiotic natural phenomena, the existence and form of many biological traits must be explained in reference to their “purpose.” Rain does not fall in order to make corn grow, Aristotle wrote, but of necessity: “What is drawn up must cool, and what has been cooled must become water and descend, the result of this being that the corn grows” (Physics II, part 8). Teeth, in contrast, are “admirably constructed for their general office, the front ones being sharp, so as to cut the food into bits, and the hinder ones broad and flat, so as to grind it to a pulp” (Parts of Animals III, part 1). Human teeth universally grow this way, so this relation of means to ends cannot be due to chance, Aristotle argued; instead, these parts of animals can only be explained by their purpose, which benefits the animal itself (Ariew, Reference Ariew, Ariew, Cummins and Perlman2002).
Two millennia later, William Paley described the organism as an intricate machine, “a cluster of contrivances” whose physical structures are best comprehended in relation to the useful functions they provide the organism (Paley, Reference Paley1803, p. 185). He recognized that these contrivances must be understood in relation to their environments: “Can it be doubted, whether the wings of birds bear a relation to air, and the fins of fish to water?” (Paley, Reference Paley1803, p. 291). Whereas Paley, arguing by exclusion, took evidence of design to be evidence for God, Darwin instead proposed that design evolved via heritable variation and differential reproduction, that is, adaptation by natural selection (Darwin, Reference Darwin1859).
Hypotheses for adaptation can be evaluated using criteria not so different from those of Aristotle, Darwin, or even Paley. Adaptations are generally characteristics of an entire species recognizable from a tight relation of means to ends: a fit between the features of the proposed adaptation and the features of the adaptive problem that it putatively solved. This constitutes evidence of design (Williams, Reference Williams1966).
The human heart must be an adaptation to pump blood, for example, because it develops universally with properties that efficiently and reliably cause blood to circulate (e.g., muscles that compress chambers; valves; inlet and outlet ports; connections to the circulatory system), an outcome essential to survival and reproduction. Conversely, a pumping function best explains why the heart has the structure it has, instead of other tissues in other arrangements, reliably and efficiently solving an adaptive problem (Darwin, Reference Darwin1859; Williams, Reference Williams1966).
There are important differences between pre- and post-Darwinian conceptions of design, however. Selection among heritable variants generally optimized traits to increase inclusive fitness, the reproduction of self or close relatives (Hamilton, Reference Hamilton1964), contra, for example, “well-being” or “longevity”; and did so in ancestral environments but not necessarily modern ones. In Williams's words, “the degree to which an organism actually achieves reproductive survival” is “rather trivial… The central biological problem is not survival as such, but design for survival” (Williams, Reference Williams1966, p. 159).
A key issue when investigating the evolution of a trait – one central to questions of the evolution of music, as we will discuss – is the distinction between proximate and ultimate-level explanations (Mayr, Reference Mayr1961; Tinbergen, Reference Tinbergen1963). Proximate-level questions ask how a trait develops over ontogeny and what causal relationships it has with other parts of the organism. Ultimate-level questions, on the other hand, ask why a trait came to be and require identifying the phylogenetic history of the trait across ancestral and extant species, and the causal role it played, if any, in the reproduction of genes coding for it (discussion: Dickins & Barton, Reference Dickins and Barton2013; Laland, Sterelny, Odling-Smee, Hoppitt, & Uller, Reference Laland, Sterelny, Odling-Smee, Hoppitt and Uller2011; Scott-Phillips, Dickins, & West, Reference Scott-Phillips, Dickins and West2011).
A proximate-level explanation for bitter taste, for instance, is that certain chemicals bind to bitter taste receptor proteins on the tongue, increasing intracellular calcium in the taste receptor cell, thereby stimulating a sensory afferent neuron, and so on. An ultimate-level explanation accounts for the presence of bitter taste receptor genes across vertebrates, and their expression in the oral cavity and other tissues, as part of a neurophysiological system to detect and avoid dietary toxins, which, if ingested, could reduce inclusive fitness (Roper & Chaudhari, Reference Roper and Chaudhari2017). Proximate-level explanation can also be applied to dysfunctions (such as cancer) and non-functions (such as the beating sound of the heart); they do not imply that a trait is an adaptation.
Ultimate-level analyses also do not presuppose adaptation. Adaptationist claims are onerous; there are infinitely many ways a phenotype can be carved into traits, most of which are unrelated to a genetic lineage's reproductive fitness. Supporting a claim of adaptation therefore requires evidence for design: evidence that a trait is improbably well-organized to efficiently, effectively, and reliably solve an adaptive problem (Williams, Reference Williams1966).
Here we evaluate claims about the evolution of music using the approach outlined above, with particular attention to the psychological design of music.
3. Two claims regarding the origins of music are unconvincing
A successful account of music must provide evidence for the design of its principal features. Music is an auditory display built from melodies and rhythms. It can involve loud, elaborate, coordinated performances with voices and musical instruments, with many listeners; it also can involve quiet, simple lullabies between parents and infants. It appears in many behavioral contexts, across the sexes and across the lifespan, as a common element of daily life.
Before we proceed, readers should note a companion BBS target article, “Music as a coevolved system for social bonding,” by Savage et al., which presents an alternate evolutionary scenario for the origins of music. Savage and colleagues propose that musicality arose fairly recently in human ancestry as a cultural invention to enhance social bonding, and was then elaborated via gene–culture evolution over tens of thousands of years (see also Podlipniak, Reference Podlipniak2017).
We appreciate the focus on gene–culture co-evolution, a phenomenon we do not explore in detail in this paper, but which dovetails nicely with our concluding ideas concerning cultural evolution. Nevertheless, the two approaches differ substantially. The theoretical justification for music as a social bonding mechanism relies primarily on the work of Dunbar and colleagues, who argued that grooming serves this function in smaller groups of non-human primates, but that larger human groups required more efficient mechanisms, namely laughter and music. On this idea, social bonds are created by the effects of joint musical performances on the neurobiology of the performers, rather than from information encoded in music. The costs of music production do not enter into this account, and Savage et al. (2020) mostly avoid theoretical or phylogenetic connections between human musicality and similar phenomena in other species (though they do offer some predictions concerning musicality in other species).
The theory we will describe differs substantially from this view. We propose that music has deep evolutionary roots in primate vocalizations, especially contact calls and territorial advertisements that were likely present in the last common ancestor of all primates, approximately 55–85 million years ago. We see music as a credible signal conveying information to listeners with whom signalers might have conflicts of interest, in a fashion similar to most work on non-human vocalizations. We draw theoretical and phylogenetic connections between human music and similar phenomena in other primate and non-primate species. We argue that unique aspects of human lifestyle, including multilevel social organization and high levels of parental investment (including from alloparents), selected for especially elaborate vocal signaling relative to most other species. Finally, we propose that the key features of musicality arising from adaptations in the proper domain of credible signaling serve as building blocks for cultural evolution, which shapes music into its actual domain.
To begin, we review two popular ideas about the origins of music, and ask whether they explain the core properties of music.
3.1 The byproduct hypothesis fails in light of six lines of evidence
The null hypothesis against which hypotheses for adaptation are tested claims that music has no evolved function, and instead is a byproduct of other adaptations that evolved for other functions unrelated to music. The byproduct hypothesis dates at least to William James, who wrote that music “is a pure incident of having a hearing organ” (James, Reference James1890, p. 627); this view echoed other scholars of his time and before (Darwin, Reference Darwin1871; Monboddo, Reference Monboddo1774; Rousseau, Reference Rousseau1781; Spencer, Reference Spencer1902), and is common in the literature. Music has been proposed to be a byproduct of linguistic or emotive communication (Bryant, Reference Bryant2013; Cattell, Reference Cattell1891; Cross & Woodruff, Reference Cross, Woodruff, Botha and Knight2009; Jackendoff, Reference Jackendoff2009; Panksepp, Reference Panksepp2009; Patel, Reference Patel2008; Pinker, Reference Pinker1997; Schulkin, Reference Schulkin2013; Sievers, Polansky, Casey, & Wheatley, Reference Sievers, Polansky, Casey and Wheatley2013); auditory scene analysis and habitat selection (Pinker, Reference Pinker1997; Trainor, Reference Trainor2015); signaling vocalizations (Bryant, Reference Bryant2013; Livingstone, Reference Livingstone1973; Mithen, Reference Mithen2005; Pinker, Reference Pinker1997; Richman, Reference Richman1993); mimicry of other animals' vocalizations (Benzon, Reference Benzon2001; Krause, Reference Krause2012); physical or motor abilities (Geist, Reference Geist1978; Larsson, Reference Larsson2014; Panksepp, Reference Panksepp2009; Tierney, Russo, & Patel, Reference Tierney, Russo and Patel2011); theory of mind (Livingstone & Thompson, Reference Livingstone and Thompson2009); or general cognitive capacities (Cross, Reference Cross2012; Honing & Ploeger, Reference Honing and Ploeger2012; Jackendoff & Lerdahl, Reference Jackendoff and Lerdahl2006; Justus & Hutsler, Reference Justus and Hutsler2005; Marcus, Reference Marcus2012).
Pinker's (Reference Pinker1997) framing is the best-known: “I suspect that music is auditory cheesecake, an exquisite confection crafted to tickle the sensitive spots of … our mental faculties” (p. 534). Six lines of evidence, taken together, call the byproduct hypothesis into question, however, and motivate theories of specific adaptations for music.
First, complex, song-like vocalizations have evolved convergently across distantly-related animals, including multiple clades of birds, marine mammals, primates, and insects; and provide important benefits related to mating and territorial defense (Coen, Xie, Clemens, & Murthy, Reference Coen, Xie, Clemens and Murthy2016). In many cases, these are socially learned, like music (Schachner, Brady, Pepperberg, & Hauser, Reference Schachner, Brady, Pepperberg and Hauser2009). Moreover, at least some explicitly musical behaviors, such as entrainment to a beat, appear in many species (Phillips-Silver, Aktipis, & Bryant, Reference Phillips-Silver, Aktipis and Bryant2010; Wilson & Cook, Reference Wilson and Cook2016). Music-like adaptations can therefore evolve, in principle.Footnote 1
Second, music is a human universal: it appears throughout a representative sample of human societies (Mehr et al., Reference Mehr, Singh, Knox, Ketter, Pickens-Jones, Atwood and Glowacki2019); plays an essential role in important activities, such as rituals and ceremonies (Nettl, Reference Nettl2015); and demonstrates cross-cultural links between form and function (Bainbridge & Bertolo et al., Reference Bainbridge, Bertolo, Youngers, Atwood, Yurdum, Simson and Mehr2021; Hilton & Crowley-de Thierry et al., Reference Hilton, Crowley, Yan, Martin and Mehr2021; Mehr et al., Reference Mehr, Singh, Knox, Ketter, Pickens-Jones, Atwood and Glowacki2019, Reference Mehr, Singh, York, Glowacki and Krasnow2018; Trehub, Unyk, & Trainor, Reference Trehub, Unyk and Trainor1993a). Music is not a byproduct of traits present in only some cultures.
