THE BIRTH OF MUSIC IN SYNCHRONOUS CHORUSING AT THE HOMINID-CHIMPANZEE SPLIT
Center for Research in Music Education
Royal University College of Music
One of the few constraints available to guide a search for the origins of music is the existence of human universals (Brown, 1991) in the domain of music, that is, types of music present in every human culture without exception. Chief among these is measured music, i.e. music whose temporal patterns are based on an evenly paced (isochronous) time-giver, the musical pulse or beat (Arom 1991, 2000; Nettl 2000). This structural feature of music avails itself of the universal human capacity to entrain to isochronous pulse trains, or to "keep time" (Fraisse 1982; McNeil 1995), an ability virtually confined to humans among higher animals. As a species-wide behavioral capacity not shared with a relative as close to us as the common chimpanzee (Williams 1967), our ability to entrain to an isochronous pulse is a candidate diagnostic feature of our species relevant to the reconstruction of the origins of music.
Though isochronous entrainment is rare among higher animals, it does occur spottily distributed among lower animals. It is present in certain species of fireflies, cicadas, crickets, crabs and frogs under the name of synchronous chorusing (see review by Greenfield, 1994). In these cases males synchronize their behavior to a common, isochronous pulse during courtship behavior to attract mobile females in the breeding season. Such behavioral synchrony need not be a cooperative behavior aimed at synchrony itself, but may be a default position used by males as a baseline for re-setting the timing of their signal to occur just ahead of neighbors in the sequence. This exploits a mechanism of temporal contrast in the perceptual system of the female which makes her turn to that out of several competing sounds which occurs just ahead of the others (Greenfield and Roizen 1993; Backwell et al. 1998). A few species do, however, appear to engage in genuinely cooperative synchrony. For example, when males - for any of a variety of reasons - live in groups which compete with one another for mobile females, cooperative behavioral synchrony to an isochronous pulse can increase the geographic reach of the courtship signal by amplitude summation of individual signals superimposed through temporal synchrony. The more exact the synchrony - the more perfect the entrainment to the common pulse - the more effective the amplitude summation and thus the greater the geographic reach of the signal, and the greater the number of female ears it will reach (Wells 1977; Buck and Buck 1978; Morris et al. 1978. See also Greenfield, 1994). Well synchronized groups of males would thus tend to attract females to themselves at the expense of less well synchronized groups.
Chimpanzee sociality differs from that of many other primates and mammals by being based on male group territoriality and female exogamy. Females leave their natal territory to settle in a new territory where they mate and rear their young (Pusey 1979; see also Pusey et al 1997). Males, on the other hand, generally do not leave their natal territory but defend it by patrolling its borders against neighboring groups (Wrangham 1975; Ghiglieri 1984). Stable groups of territorial males must, in other words, attract females from other territorial groups, and are thus in competion with one another regarding migrating females. Female exogamy is also present in humans, characterizing a majority of hunter-gatherer societies (Ember 1978). A trait shared by humans and chimpanzees can be assumed to have been present in their common ancestor as well, whose social system accordingly featured groups of associated males competing for migrating females. This global pattern of sociality is strikingly reminiscent of the pattern of sociality associated with the evolution of genuinely cooperative chorus synchrony in insects (Morris et al. 1978; Greenfield 1994), and raises the question of whether synchronous chorusing might have been a factor in the evolutionary history of the hominid-chimpanzee clade.
There are two major points of divergence in this history which have left representatives surviving to this day, one being the late miocene split between hominids - who eventually gave rise to homo - and ancestral chimpanzees some five to six million years ago, the other being the split between ancestral common chimpanzees and ancestral bonobos a few million years later. Synchronous chorusing appears to have played a role in both speciation events, since in either case the present day representatives of one branch of either split, namely human beings and bonobos, appear to possess the capacity for synchronous chorusing, a capacity not possessed by the common chimpanzee. As already noted, the capacity for entrainment to an isochronous pulse is a cross-cultural human universal, and it has been reported that bonobos engage in a unique vocal behavior without homologue in common chimpanzees - so called "staccato hooting" - in which multiple individuals synchronize their hooting to a common steady beat (de Waal 1988).