Third, music shows evidence for complex design, including grammar-like structures analogous to those of language (Lerdahl & Jackendoff, Reference Lerdahl and Jackendoff1983), some of which may be universal (Jacoby et al., Reference Jacoby, Undurraga, McPherson, Valdés, Ossandón and McDermott2019; Mehr et al., Reference Mehr, Singh, Knox, Ketter, Pickens-Jones, Atwood and Glowacki2019). Moreover, music perception is computationally complex, such that artificial intelligence is currently at pains to emulate it (Benetos, Dixon, Giannoulis, Kirchhoff, & Klapuri, Reference Benetos, Dixon, Giannoulis, Kirchhoff and Klapuri2013). Music is unlikely to occur as a result of random chance.
Fourth, the motivation and ability to perceive music appear early in ontogeny: neonates are sensitive to rhythms (Winkler, Háden, Ladinig, Sziller, & Honing, Reference Winkler, Háden, Ladinig, Sziller and Honing2009) and melodies (Granier-Deferre, Bassereau, Ribeiro, Jacquet, & DeCasper, Reference Granier-Deferre, Bassereau, Ribeiro, Jacquet and DeCasper2011) and infant music cognition is precocial (e.g., infants have detailed long-term memory for music; Mehr, Song, & Spelke, Reference Mehr, Song and Spelke2016; Mehr & Spelke, Reference Mehr and Spelke2017; Trainor, Wu, & Tsang, Reference Trainor, Wu and Tsang2004; reviews: Hannon & Trainor, Reference Hannon and Trainor2007; Trehub, Reference Trehub2001). Music perception develops naturally, does not require extensive training, and is not a byproduct of traits specific to adults.
Fifth, music perception displays evidence for neural specialization (Norman-Haignere, Kanwisher, & McDermott, Reference Norman-Haignere, Kanwisher and McDermott2015, Reference Norman-Haignere, Feather, Brunner, Ritaccio, McDermott, Schalk and Kanwisher2019) and is impaired in specific deficits, such as tone-deafness (Peretz, Ayotte, Zatorre, & Jutras, Reference Peretz, Ayotte, Zatorre, Mehler, Ahad, Penhune and Jutras2002; Peretz & Vuvan, Reference Peretz and Vuvan2017). Music is unlikely to be a byproduct of other neural systems.
Last, music is ancient: flutes are at least 40,000 years old (Conard, Malina, & Münzel, Reference Conard, Malina and Münzel2009) and the human auditory and vocal production systems are far older (Fitch, Reference Fitch2006; Martínez, Rosa, Arsuaga, & Carbonell, Reference Martínez, Rosa, Arsuaga, Jarabo, Quam, Lorenzo and Carbonell2004; Quam et al., Reference Quam, de Ruiter, Masali, Arsuaga, Martínez and Moggi-Cecchi2013). Music is not a recent cultural invention.
While no one of these pieces of evidence is a sufficient condition for rejecting the byproduct hypothesis, taken together, they motivate a search for an alternative.
3.2 The social bonding hypothesis fails in light of three theoretical issues
The best-known evolutionary hypothesis for music is that it evolved to create and maintain “social bonds.” Roederer (Reference Roederer1984), for example, argued that music established “behavioral coherency in masses of people” to meet the demands of “coherent, collective actions on the part of groups of human society” (p. 356). Brown (Reference Brown, Wallin, Merker and Brown2000b) asserted that “music-making has all the hallmarks of a group adaptation and functions as a device for promoting group identity, coordination, action, cognition, and emotional expression” (p. 296). These and similar claims (Barrow, Reference Barrow2005; Benzon, Reference Benzon2001; Brown, Reference Brown, Tonneau and Thompson2000a; Conard et al., Reference Conard, Malina and Münzel2009; Cross & Morley, Reference Cross, Morley, Malloch and Trevarthen2009; Dissanayake, Reference Dissanayake, Wallin, Merker and Brown2000, Reference Dissanayake2008, Reference Dissanayake, Malloch and Trevarthen2009; Dunbar, Reference Dunbar1998, Reference Dunbar2012a; Freeman, Reference Freeman, Wallin, Merkur and Brown2000; Fritz, Hardikar, Demoucron, & Leman, Reference Fritz, Hardikar, Demoucron, Niessen, Demey, Giot and Leman2013; Geissmann, Reference Geissmann, Wallin, Merker and Brown2000; Huron, Reference Huron2001; Jourdain, Reference Jourdain1997; Kirschner & Tomasello, Reference Kirschner and Tomasello2009, Reference Kirschner and Tomasello2010; Koelsch & Siebel, Reference Koelsch and Siebel2005; Kogan, Reference Kogan1994; Launay, Tarr, & Dunbar, Reference Launay, Tarr and Dunbar2016; Loersch & Arbuckle, Reference Loersch and Arbuckle2013; McNeill, Reference McNeill1995; Merker, Madison, & Eckerdal, Reference Merker, Madison and Eckerdal2009; Morley, Reference Morley and Bannan2012; Pearce, Launay, & Dunbar, Reference Pearce, Launay and Dunbar2015; Reddish, Fischer, & Bulbulia, Reference Reddish, Fischer and Bulbulia2013; Richman, Reference Richman1993; Schulkin, Reference Schulkin2013; Schulkin & Raglan, Reference Schulkin and Raglan2014; Weinstein, Launay, Pearce, Dunbar, & Stewart, Reference Weinstein, Launay, Pearce, Dunbar and Stewart2016; Wiltermuth & Heath, Reference Wiltermuth and Heath2009) together form the social bonding hypothesis.
This view was popularized in part by Dunbar's proposal of a role for social bonding in the evolution of many human social traits (Dunbar, Reference Dunbar1991): he argued that in primates, manual grooming serves a social bonding function; as group size increased in the hominin lineage, manual grooming became prohibitively time-consuming, creating a selection pressure for a less costly bonding mechanism; and, as a consequence, new bonding mechanisms evolved. These mechanisms were first proposed to be language and gossiping, which could be broadcast to multiple individuals while doing other tasks, replacing grooming as the primary means of social bonding in humans (Dunbar, Reference Dunbar1998). Later, Dunbar and colleagues revised this position (Dunbar & Lehmann, Reference Dunbar and Lehmann2013), arguing that musical chorusing and laughter evolved instead (Dunbar, Reference Dunbar, Oller and Griebel2004, Reference Dunbar2012a; Dunbar, Kaskatis, MacDonald, & Barra, Reference Dunbar, Kaskatis, MacDonald and Barra2012; Pearce et al., Reference Pearce, Launay and Dunbar2015, Reference Pearce, Launay, van Duijn, Rotkirch, David-Barrett and Dunbar2016, Reference Pearce, Launay, MacCarron and Dunbar2017; Tarr, Launay, Cohen, & Dunbar, Reference Tarr, Launay, Cohen and Dunbar2015; Tarr, Launay, & Dunbar, Reference Tarr, Launay and Dunbar2014; Tarr, Launay, & Dunbar, Reference Tarr, Launay and Dunbar2016).
Most empirical tests of the hypothesis examine music's impact on prosociality and its hormonal mediators in laboratory experiments: participants are randomized into groups that engage in synchronized musical behavior (treatment) or another activity (control). The general finding is greater levels of prosociality and cooperation in the music/dancing conditions relative to controlsFootnote 2 (Anshel & Kipper, Reference Anshel and Kipper1988; Cirelli, Einarson, & Trainor, Reference Cirelli, Einarson and Trainor2014; Kirschner & Tomasello, Reference Kirschner and Tomasello2009, Reference Kirschner and Tomasello2010; Pearce et al., Reference Pearce, Launay and Dunbar2015, Reference Pearce, Launay, van Duijn, Rotkirch, David-Barrett and Dunbar2016, Reference Pearce, Launay, MacCarron and Dunbar2017; Reddish et al., Reference Reddish, Fischer and Bulbulia2013; Schellenberg et al., Reference Schellenberg, Corrigall, Dys and Malti2015; Tarr et al., Reference Tarr, Launay, Cohen and Dunbar2015).
The social bonding hypothesis has at least three key issues, however.
3.2.1 A “stress-reducing” social bonding mechanism is superfluous
The ultimate-level problem of sociality is that it imposes difficult-to-overcome inclusive fitness costs: increased competition with conspecifics for essential, limited resources; inbreeding depression; and increased exposure to pathogens (Alexander, Reference Alexander1974). Living with and cooperating with conspecifics requires that the inclusive fitness benefits of sociality outweigh its fitness costs. Often they do not: dispersal and solitary living are ubiquitous across species (Benton, Baguette, Clobert, & Bullock, Reference Benton, Baguette, Clobert and Bullock2017; Bowler & Benton, Reference Bowler and Benton2005; Duputié & Massol, Reference Duputié and Massol2013).
In primates, diurnal social living evolved about 52 million years ago (Shultz, Opie, & Atkinson, Reference Shultz, Opie and Atkinson2011). Because diurnal foraging increases predation risk, the joint evolution of diurnality and sociality supports the long-standing idea that primate sociality evolved as a defense against predators (Silk & Kappeler, Reference Silk, Kappeler, Rubenstein and Abbot2017; Van Schaik, Reference Van Schaik1983). Advocates of the social bonding hypothesis claim that social living creates psychological stresses that threaten the cohesion of the group, necessitating a “bonding mechanism” (in non-human primates, grooming; in humans, music) that reduces stress:
Since living in groups of any kind creates stresses that would normally result in the group disbanding, species that live in stable social groups have to circumvent this problem if they are to prevent group size collapsing. (Dunbar, Reference Dunbar2012a, p. 1838)
This idea is superficially appealing because it draws attention to the fitness costs of social living, presenting them as proximate-level stresses, and implying a need for a behavioral response to relieve the stress.
But an ultimate-level analysis must consider alternative strategies. The alternative to sociality is solitary living, seen in ~70% of mammal species (Wilson & Reeder, Reference Wilson and Reeder2005). On the hypothesis that sociality solves the adaptive problem of defense against predators, the net fitness benefits of sociality exceed those of solitary life (with its attendant high risk of predation). The stress-related benefits of a “social bonding mechanism” are superfluous.
For an analogy, consider a group of friends walking close together in a dangerous neighborhood at night. There are costs to this sociality: they bump into each other; they don't fit on the sidewalk, forcing some to risk injury from oncoming cars; it's harder for them to converse, and so on. An ultimate-level analysis recognizes that the benefits of their sociality – defense against getting mugged – outweigh the costs, and no bonding mechanism, such as grooming or singing, is required to keep them together.
Grooming does provide hygienic benefits to primates, such as removal of ectoparasites (Barton, Reference Barton1985); perhaps with social functions beyond hygiene (McKenna, Reference McKenna1978; Seyfarth, Reference Seyfarth1977; Seyfarth & Cheney, Reference Seyfarth and Cheney1984), because across species the proportion of time spent grooming is positively correlated with group size (Dunbar, Reference Dunbar1991). But this association, the core empirical finding underlying the social bonding hypothesis, is poorly evidenced: its strength is modest, and, when adjusting for terrestriality and other ecological factors, is not distinguishable from zero (Jaeggi, Kramer, Hames, & Gurven, Reference Jaeggi, Kramer, Hames, Kiely, Gomes, Kaplan and Gurven2017). This may be because primate group size is confounded with terrestriality; if so, increased grooming time could instead be explained by some property of a terrestrial niche, such as increased parasite load (Grueter, Bissonnette, Isler, & van Schaik, Reference Grueter, Bissonnette, Isler and van Schaik2013; Jaeggi et al., Reference Jaeggi, Kramer, Hames, Kiely, Gomes, Kaplan and Gurven2017; cf. Dunbar & Lehmann, Reference Dunbar and Lehmann2013).
Whatever evolved social functions grooming might have, it is unlikely that they include stress reduction. Predation risk and perhaps communal resource defense, not grooming, are the ultimate-level “bonding forces” that likely explain primate sociality (Port et al., Reference Port, Hildenbrandt, Pen, Schülke, Ostner and Weissing2020), and the additional benefits of cooperative endeavors such as hunting, parenting, and territorial defense likely explain human sociality.
3.2.2 The social bonding hypothesis conflates proximate- and ultimate-level reasoning
Might grooming solve other problems of sociality? Defense against predators, territory defense, hunting, and parenting are compelling examples of cooperation whose benefits could offset sociality's costs. They raise profound theoretical challenges, however, involving free-riders: agents that receive benefits from others but do not provide any. Without countermeasures, free-riding is favored by natural selection (Nowak, Reference Nowak2006), so forming cooperative relationships with arbitrary individuals is untenable. Instead, these relationships must be targeted at specific categories of individuals, such as kin, neighbors, or those likely to reciprocate, and adjusted to local socio-ecological conditions (Markham, Gesquiere, Alberts, & Altmann, Reference Markham, Gesquiere, Alberts and Altmann2015) such that long-run benefits are provided only when they exceed long-run costs to the donor (Nowak, Reference Nowak2006).
Proponents of the social bonding hypothesis offer a proximate-level explanation, wherein the neurohormonal effects of music are a solution to the impediments to sociality and cooperation described above:
My proposal is that music arose originally because it allows individuals to become more group-oriented. Music seems to achieve this through a capacity to produce endorphins which have a positive effect on our attitudes towards others. (Dunbar, Reference Dunbar and Bannan2012b, p. 208)
We propose that synchrony might act as direct means to encourage group cohesion by causing the release of neurohormones that influence social bonding. (Launay et al., Reference Launay, Tarr and Dunbar2016, p. 779)
There are two problems with these claims. First, evidence that X causes Y is weak evidence that X evolved to cause Y. Recall Aristotle: rain causes corn to grow without implying any “purpose” for rain. Rain shows little evidence of special design for solving corn's hydration problem, it has many other, unrelated effects, and so on. By analogy, a proximate-level analysis shows that petting animals reduces human anxiety via hormonal and physiological effects (Beetz, Uvnäs-Moberg, Julius, & Kotrschal, Reference Beetz, Uvnäs-Moberg, Julius and Kotrschal2012), but animal-petting did not evolve to reduce anxiety or the threats that trigger it, of course.
Second, proximate mechanisms, such as release of neurohormones, are themselves subject to selection, and therefore cannot serve as ultimate-level explanations for the genetic evolution of a social bonding strategy. In order for a social strategy to evolve, it must outperform conceivable mutant strategies (a well-recognized criterion for claims of adaptation, the evolutionarily stable strategy; Smith & Price, Reference Smith and Price1973). A mutation that prevented music from increasing endorphins and/or reduced endorphins' effects on prosociality would have allowed humans with that mutation to free-ride: they could gain from the prosocial behavior of others (becoming more bonded with the group) without being prosocial themselves. Such a free-rider mutation would be selected for (Nowak, Reference Nowak2006).
How can an unconditional social bonding mechanism like music be stabilized against free-riders? Confusion between proximate- and ultimate-level analyses in the social bonding hypothesis leaves this question unanswered.
3.2.3 Music is poorly designed to coordinate groups
Another version of the social bonding hypothesis proposes that music evolved by genetic group selection to enable humans to act as coordinated superorganisms: music increased group fitness by promoting group identity, cognition, coordination, and catharsis. These within-group functions are proposed to increase the ability of groups to compete with other groups (Brown, Reference Brown, Tonneau and Thompson2000a).
While music does play a universal role in rituals (e.g., shamanistic trance; Mehr et al., Reference Mehr, Singh, Knox, Ketter, Pickens-Jones, Atwood and Glowacki2019; Singh, Reference Singh2018), the problem with this view is that it equates proximate social “functions” or “effects” with adaptations shaped by natural selection.Footnote 3 Because any behavior has effects, and some of those effects may be incidentally “useful” (e.g., animal-petting reduces anxiety), the proper criterion is that music be well-designed for the proposed within-group function.
The superorganism model is based on an explicit analogy with multicellular organisms, where energy and time are sharply constrained resources. Within-organism signaling, cognition, and coordination evolved to be as efficient as possible, to maximize between-organism competitiveness. In neural signaling, for example, time and energy trade off: higher information rates use more energy, so at all levels of neural organization, strategies evolved to reduce energy consumption by filtering out predictable inputs, reducing the amount of redundant encoding (Laughlin, Reference Laughlin2001; Niven, Reference Niven2016; Niven & Laughlin, Reference Niven and Laughlin2008).
But music takes considerable time and energy to produce. People who produce music incur opportunity costs (Mehr & Krasnow, Reference Mehr and Krasnow2017) and expend energy that could be used for other activities that directly increase reproductive success, such as food production (Hagen & Bryant, Reference Hagen and Bryant2003). Music is also often loud, and could attract predators or allow competing groups to eavesdrop. These costs also accrue to the variants of the social bonding hypothesis discussed earlier. Indeed, while music and other synchronous, ritualistic behaviors are often argued to be unambiguously beneficial for groups, the “neglected dark side of synchrony” (p. 3) shows that synchrony increases conformity and groupthink while reducing creativity and productive dissent (Gelfand, Caluori, Jackson, & Taylor, Reference Gelfand, Caluori, Jackson and Taylor2020).
Because natural selection shapes traits to perform specific functions by selecting among alternatives, a criterion for claiming adaptation is that a trait is uniquely suited to causing certain effects, relative to feasible alternatives. In the case of the social bonding hypothesis, an obvious alternative to music that serves the same proposed within-group functions is language, a low-cost signaling system that efficiently facilitates the coordination of collective action and other social behaviors (Pinker & Bloom, Reference Pinker and Bloom1990). Consider that the coxswain, whose job is to maintain the coordination of rowers, does not sing, nor does the crew; the efficient vocalization “row!” minimizes the energy required for within-group coordination, while maximizing the rowers' ability to win a race.Footnote 4 Moreover, in a sample of six small-scale human societies, conversation time was close to the expected grooming time for a terrestrial primate with recent ape ancestry (Jaeggi et al., Reference Jaeggi, Kramer, Hames, Kiely, Gomes, Kaplan and Gurven2017), suggesting that language adequately provides whatever social functions grooming may have. As a social coordination or bonding mechanism, music thus appears to have no advantages over language and many disadvantages.
The weak case for music as an adaptation for social bonding does not mean that music has no evolved social functions. In the rest of this paper, we outline an alternative social hypothesis for the origins of music.
4. Origins of music in credible signaling
The social bonding hypothesis proposes that the fitness benefit of music arises from the neurophysiological effects of music production on music-makers themselves. Signaling hypotheses, in contrast, propose that fitness benefits arise from the information communicated by music-makers, via their music, to various categories of listeners.
Acoustic communication has evolved repeatedly and independently in many clades of tetrapods. It appeared 200 million years ago in therian mammals and is found in ~95% of mammal species (Chen & Wiens, Reference Chen and Wiens2020). If music is an adaptation, it likely evolved from ancestral vocalizations, an idea foreshadowed by Lucretius two millennia ago:
To imitate the liquid notes of birds
Was earlier far amongst men than power to make
By measured song, melodious verse and give
Delight to ears.
(De Rerum Natura, Book V)Footnote 5
In non-human animals, most vocal adaptations evolve to send signals, which are defined as “any act or structure which alters the behaviour of other organisms, which evolved because of that effect, and which is effective because the receiver's response has also evolved” (Maynard Smith & Harper, Reference Maynard Smith and Harper2003, p. 3).Footnote 6 On average, receivers benefit from responding to the signal, and signalers benefit from the receivers' response.Footnote 7 Cues, in contrast, convey information about one organism to another but did not evolve to do so (e.g., bleeding is a cue of injury but did not evolve to signal injury). Common functions of signals include species identification in mate choice; individual recognition in interactions among conspecifics, such as territoriality, dominance, and cooperation; and conveying information on formidability, health, or behavioral type (Tibbetts, Mullen, & Dale, Reference Tibbetts, Mullen and Dale2017).
Why do animals believe the vocal signals they hear? What maintains their credibility? If the interests of signaler and receiver are aligned, as in cells in an organism or agents in a superorganism, then selection for dishonesty is absent and signals evolve to be as efficient as possible (Maynard Smith & Harper, Reference Maynard Smith and Harper2003). If not, then selection can drive signalers to deceive and receivers to be vigilant against manipulation.