The possibility that the behavioral adaptation of entrainment for vocal synchrony arose twice by independent evolution from the common ancestor of humans and chimpanzees implies that there must have been strong predisposing factors for it in ancestral behavior, making the step to cooperative synchrony a short one, taken twice by independent evolution. Beyond the global pattern of sociality already referred to there is an additional feature of chimpanzee behavior which might provide the key in this regard, namely the so called "carnival display" (Reynolds and Reynolds 1965; Sugiyama 1969, 1972; Wrangham 1975, 1979; Ghiglieri 1984). When a subgroup of foraging chimpanzee males discovers a ripe fruiting tree they tend to launch a noisy display of combined vocal and locomotor excitement which attracts other males and females to the site. The new arrivals join the group display before eventually settling down to feed on the newly discovered resource. The display is cooperative in that it attracts additional mouths to the resource, and it is an honest signal of resource discovery, since false alarm are likely to provoke retaliation.
Over time, the frequency of carnival displays on a given territory would tend to reflect a combination of male cooperativity and abundance of fruit trees on that territory. This signal would provide a source of potentially important information not only for other members of the territorial group, but for those inter-territorially migrating females who are in the process of choosing a territory on which to settle permanently for the rearing of their young. However, the voice resources of common chimpanzees are such that the chaotic noise of their carnival display is unlikely to span even the diameter of their home territory - measuring some four kilometers across - let alone penetrate the circle of surrounding territories. This constraint imposed by vocal limitations could, however, be overcome by cooperative male synchronous chorussing, allowing the amplitude of individual voices to sum to the extent of the precision of their entrainment to a common, isochronous pulse. Since sound attenuates according to the square of the distance, 4 perfectly synchronized males would double the reach of their signal, while 16 synchronized males would quadruple its reach, allowing the carnival display to penetrate beyond territorial boundaries to reach the ears of migrating females.
It only remains to suggest that humans are the direct descendants of a subpopulation of the human-chimpanzee common ancestor in which the selection pressure of female choice of migratory target territory led to the evolution of temporal synchrony in the vocal behavior of males performing the carnival display. Since the extent of summation of individual voices, and thereby the geographic reach of the summed signal, depends on the precision of synchrony, a premium attached to timing precision. It is further suggested that just as today we assist the precision of our musical timing by relying on repetitive locomotor rhythms like foot tapping, this early break-away subpopulation of the common ancestor supported the precision of its vocal timing by repetitively rhythmic locomotor movements performed in place, that is, by a form of dancing. Synchronous chorusing and dancing to an isochronous pulse in a group setting would seem to qualify as a form of music by a wide range of construals of that elusive term (see, for example, the Bantu term "ngoma" (Keil 1979), the Blackfoot "saapup" (Nettl 2000) and the old greek "mousiké" (Merker 1999)).
If the above evolutionary scenario has any merit, the ultimate roots of human music extend back to the parting of ways between pre-chimpanzees and hominids through a late miocene "breakthrough adaptation" (synchronous chorusing) which allowed a subpopulation of group-living ancestors to broadcast the witness of their voices regarding their own cooperativity and the resource-richness of their territory to the tuned ears of migrating females in search of a suitable territory on which to settle to rear their young. The present-day universal human capacity to keep time (entrain to an isochronous pulse) would accordingly be an adaptation retained from this time, informing the cross-cultural ubiquity of measured music as well as other uses of the human capacity to entrain to an isochronous pulse. This evolutionary scenario for the origins of human music harbors farreaching consequences for numerous issues pertaining to the subsequent trajectory of human evolution, including the issues of brain expansion, the evolution of language, and the relationship between human music and language (see Merker, 2000).
Work on this paper was supported by a grant from the Bank of Sweden Tercentenary Foundation to Bjorn Merker.
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