Some signals are necessarily credible because they are causally related to the quality being signaled. A wolf howl credibly indicates that a wolf is present, for example, and the number of distinct, simultaneous wolf howls credibly indicates a lower bound on the size of the pack (an “index”; Maynard Smith & Harper, Reference Maynard Smith and Harper2003). Costly signals, in contrast, are credible because to send them imposes a fitness cost that is lower for individuals with the quality than those without it: faking the signal is more costly than it is worthFootnote 8 (Spence, Reference Spence1973; Zahavi, Reference Zahavi1975).
In addition to credibility, multiple selection pressures can shape signals, including biases in the sensory systems of receivers; receiver abilities to discriminate signals; the structure of the environment; social challenges; and arms races between signalers and receivers (Krebs & Dawkins, Reference Krebs, Dawkins, Krebs and Davies1984), where signalers are selected to produce the signal at lower cost and receivers are selected to better discriminate the quality of signalers (Bradbury & Vehrencamp, Reference Bradbury and Vehrencamp1998; Cummings & Endler, Reference Cummings and Endler2018; Hill, Reference Hill1994; Lindsay, Andersson, Bererhi, & Edwards, Reference Lindsay, Andersson, Bererhi, Höglund, Johnsen, Kvarnemo and Edwards2019; McCoy & Haig, Reference McCoy and Haig2020; van Doorn & Weissing, Reference van Doorn and Weissing2006).
Here, we emphasize the importance of conflicts of interest between music producers and the audience, private information, and the features of music that underlie its ability to overtly signal covert information about the minds of those producing it.
4.1 The mate quality hypothesis is poorly supported
An early theory of music, first proposed by Darwin (Reference Darwin1871) and endorsed by many others (Barrow, Reference Barrow2005; Charlton, Reference Charlton2014; Dutton, Reference Dutton2009; Merker, Reference Merker, Wallin, Merker and Brown2000a; Miller, Reference Miller, Wallin, Merker and Brown2000a, Reference Miller2000b; Miranda, Kirby, & Todd, Reference Miranda, Kirby and Todd2003; Orians, Reference Orians2014; Sluming & Manning, Reference Sluming and Manning2000; Todd, Reference Todd, Wallin, Merker and Brown2000; Todd & Werner, Reference Todd, Werner, Griffith and Todd1999; van den Broek & Todd, Reference van den Broek and Todd2009) is that male musical abilities and female musical preferences coevolved, with music functioning as a credible signal of male mate quality.
If musical production requires a brain and body relatively unperturbed by genetic mutation, infection, or developmental instability, plus time to cultivate one's talent (properties that are difficult to perceive directly), the mate quality hypothesis argues that mates who prefer music-producers will benefit. This increases selection for music-producers to generate more impressive, complex, or interesting music (so as to improve the chance of being chosen as a mate). Given the sex difference in the amount of investment required of human parents for an offspring to be reproductively viable (Trivers, Reference Trivers and Campbell1972), signal production should be accentuated in the sex with lower obligate parental investment (males) and choosiness should be accentuated in the sex with the higher obligate parental investment (females).
Sexually dimorphic signals of mate quality are common across species (e.g., coloration, ornaments) and they play key roles in mate attraction (Andersson, Reference Andersson1994; Dale, Dey, Delhey, Kempenaers, & Valcu, Reference Dale, Dey, Delhey, Kempenaers and Valcu2015; O'Brien, Allen, Van Kleeck, & Emlen, Reference O'Brien, Allen, Van Kleeck, Hone, Knell, Knapp and Emlen2018); for example, male birdsong functions in part to attract mates (Catchpole & Slater, Reference Catchpole and Slater2018). Some mammals show this pattern too. In sac-winged bats, males produce complex songs that females may use in mating decisions (Behr et al., Reference Behr, von Helversen, Heckel, Nagy, Voigt and Mayer2006). Adult house mice produce sexually dimorphic ultrasonic vocalizations with song-like features in response to the presence of novel female urine, but not the scents of immature females or other males (Musolf, Hoffmann, & Penn, Reference Musolf, Hoffmann and Penn2010). If human music evolved in the context of signaling mate quality, it should have retained similar, signature features of a sexually selected adaptation.Footnote 9
Ironically, the mate quality hypothesis is easy to refute precisely because it is so well-specified. Music is tenuously linked to mate quality. While love/courtship songs are common across cultures, they are only weakly identifiable as such relative to other forms of song (Mehr et al., Reference Mehr, Singh, Knox, Ketter, Pickens-Jones, Atwood and Glowacki2019, Reference Mehr, Singh, York, Glowacki and Krasnow2018). A large twin study found inconsistent relations between measures of reproductive success or sociosexuality and measures of music production or music perception abilities (Mosing et al., Reference Mosing, Verweij, Madison, Pedersen, Zietsch and Ullén2015); the few positive relations reported were weak, and no stronger in men than in women. In another study, music performance quality was positively associated with indices of mate quality and attractiveness, but the effects and sample size were small and did not differ by sex (Madison, Holmquist, & Vestin, Reference Madison, Holmquist and Vestin2018). Musical preferences can vary across the menstrual cycle (Charlton, Reference Charlton2014), perhaps indicating a role in mating, but this effect has failed to replicateFootnote 10 (Charlton, Filippi, & Fitch, Reference Charlton, Filippi and Fitch2012).
Sexually-selected traits that function for display and choice in mating contexts are often developmentally and contextually calibrated to mating (Kokko, Reference Kokko1997). In humans, for instance, puberty coordinates the developmental timing of physical and psychological traits that support mating: menarche and spermarche coincide with the development of secondary sexual characteristics and the relative onset of mating psychological systems (Kaplan & Gangestad, Reference Kaplan, Gangestad and Buss2005). But humans of all ages produce and listen to music; no part of the music faculty emerges at or around puberty.Footnote 11 Young children enjoy the music of sexually mature conspecifics, and vice versa, a pattern contrasting with that of sexual attraction, which begins in late childhood (Herdt & McClintock, Reference Herdt and McClintock2000). While mating-related behaviors tend to be produced only in mating-relevant contexts, music is produced and consumed in multifarious contexts, universally, that are completely unrelated to mating (e.g., work, healing, greeting visitors, mourning; Mehr et al., Reference Mehr, Singh, Knox, Ketter, Pickens-Jones, Atwood and Glowacki2019).
Last, many mating-related traits in humans are sexually dimorphic, such as male biases for traits useful in physical competition (Puts, Reference Puts2010; Sell et al., Reference Sell, Cosmides, Tooby, Sznycer, von Rueden and Gurven2008) and female biases for traits useful in discerning investment potential (Buss, Reference Buss1989; Conroy-Beam, Buss, Pham, & Shackelford, Reference Conroy-Beam, Buss, Pham and Shackelford2015; Kenrick & Keefe, Reference Kenrick and Keefe1992). If music evolved to signal mate quality, then adaptations for music production should be more developed in men and adaptations for music perception should be more developed in women.
Little evidence supports this pattern. Dimorphisms in human vocalizations and vocal anatomy – lower voices in males, signaling threat potential (Puts, Apicella, & Cárdenas, Reference Puts, Apicella and Cárdenas2011), and higher voices in females, signaling fecundity (Apicella & Feinberg, Reference Apicella and Feinberg2009) – appear beginning at puberty (McDermott, Reference McDermott2012), but are neither more exaggerated nor more honestly signaled via song rather than via speech (cf. Keller, König, & Novembre, Reference Keller, König and Novembre2017). Auditory perception skills are comparable in males and females, with only small and inconsistent sex differences (Müllensiefen, Gingras, Musil, & Stewart, Reference Müllensiefen, Gingras, Musil and Stewart2014; Shuter-Dyson & Gabriel, Reference Shuter-Dyson and Gabriel1981). Musical disorders, such as specific musical anhedonia and congenital amusia, are found just as frequently in males as in females (Mas-Herrero, Zatorre, Rodriguez-Fornells, & Marco-Pallarés, Reference Mas-Herrero, Zatorre, Rodriguez-Fornells and Marco-Pallarés2014; Peretz & Vuvan, Reference Peretz and Vuvan2017). A lone report of sex differences in the frequency of music performance across human societies (Savage, Brown, Sakai, & Currie, Reference Savage, Brown, Sakai and Currie2015) is likely the result of sampling bias (for discussion, see Mehr et al. Reference Mehr, Singh, York, Glowacki and Krasnow2018, Reference Mehr, Singh, Knox, Ketter, Pickens-Jones, Atwood and Glowacki2019). If anything, female musicians produce more novel songs than their male peers (Askin, Mauskapf, Koppman, & Uzzi, Reference Askin, Mauskapf, Koppman and Uzzi2020).
The pervasiveness of music across the sexes is evident in daily life: both males and females seek out and enjoy the performances of both male and female musicians (Hagen & Bryant, Reference Hagen and Bryant2003), and some evidence suggests that musical preferences are biased toward performers of the same sex as the listener (Greenberg, Matz, Schwartz, & Fricke, Reference Greenberg, Matz, Schwartz and Fricke2020). Male and female performers are both well-represented, historically, on the Billboard Top 100, albeit with an advantage toward males (Lafrance, Worcester, & Burns, Reference Lafrance, Worcester and Burns2011). While many of the highest-grossing musical artists of all time are male, sex differences in success as a musician likely have little to do with biology – a half-century ago, virtually all professional orchestral musicians were male, for example, whereas now the world's top orchestras are approaching gender parity (Sergeant & Himonides, Reference Sergeant and Himonides2019).
This pattern of evidence has contributed to a growing consensus that links between music and mate quality are weak (Mosing et al., Reference Mosing, Verweij, Madison, Pedersen, Zietsch and Ullén2015; Ravignani, Reference Ravignani2018).
4.2 Music as a credible signal of cooperative intent
We agree with proponents of the mate quality hypothesis that music is a credible signal. But song-like vocalizations in non-human animals often signal much more than mate quality. Even in songbirds, the poster-species for the sexual selection of male song, singing can serve other functions, such as territorial advertisements (Tobias et al., Reference Tobias, Sheard, Seddon, Meade, Cotton and Nakagawa2016).
We also agree with the proponents of the social bonding hypothesis that musical abilities evolved because musical performances played an important role in cooperative sociality. But given the issues described above, we find it more likely that music evolved to credibly signal decisions to cooperate that were already reached by other means, not to determine them. Cooperation often fails, making it useful to have a credible signal indicating that, by various (non-musical) means, one or more agents have decided to cooperate. Credible signals of cooperative intent, in turn, can produce decisions by signal receivers that benefit the signalers.
We will discuss two behavioral contexts where complex vocal signals have evolved in numerous other species; where unique characteristics of the human species created selection pressures for an elaborate credible signal; and where music universally appears.
First, in the context of territorial advertisements, we consider pressures of coordinated territorial defense across coalitions and in the context of cooperative alliances with other groups. We propose that music could function as a credible signal of coalition strength, size, and coordination ability.
Second, in the context of contact calls, we consider pressures of helpless infants requiring substantial parental investment, relative to other primates; and multiple dependent siblings competing for parental investment. We propose that music could function as a credible signal of parental attention.
4.2.1 Synchronous coordinated music as a credible signal of coalition strength, size, and cooperation ability
In mammals, loud auditory signals are frequently agonistic, and territorial advertisements are a prime example (Gustison & Townsend, Reference Gustison and Townsend2015). Territoriality is common in taxa ranging from bacteria to vertebrates (Maher & Lott, Reference Maher and Lott2000; Smith & Dworkin, Reference Smith and Dworkin1994), including primates (Willems & van Schaik, Reference Willems and van Schaik2015). Territory owners have a consistent advantage over intruders, often retaining their territory without a fight (Kokko, Jennions, & Brooks, Reference Kokko, Jennions and Brooks2006). It is thus in the interest of owners to advertise their residence in a territory to deter intruders and avoid a fight.
Territorial calls, which credibly signal that a territory is occupied, are found in many species, including birds, primates, and other mammals (Bates, Reference Bates1970; Gustison & Townsend, Reference Gustison and Townsend2015; Ladich & Winkler, Reference Ladich and Winkler2017; Wich & Nunn, Reference Wich and Nunn2002). Loud primate calls are a plausible evolutionary precursor to human music (Geissmann, Reference Geissmann, Wallin, Merker and Brown2000) because they appear to have existed in the last common ancestor of all primates and are often produced by both sexes and directed at both sexes (Wich & Nunn, Reference Wich and Nunn2002). Some African apes display drumming-like behaviors as part of territorial signals (Goodall, Reference Goodall1986; Hagen & Hammerstein, Reference Hagen and Hammerstein2009). In humans, vocal and instrumental music are reliably associated with war, procession, and ritual across a representative sample of societies (Mehr et al., Reference Mehr, Singh, Knox, Ketter, Pickens-Jones, Atwood and Glowacki2019, Table 1); appears in political and military contexts with analogs to territorial signaling (Hagen & Bryant, Reference Hagen and Bryant2003; Hagen & Hammerstein, Reference Hagen and Hammerstein2009); is generally not sexually differentiated (see above); and, of course, is often loud.
Social species that collectively defend territory, such as chimpanzees and several species of social carnivores (e.g., lions, wolves), produce coordinated vocal territorial advertisements (e.g., roars, howls), which credibly signal group size to potential intruders (Harrington, Reference Harrington1989; Harrington & Mech, Reference Harrington and Mech1979; Krebs, Reference Krebs1977; McComb, Packer, & Pusey, Reference McComb, Packer and Pusey1994; Wilson, Hauser, & Wrangham, Reference Wilson, Hauser and Wrangham2001). In a study of nearly 10,000 bird species, the presence of communal signaling was associated with territoriality, typically in conjunction with stable social bonds (Tobias et al., Reference Tobias, Sheard, Seddon, Meade, Cotton and Nakagawa2016). Moreover, the effect of territoriality was more than twice the size of that of social bonds, and territoriality was a crucial precursor to communal signaling, suggesting that long-term social bonds might evolve after communal signaling.
Some coordinated vocal signals, like bird duets, involve complex, temporally synchronized displays. A high level of synchronous coordination among signalers requires considerable effort to achieve, and thus credibly signals a willingness and ability to cooperate over time, thereby serving as an index of the quality of the coalition defending the territory, above and beyond coalition size (critical information otherwise not apparent to intruders; Hagen & Bryant, Reference Hagen and Bryant2003; Hall & Magrath, Reference Hall and Magrath2007; Wiley & Wiley, Reference Wiley and Wiley1977). If synchronous coordination is a signal of coalition quality, selection should push receivers to better discriminate differences in degrees of coordination, and signalers to produce more complex coordinated signals, leading to signal elaboration.
Several primate species also produce highly synchronized song-like duets. As in birds, song-like calls are characteristic of species living in small, monogamous groups (Schruth, Templeton, & Holman, Reference Schruth, Templeton and Holman2019). Although duetting and coordinated vocalizations might have some role in pair-bond formation and strengthening in a few monogamous species, such as gibbons and titi monkeys, most evidence suggests these calls primarily function to exclude intruders and maintain spacing: they are territorial advertisements (Snowdon, Reference Snowdon, Quam, Ramsier, Fay and Popper2017).
Experimental evidence suggests that higher levels of coordination in such signals indicate higher coalition quality. Duetting magpie-larks that had been paired for a longer time were more likely to produce highly coordinated displays, and in an experimental loudspeaker study on natural territories, playbacks of highly coordinated duets, which simulated territorial intrusions, evoked significantly higher song rates by resident males than poorly coordinated duets (indicating that the highly coordinated duets were perceived as more threatening; Hall & Magrath, Reference Hall and Magrath2007).
Humans are both primates and social hunters, so we expect human ancestors to have advertised territory ownership in a similar fashion: using loud, coordinated vocalizations, perhaps with drumming. We propose that such territorial vocalizations are an evolutionary precursor to music, especially rhythmic music (Hagen & Bryant, Reference Hagen and Bryant2003; Hagen & Hammerstein, Reference Hagen and Hammerstein2009; cf. Merker, Reference Merker2000b). Signatures of this function might persist in modern humans in coordinated group dances that are universal across cultures (Mehr et al., Reference Mehr, Singh, Knox, Ketter, Pickens-Jones, Atwood and Glowacki2019; Nettl, Reference Nettl2015; e.g., the Māori haka; Best, Reference Best1924). The group music of Aka Congo Basin hunter-gatherers, for instance, is audible to groups living some distance away.Footnote 12
Complex forms of social organization likely set the stage for the evolution of complex credible signals, including synchronized and coordinated vocalizations. Sometime after diverging from other apes, the human lineage underwent a major transition to a multilevel society. In multilevel societies, small family units regularly aggregate with other family units, forming a higher-level unit, which in some species aggregate to form an even higher-level unit. This societal structure occurs in some other primate species (e.g., hamadryas baboons; Swedell & Plummer, Reference Swedell and Plummer2019), and some evidence suggests that higher degrees of social complexity are correlated with increased vocal diversity and flexibility (e.g., in macaques; Rebout, De Marco, Lone, & Thierry, Reference Rebout, De Marco, Lone, Sanna, Cozzolino, Micheletta and Thierry2020).
A notable attribute of some multilevel-society species is that, in addition to the agonism or tolerance exhibited between units, units also cooperate. Homo sapiens exhibits particularly rich cooperative behavior between units: cooperative families are nested within cooperative residential groups that often form cooperative alliances with other residential groups to obtain food, buffer resource variation, raise children, defend territory, and so on (Chapais, Reference Chapais2013; Hamilton, Milne, Walker, Burger, & Brown, Reference Hamilton, Milne, Walker, Burger and Brown2007; Pisor & Surbeck, Reference Pisor and Surbeck2019; Rodseth, Wrangham, Harrigan, & Wolpoff, Reference Rodseth, Wrangham, Harrigan, Smuts, Dare, Fox and Wolpoff1991; Swedell & Plummer, Reference Swedell and Plummer2019).
Between-group cooperation likely created many new selection pressures. In particular, if human groups varied in the benefits they could provide other groups as allies, and the number of alliances a group could maintain was limited, a biological market would have arisen (Hammerstein & Noë, Reference Hammerstein and Noë2016), wherein groups evaluated the coalition quality of potential allies by assessing their size, cooperation ability, and willingness to cooperate, and potential allies had incentives to exaggerate these qualities (Hagen & Bryant, Reference Hagen and Bryant2003; Hagen & Hammerstein, Reference Hagen and Hammerstein2009). Common properties of music, especially those found in rhythmic, coordinated performances, provide a close fit to the necessary criteria for a credible signal of such otherwise difficult-to-observe group-level features. The time needed to create and practice group complex musical performances and achieve complex synchrony necessarily corresponds to a dimension of the underlying quality of the coalition: the amount of time coalition members have cooperated with one another.
In summary, we propose that music evolved, in part, as a means for groups to credibly show off their qualities to other groups.
There is substantial ethnographic, historical, and archaeological evidence of credible signaling of coalition quality among human groups, typically in the context of feasting. In feasting, two or more individuals share special types or quantities of foods, for a special purpose or event (Hayden, Reference Hayden2014). In addition to food, feasting often includes special clothing, ornaments, and other artifacts – and music and dance. Feasting has been documented in societies of all levels of social complexity, ranging from band-level hunter-gatherers to nation-states, including at archaeological sites throughout the Holocene (reviewed in Hayden, Reference Hayden2014; Hayden & Villeneuve, Reference Hayden and Villeneuve2011). While many functions of feasting have been proposed (Hayden, Reference Hayden2014; Wiessner & Schiefenhövel, Reference Wiessner and Schiefenhövel1998), there is widespread agreement that feasts play a critical role in the formation of alliances between groups (reviewed in Hayden, Reference Hayden2014; Hayden & Villeneuve, Reference Hayden and Villeneuve2011). As Sosis (Reference Sosis2000) observed, the goal of ritualized foraging and feasting and other forms of food distribution is often to enhance the reputation of an entire group by displaying its productivity.
It is notable, then, that music and dance co-occur with feasting frequently in the ethnographic record.Footnote 13 For example, Congo Basin hunter-gatherers are renowned for their music, which they perform in many social contexts, including at spirit plays and large inter-community dances following big game kills (Fürniss, Reference Fürniss and Hewlett2017; Lewis, Reference Lewis and Arbib2013, Reference Lewis and Hewlett2017), as in the Mbendjele BaYaka:
Sharing [food] between camps is less frequent, but will occur when big game is killed and during massana forest spirit performances. When an elephant is killed, Mbendjele in the area go rapidly to where the carcass is lying. Large camps grow, and feasting and dancing go on until the elephant has been consumed. (Lewis, Reference Lewis and Hewlett2017, p. 227)
It is plausible that similar events regularly occurred during human evolution because there is archaeological evidence for domestic spaces, large game hunting, mass kills, cooking, large aggregations, burials, ornaments, use of pigments, and musical instruments throughout the Upper Pleistocene, with some evidence appearing earlier (Barham, Reference Barham2002; Conard et al., Reference Conard, Malina and Münzel2009; Kuhn, Reference Kuhn2014; Kuhn & Stiner, Reference Kuhn and Stiner2019; Maher & Conkey, Reference Maher and Conkey2019; Stiner, Reference Stiner2019, Reference Stiner2013, Reference Stiner2017).
A function of music in the context of alliance formation may also help to explain why music is often produced for and enjoyed by strangers, as in modern recorded music and live concerts. The selective dynamics of “social foraging” in the hominin niche, where strangers have an uncertain but non-zero possibility of becoming lucrative social partners, frame strangers as the appropriate targets of social foraging tactics (Delton, Krasnow, Cosmides, & Tooby, Reference Delton, Krasnow, Cosmides and Tooby2011; Delton & Robertson, Reference Delton and Robertson2012; Rand, Peysakhovich, Kraft-Todd, & Greene, Reference Rand, Peysakhovich, Kraft-Todd, Newman, Wurzbacher, Nowak and Greene2014). Moreover, some data show that observers can infer coalition quality and fighting ability from observations of musical performances and other coordinated behaviors. People who listened to a musical performance with instruments mixed either in-sync, consistently out-of-sync, or scrambled rated coalition quality higher in the in-sync versus out-of-sync (but not scrambled) conditions (Hagen & Bryant, Reference Hagen and Bryant2003). When listening to people marching asynchronously or synchronously, judges rated the synchronous groups as more formidable, better able to coordinate a physical attack, and higher in social closeness; judgments of formidability were mediated by judgments of coordination, not bonding (Fessler & Holbrook, Reference Fessler and Holbrook2016).
On this view, music is clearly rooted in sociality. In contrast to the social bonding hypothesis, however, we predict that music does not directly cause social cohesion: rather, it signals existing social cohesion that was obtained by other means (Hagen & Bryant, Reference Hagen and Bryant2003, p. 30).
We do not think this is the only social context in which music can act as a credible signal. Within groups, musical performances might also create common knowledge of decisions to cooperate, which could serve group coordination and cooperation (Chwe, Reference Chwe2001; Freitas, Thomas, DeScioli, & Pinker, Reference Freitas, Thomas, DeScioli and Pinker2019; see Hagen & Bryant, Reference Hagen and Bryant2003 for other possibilities); credibly signal qualities guiding same-sex partner choice in a biological market (Hammerstein & Noë, Reference Hammerstein and Noë2016), and perhaps informing mate choice by both sexes; and as a group analog of emotional expression (Hagen & Bryant, Reference Hagen and Bryant2003; Hagen & Hammerstein, Reference Hagen and Hammerstein2009). Producing music that is specific to a group might also credibly signal membership in that group (Mehr et al., Reference Mehr, Song and Spelke2016; Mehr & Spelke, Reference Mehr and Spelke2017) in a fashion similar to food preferences and dialects (see Kinzler, Dupoux, & Spelke, Reference Kinzler, Dupoux and Spelke2007; Liberman, Woodward, Sullivan, & Kinzler, Reference Liberman, Woodward, Sullivan and Kinzler2016).
Next, we examine a case where we believe within-group rather than between-group credible signaling has shaped music.
4.2.2 Infant-directed song as a co-evolved system for negotiating parental investment of attention
Contact calls are a common vocalization across many species, distinct from territorial signals. In primates, these include loud calls between separated group members, and frequent quiet calls during heightened risk of separation (e.g., in dense vegetation). Contact calls rank among the most diverse and complex call types across species (Bouchet, Blois-Heulin, & Lemasson, Reference Bouchet, Blois-Heulin and Lemasson2013; Leighton, Reference Leighton2017), enabling individuals to recognize, estimate distance to, and maintain contact with their social partners (Kondo & Watanabe, Reference Kondo and Watanabe2009; Rendall, Cheney, & Seyfarth, Reference Rendall, Cheney and Seyfarth2000).
One important class of contact calls are those between parents and offspring. These serve functions of mutual interest to parents and offspring, for example, enabling parents to be available to solve problems their offspring are ill-suited to solve on their own. Chacma baboon barks, for example, range from tonal, harmonically rich variants that are used for contact calls, to barks with a noisier, harsher structure that are used for alarm calls. By the age of 6 months, infants learn to discriminate call types and to discriminate their mothers' contact barks from those of unrelated females (Fischer, Cheney, & Seyfarth, Reference Fischer, Cheney and Seyfarth2000); and mothers recognize their infants' contact calls (Rendall et al., Reference Rendall, Cheney and Seyfarth2000).
We propose that in the human lineage, maternal contact calls evolved to encode credible information beyond identity and distance, namely attention to the infant. There are few relationships where inclusive fitness interests overlap as much as they do between parents and offspring – but even these are not perfectly aligned. Because of the mechanics of diploid sexual reproduction, a parent is equally related to all her offspring, whereas each offspring is twice and four times as related to itself as it is to each of its full and half siblings, respectively. A strategy that optimizes the parent's inclusive fitness (e.g., equal food distribution across offspring) does not necessarily optimize an offspring's inclusive fitness, and vice versa. This possibility, parent–offspring conflict (Trivers, Reference Trivers1974), implies differences in the interests of supply genes in the parent and demand genes in the offspring (Bossan, Hammerstein, & Koehncke, Reference Bossan, Hammerstein and Koehncke2013).
Some aspects of human reproduction suggest that selection pressures for complex contact calls have increased relative to those in apes. First, human brain size is about triple that of other apes (Schoenemann, Reference Schoenemann2006) and most brain growth occurs postnatally, implying that human infants are born helpless and have a very long juvenile period. Second, human forager interbirth intervals are about half those of chimpanzees (Marlowe, Reference Marlowe2005; Thompson, Reference Thompson2013), requiring ancestral human mothers to simultaneously care for multiple dependent offspring (in contrast to chimpanzee mothers, who typically care for a single dependent offspring). Third, unlike other great apes, humans rely heavily on alloparenting in a multilevel society, requiring ancestral human infants to establish relationships with multiple caregivers and vice versa (Hrdy, Reference Hrdy2009). Unlike chimpanzee infants, ancestral human infants typically competed with multiple juveniles for the attention of multiple caregivers.
Human parents increase their offspring's fitness by attending to them and protecting them from harm. Attention is a limited resource, however; many other challenges require attention, and solving those may benefit the parent more than the infant (relative to the provisioning of attention to maintain infant safety). The interests of infants and their parents conflict, in terms of the optimal provisioning of attention: infants often “prefer” more attention than a parent would “prefer” to provide.
How does this conflict of interests play out? Infants have bargaining power to extract parental investment (in the form of material investment, like nursing, or parental attention); they demand attention by crying (for discussion of evolutionary scenarios, see Soltis, Reference Soltis2004 and commentaries). Parents lack perfect access to their infant's internal state, so crying provides information about when investment can be provided. Care-eliciting infant vocalizations (e.g., distress calls, separation calls) are common across mammals (Newman, Reference Newman2007), including humans, and mothers reliably respond to these vocalizations by providing care (Bornstein, Putnick, Rigo, & Venuti, Reference Bornstein, Putnick, Rigo, Esposito, Swain, Suwalsky and Venuti2017).
Whereas infants can easily detect when material investment has been provisioned, attention is a covert property of the parent's mind, with unreliable cues. Infants can infer that parents are attending to them from estimating the parent's gaze direction, but this only provides partial information (the parent could be concentrating on something else). Touch is also a good cue that a parent is nearby; but the parent could be asleep, or attending to something else.
Better than these cues would be a credible signal from the parent, reliably indicating that the infant has their attention (Mehr & Krasnow, Reference Mehr and Krasnow2017). A vocal signal is a good candidate because its acoustic properties allow the proximity of the producer of the signal to be reliably inferred by the target. To the extent that the signal monopolizes the vocal apparatus, producing it is incompatible with other activities (such as speaking to another adult) that could co-opt the parent's attention. And aspects of the vocal signal can be modulated in real time, in response to the infant's state and behavior, which cannot be done without attending to the infant.
Here again we expect an evolutionary arms race, driven by partially conflicting fitness interests between senders and receivers, producing an elaborated signal. We propose that this process could lead to key features of music: in particular, contrasting with the rhythmic features developing from territorial signals, we expect the rather more subdued context of soothing parent–infant contact calls to give rise to melodic features, tokens of which are the lullabies we sing to infants today (Mehr & Krasnow, Reference Mehr and Krasnow2017).
Three sets of results support this idea. First, if adaptations support the production of song in parents and alloparents, and the appetite for and ability to perceive song in infants, then music should appear universally in the context of infant care and infant-directed songs should share features worldwide. These predictions, long discussed in the music cognition literature (Hannon & Trainor, Reference Hannon and Trainor2007; Peretz, Reference Peretz2006; Trehub, Reference Trehub2001), are well-evidenced. In an analysis of high-quality ethnography from a representative sample of human societies, text concerning vocal music was significantly associated with infant care and children, over and above base rates of reporting (this finding replicates both with expert annotations of the ethnography and automated text analysis; Mehr et al., Reference Mehr, Singh, Knox, Ketter, Pickens-Jones, Atwood and Glowacki2019). Moreover, infant-directed songs are found in 100% of a pseudorandom sample of field recordings in mostly-small-scale societies; and naïve listeners, who are unfamiliar with the languages or cultures involved, reliably recognize them as infant-directed, with remarkable consistency (Mehr & Singh et al., Reference Mehr, Singh, York, Glowacki and Krasnow2018, Reference Mehr, Singh, Knox, Ketter, Pickens-Jones, Atwood and Glowacki2019). This finding replicates prior cross-cultural work (Trehub, Unyk, & Trainor, Reference Trehub, Unyk and Trainor1993a, Reference Trehub, Unyk and Trainor1993b).
Second, the genetic architecture of musical perception and motivation should be regulated, in part, by parent-of-origin epigenetic mechanisms, such as genomic imprinting. Humans are sexually reproducing but not obligately monogamous, which differentiates the conflict of interest between parents and offspring by parental sex: because maternity certainty is greater than paternity certainty, genes of maternal origin are more likely to be found in an offspring's siblings than genes of paternal origin (Haig & Wilkins, Reference Haig and Wilkins2000). Genes of maternal origin are thus under selection to bias the tradeoff in demand for parental investment in the direction of the offspring's siblings and away from the offspring; on average, maternally inherited genes should reduce investment demands on mothers, and vice versa. This prediction is confirmed by the fact that genes with parent-of-origin effects tend to affect demands for parental investment, such as intrauterine growth (Haig, Reference Haig1993).
Genomic imprinting disorders, where genetic dysregulation is differentiated by parent-of-origin, provide a unique test of the relation between a trait and its putative link to parental investment (Haig & Wharton, Reference Haig and Wharton2003). Angelman and Prader-Willi syndromes result from opposing dysregulation at the same genetic region (15q11-13), with a loss of genes expressing maternal interest resulting in Angelman syndrome, and the reverse, a relative loss of genes expressing paternal interest resulting in Prader-Willi syndrome. The behavioral phenotypes reflect the different effects of maternally versus paternally inherited genes: infants with Angelman syndrome have a voracious appetite while nursing, are awake for more hours of the day than typically developing infants, and attract more attention via smiling than do typically developing children (Ubeda, Reference Ubeda2008; Williams, Beaudet, Clayton-Smith, & Wagstaff, Reference Williams, Beaudet, Clayton-Smith, Knoll, Kyllerman, Laan and Wagstaff2006), increasing investment demands on the mother. Infants with Prader-Willi syndrome, in contrast, are born with low birth weight, sleep more than typically developing infants, and often lack a suckle reflex (Cassidy & Driscoll, Reference Cassidy and Driscoll2008; Holm, Cassidy, Butler, & Greenberg, Reference Holm, Cassidy, Butler, Hanchett, Greenswag, Whitman and Greenberg1993; Peters, Reference Peters2014), with the opposite effect (decreasing investment demands on the mother).
Recent findings show that these effects extend to the domain of music, demonstrating a genetic link between music perception and parental investment. People with Angelman syndrome have a suppressed relaxation response to music (Kotler, Mehr, Egner, Haig, & Krasnow, Reference Kotler, Mehr, Egner, Haig and Krasnow2019); while people with Prader-Willi syndrome have a potentiated relaxation response to music, along with pitch perception deficits (Mehr, Kotler, Howard, Haig, & Krasnow, Reference Mehr, Kotler, Howard, Haig and Krasnow2017). These results support the idea that music signals attention: suppressed relaxation in Angelman syndrome implies increased maternal demands, while potentiated relaxation in Prader-Willi syndrome implies reduced maternal demands, in line with other findings concerning parental investment demands in genomic imprinting disorders.
Last, we also expect relationships between the acoustic features of non-human primate contact calls and human infant-directed song. While few data exist with which to test these relationships, preliminary findings suggest that similarities do exist. For example, baboon contact calls are harmonically rich, whereas alarm calls are harsh and noisy (Fischer et al., Reference Fischer, Cheney and Seyfarth2000); in a vocalization corpus from 21 human societies, infant-directed song was acoustically distinct from infant-directed speech across many pitch, rhythmic, phonetic, and timbral attributes (Moser et al., Reference Moser, Lee-Rubin, Bainbridge, Atwood, Simson, Knox and Mehr2020), with a similar pattern of results to the acoustic differences between baboon contact calls and alarm calls. Moreover, several acoustic features driving these effects were related to vocal exertion (e.g., temporal modulation, pitch rate, vowel rate), perhaps honestly signaling additional costs incurred by the signaler.
5. Discussion
A comprehensive understanding of music requires that proximate-level explanations are distinguished from ultimate-level explanations uniquely linked to music; that proposed adaptations explain the core features of music that are putatively shaped by natural selection, and distinguish them from features that are byproducts of other adaptations; and finally, that the results of evolutionary analyses provide a foundation on which cultural-evolutionary processes can plausibly act.
The credible signaling account meets these criteria, whereas other accounts of the origins of music do not.
5.1 Credible signaling may explain some basic features of music
Early in this paper we noted some properties of human music that need explanation. While we find it implausible that any one theory can explain all of them, two core features of music are directly related to the ideas presented here.
An evolved system for quickly and reliably signaling coalition quality, which might otherwise be difficult to perceive, especially during territorial advertisements, agonistic intergroup encounters (e.g., war songs, dances), and alliance-forging feasts, provides a functional explanation for rhythm: selection pressures toward synchronized isochronous sounds, with complex internal design. An evolved system for credibly signaling parental attention to infants provides a functional explanation for melody: selection pressures toward manipulating affective prosody in vocalizations, constrained by the physics of the vocal production system and inherent features of the auditory world.
These “building blocks” appear universally in music (Mehr et al., Reference Mehr, Singh, Knox, Ketter, Pickens-Jones, Atwood and Glowacki2019; Nettl, Reference Nettl2015; Savage et al., Reference Savage, Brown, Sakai and Currie2015), like “building blocks” of language (e.g., Baker, Reference Baker2001). They provide a grammar-like, combinatorially generative interface through which musical content can be created, improvised, and elaborated upon, through hierarchical organization of meter and tonalityFootnote 14 (Krumhansl, Reference Krumhansl2001; Lerdahl & Jackendoff, Reference Lerdahl and Jackendoff1983), in fashions that themselves have universal signatures (Jacoby & McDermott, Reference Jacoby and McDermott2017; Jacoby et al., Reference Jacoby, Undurraga, McPherson, Valdés, Ossandón and McDermott2019; Mehr et al., Reference Mehr, Singh, Knox, Ketter, Pickens-Jones, Atwood and Glowacki2019).
The importance of rhythm and pitch in human music perception – and the degree to which these features of music are unique to human vocalizations – may be directly tied to their evolutionary history.
5.2 Music is culturally evolved but cultural evolution has to start somewhere
We understand culture as information that affects individuals' behavior and that is acquired from conspecifics through teaching, imitation, and other types of social transmission (Boyd & Richerson, Reference Boyd and Richerson2004; Tooby & Cosmides, Reference Tooby and Cosmides1992). Because information is transmitted with some degree of fidelity through non-genetic means (e.g., memory, learning), information is cumulative. Some cultural information is passed on with greater frequency and higher regularity than other information. For example, social learners tend to pay attention to information sources that have established prestige more than sources that do not (Henrich & Boyd, Reference Henrich and Boyd2002). Similarly, some information is easier to learn than other information; children exhibit interest about information associated with danger and retain it with greater fidelity and over longer periods than related information unassociated with danger (Barrett, Peterson, & Frankenhuis, Reference Barrett, Peterson and Frankenhuis2016; Wertz, Reference Wertz2019).
One characteristic of cumulative culture is ritualization (Lorenz, Reference Lorenz1966), analogous to co-evolutionary processes underlying animal communication systems (Krebs & Dawkins, Reference Krebs, Dawkins, Krebs and Davies1984). Cultural signals can develop extravagant physical features resulting from arms race dynamics, particularly in cases when there is a conflict of interest between senders and receivers. Examples from modern environments include conspicuously branded luxury goods, which can signal wealth (Han, Nunes, & Drèze, Reference Han, Nunes and Drèze2010); or businesses that engage in one-upmanship by incorporating exaggerated sensory features in competitive advertisements (Dunham, Reference Dunham and Saad2011).
Such cultural ritualization is likely at play in the musical domain, especially given the increasingly important role of elaborate feasting in the cultural evolution of social complexity across the globe throughout the Holocene (Hayden, Reference Hayden2014), and given the highly variable musical features that continually unfold over time across compositional styles, instrumentation and orchestration, improvisatory motifs, setting lyrics to music, and so on. Music must be shaped by culture in all contexts, however, not only those of coalition signaling and parental care.
We propose that the adaptations proposed here provide a foundation for cultural-evolutionary processes. These traits – particular grammar-like structures, for instance, such as tonalities and meters – gravitate toward certain forms, or “attractors,” and away from others (Sperber, Reference Sperber1996; Sperber & Hirschfeld, Reference Sperber and Hirschfeld2004). These attractors will interact with evolved capacities for non-musical traits resulting in sensitivity and attraction to features in communicative acts that trigger them (e.g., musical phenomena that evoke the sound of an emotional voice), increasing variability in music.
As this process repeats within and across cultures, the diversity of music increases, while underlaid by universals that can be traced back to music's adaptive functions in credible signaling. This pattern of universality and diversity is exactly what is observed in systematic analyses of music across cultures (Mehr et al., Reference Mehr, Singh, Knox, Ketter, Pickens-Jones, Atwood and Glowacki2019) and, we believe, is what continues to shape music, worldwide, today.
Understanding this variability has been a longstanding interest of ethnomusicologists, who document musical traditions as they are shaped by social environments, politics, and ethnolinguistic history (Blacking, Reference Blacking1973; Feld, Reference Feld1984; Nettl, Reference Nettl2015), but it has strong parallels in the study of cultural evolution and social transmission. In particular, the prevalence of specific musical features (a particular scale, musical instrument, ornament, vocal practice, and so on) in a given society's music is likely to be shaped by that society's relation to other societies, just as the presence or absence of linguistic features is predictable by lineage (Dunn, Greenhill, Levinson, & Gray, Reference Dunn, Greenhill, Levinson and Gray2011).
We expect that studying the cultural evolution of musical features will be a productive endeavor (with promising first steps already underway; e.g., Savage et al., Reference Savage, Brown, Sakai and Currie2015). We predict, however, that those features least likely to be shaped by culture are those core features predicted by the evolutionary account described here. For example, whereas we expect few musical systems worldwide to lack melody and rhythm as core features, we expect many to have rather different instantiations of those features. This is uncontroversial: while scales commonly used in music differ across cultures, they nevertheless are mutually intelligible, implying shared psychological mechanisms for music perception surrounding the interpretation of melodies (Castellano, Bharucha, & Krumhansl, Reference Castellano, Bharucha and Krumhansl1984; Krumhansl et al., Reference Krumhansl, Toivanen, Eerola, Toiviainen, Järvinen and Louhivuori2000; Mehr et al., Reference Mehr, Singh, Knox, Ketter, Pickens-Jones, Atwood and Glowacki2019).
5.3 Auditory cheesecake: not wrong, but not right either
A key difficulty of studying the evolution of music, to which we alluded throughout this paper, is that the present environment has diverged from the environment in which humans evolved. In this context, Pinker's (Reference Pinker1997) “auditory cheesecake” analogy for a byproduct account of music is neither surprising nor controversial. We should expect many human behaviors to have cheesecake-like features. Just as the world's great writers have stretched the bounds of human language far beyond language's original adaptive functions, the boundless creativity of composers and performers have created an actual domain of music that, we believe, is quite far from its proper domain.
In this sense, we agree with Pinker that many musical inventions are byproducts, plain and simple: auditory cheesecake is not wrong. But in light of the adaptations proposed here, auditory cheesecake isn't right either: as we have argued, in at least two contexts, music exhibits design features consistent with adaptations for credible signaling, which give rise to a universal human psychology of music.
6. Conclusions
Why study the origins of music, language, or any other human behavior? It's unlikely that anyone will ever explain the full extent to which a particular behavior is accounted for by one or more adaptations because, given its complexity, human behavior cannot be exhaustively measured.
Nevertheless, we think that inching toward a functional understanding of complex behavior helps determine what the phenomena in question are, exactly, by isolating the core psychological representations and cultural processes underlying the phenomena from those that are merely associated with them. In the case of music, the analyses presented here lay out a roadmap for understanding the phenomenon of human musicality.
Music-like behaviors occur in a broad swath of species, including our ape relatives, and increasing evidence indicates that these serve important credible signaling functions among agents with conflicts of interest, such as territorial advertisements and mate attraction. In humans, across cultures, music is associated with social behaviors that directly involve credible signaling private information among agents with conflicts of interest, especially coalitional interactions and infant care, but perhaps others too. Accordingly, the psychological mechanisms for processing and producing features of music that are implied by those contexts, such as melody and rhythm, should also be universal; all of this is proposed to constitute music's proper domain. In music's actual domain, in contrast, we should expect the engine of cultural evolution to develop and expand these features, producing a diverse set of musical manifestations worldwide that retain some key features of their evolved functions.
Additional mechanisms likely interact with these core features. These may include psychological mechanisms that enable the perception of higher-level features of music, such as implied harmony or musical emotions; linguistic mechanisms that shape the ways in which language and music are intertwined; cultural mechanisms that drive musical traditions and are shaped historically as cultures mix and combine to form new cultures; technological mechanisms that directly alter the feature space of musicality, including musical inventions, such as instruments and music production software, or new musical forms, such as microtonal music; and, not least, aesthetic mechanisms that drive the preferences and interests of those who make and listen to music worldwide.
Understanding these mechanisms in isolation and as they interact with each other to produce the phenomenon of human musicality is a key challenge for the field – a challenge that will be served well by a clear explanation for the origins of music, which can then be built upon using the interdisciplinary toolkit of modern science.
Acknowledgments
We thank Paul Seabright, the Institute for Advanced Study in Toulouse, and the Fondation Royaumont, whose meeting “Origins of music in non-state societies” sparked the idea for this paper; and Mila Bertolo, Courtney Hilton, Cody Moser, and Manvir Singh for feedback on the manuscript.
Financial support
The “Origins of music in non-state societies” meeting was supported by the Agence Nationale de la Recherche (Investissements d'Avenir ANR-17-EURE-0010). S.A.M. is supported by the Harvard Data Science Initiative and the National Institutes of Health Director's Early Independence Award DP5OD024566.
Conflict of interest
None.
Treatment Index
TARGET ARTICLES
Savage, Patrick E., Psyche Loui, Bronwyn Tarr, Adena Schachner, Luke Glowacki, Steven Mithen, and W. Tecumseh Fitch, Music as a coevolved system for social bonding, 1
Mehr, Samuel A., Max M. Krasnow, Gregory A. Bryant, and Edward H. Hagen, Origins of music in credible signaling, 23
COMMENTARIES
Akkermann, Miriam, Ugur Can Akkaya, Cagatay Demirel, Dirk Pflüger, and Martin Dresler, Sound sleep: Lullabies as a test case for the neurobiological effects of music, 95
Ashley, Richard D., Music, groove, and play, 39
Atzil, Shir, and Lior Abramson, Musicality was not selected for, rather humans have a good reason to learn music, 41
Belfi, Amy M., Social bonding and music: Evidence from lesions to the ventromedial prefrontal cortex, 43
Benítez-Burraco, Antonio, Evolutionary linguistics can help refine (and test) hypotheses about how music might have evolved, 44
Bowling, D. L., M. Hoeschele, and J. C. Dunn, Progress without exclusion in the search for an evolutionary basis of music, 96
Brown, Steven, Music and dance are two parallel routes for creating social cohesion, 46
Cross, Ian, Music, attachment, and uncertainty: Music as communicative interaction, 48
Del Mastro, Maria Chiara, Maria Rosaria Strollo, and Mohamad El Haj, Music as a social bond in patients with amnesia, 49
Dissanayake, Ellen, Ancestral human mother–infant interaction was an adaptation that gave rise to music and dance, 51
Dubourg, Edgar, Jean-Baptiste André, and Nicolas Baumard, The evolution of music: One trait, many ultimate-level explanations, 98
Eirdosh, Dustin, and Susan Hanisch, The music and social bonding hypothesis does require multilevel selection, 52
Fritz, Jonathan B., Is the MSB hypothesis (music as a coevolved system for social bonding) testable in the Popperian sense?, 53
Gabriel, Shira, and Elaine Paravati, If music be the food of love, play on: Four ways that music may lead to social connection, 55
Gardiner, Martin F., Human evolution of gestural messaging and its critical role in the human development of music, 99
Gingras, Bruno, Music's putative adaptive function hinges on a combination of distinct mechanisms, 57
Grahn, Jessica A., Anna-Katharina R. Bauer, and Anna Zamm, Is neural entrainment to rhythms the basis of social bonding through music?, 58
Hannon, Erin E., Alyssa N. Crittenden, Joel S. Snyder, and Karli M. Nave, An evolutionary theory of music needs to care about developmental timing, 60
Hansen, Niels Chr. and Peter E. Keller, Oxytocin as an allostatic agent in the social bonding effects of music, 61
Harrison, Peter M. C. and Madeleine Seale, Against unitary theories of music evolution, 64
Hattori, Yuko, Bonding system in nonhuman primates and biological roots of musicality, 65
Hilton, Courtney B., Rie Asano, and Cedric Boeckx, Why musical hierarchies?, 101
Honing, Henkjan, Unravelling the origins of musicality: Beyond music as an epiphenomenon of language, 66
Iyer, Vijay, What's not music, but feels like music to you?, 68
Juslin, Patrik N., Mind the gap: The mediating role of emotion mechanisms in social bonding through musical activities, 69
Kasdan, Anna, Reyna L. Gordon, and Miriam D. Lense, A neurodevelopmental disorders perspective into music, social attention, and social bonding, 103
Kennedy, Patrick, and Andrew N. Radford, Credible signalling and social bonds: Ultimately drawing on the same idea, 105
Killin, Anton, Carl Brusse, Adrian Currie, and Ronald J. Planer, Not by signalling alone: Music's mosaicism undermines the search for a proper function, 107
Kraus, Nils, and Guido Hesselmann, Musicality as a predictive process, 71
Leivada, Evelina, The origins of music in (musi) language, 108
Levitin, Daniel J., Knowledge songs as an evolutionary adaptation to facilitate information transmission through music, 109
Lieberman, Debra, and Joseph Billingsley, If it quacks like a duck: The by-product account of music still stands, 111
Lumaca, Massimo, Elvira Brattico, and Giosuè Baggio, Signaling games and music as credible signal, 112
Margulis, Elizabeth Hellmuth, Pluralism provides the best chance for addressing big questions about music, 73
Merker, Bjorn, Music, bonding, and human evolution: A critique, 74
Morrison, Steven J., Bonds and signals underlie the music learning experience, 76
Moser, Cody, Jordan Ackerman, Alex Dayer, Shannon Proksch, and Paul E. Smaldino, Why don't cockatoos have war songs?, 113
Patel, Aniruddh D., Chris von Rueden, Where they sing solo: Accounting for cross-cultural variation in collective music-making in theories of music evolution, 77
Pfordresher, Peter Q., Music production deficits and social bonding: The case of poor-pitch singing, 79
Pinker, Steven, Sex and drugs and rock and roll, 115
Popescu, Tudor, Nathan Oesch, and Bryony Buck, Musical features emerging from a biocultural musicality, 80
Ravignani, Andrea, Isochrony, vocal learning, and the acquisition of rhythm and melody, 82
Rendell, Luke, Emily L. Doolittle, Ellen C. Garland, and Alex South, A boldly comparative approach will strengthen co-evolutionary accounts of musicality's origins, 84
Sachs, Matthew E., Oriel FeldmanHall, and Diana I. Tamir, Clarifying the link between music and social bonding by measuring prosociality in context, 86
Scott-Phillips, Thom, Atsuko Tominaga, and Helena Miton, Ecological and psychological factors in the cultural evolution of music, 116
Sievers, Beau R. and Thalia Wheatley, Rapid dissonant grunting, or, but why does music sound the way it does?, 117
Snyder, Kate T., and Nicole Creanza, Functional and evolutionary parallels between birdsong and human musicality, 119
Stewart-Williams, Steve, Making music: Let's not be too quick to abandon the byproduct hypothesis, 120
Számadó, Szabolcs, Pre-hunt charade as the cradle of human musicality, 121
Tichko, Parker, Kevin A. Bird, and Gregory Kohn, Beyond “consistent with” adaptation: Is there a robust test for music adaptation?, 123
Trainor, Laurel J., Understanding the origins of musicality requires reconstructing the interactive dance between music-specific adaptations, exaptations, and cultural creations, 124
Trehub, Sandra E., Challenging infant-directed singing as a credible signal of maternal attention, 126
Trevor, Caitlyn, and Sascha Frühholz, The evolutionary benefit of less-credible affective musical signals for emotion induction during storytelling, 127
van Mulukom, Valerie, The evolution of music as artistic cultural innovation expressing intuitive thought symbolically, 87
Verpooten, Jan, and Marcel Eens, Singing is not associated with social complexity across species, 89
Wald-Fuhrmann, Melanie, Lara Pearson, Tina Roeske, Christian Grüny and Rainer Polak, Music as a trait in evolutionary theory: A musicological perspective, 90
Wood, Connor, Musical bonds are orthogonal to symbolic language and norms, 129
Zentner, Marcel, Social bonding and credible signaling hypotheses largely disregard the gap between animal vocalizations and human music, 130
Zhang, Qing, and Edward Ruoyang Shi, Why language survives as the dominant communication tool: A neurocognitive perspective, 92
Zou, Ivan Yifan, and William S.-Y. Wang, Music as social bonding: A cross-cultural perspective, 93
AUTHORS' RESPONSES
Savage, Patrick E., Psyche Loui, Bronwyn Tarr, Adena Schachner, Luke Glowacki, Steven Mithen, and W. Tecumseh Fitch, Toward inclusive theories of the evolution of musicality, 132
Mehr, Samuel A., Max M. Krasnow, Gregory A. Bryant, and Edward H. Hagen, Toward a productive evolutionary understanding of music, 140
Target article
Music as a coevolved system for social bonding
Related commentaries (60)
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Why language survives as the dominant communication tool: A neurocognitive perspective
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Author response
Toward a productive evolutionary understanding of music
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