This behaviour, found in all human cultures, is known as bodily entrainment, defined by W. Tecumseh Fitch as “the capacity to move one’s limbs or body to a complex external ‘beat’.”  Music can compel us to tap our feet, click our fingers, or swing our whole bodies in circles. This movement can be purely spontaneous and unconscious or it can be highly organised. The phenomenon seen in classical concerts of people listening to rhythmic, emotive music in complete stillness is highly exceptional, and has its origins in the elite culture for which that music was originally written. In almost every society, music and movement have explicitly belonged together. Just as we can’t properly understand language if we ignore gesture, we can’t understand music without exploring its relationship to movement.
Of course, movement and music can exist independently of each other. But there is a strong relationship between the auditory system and the motor system, producing the bodily movements that seem to be an instinctive part of our response to music and become part of how we make it. This relationship is expressed most comprehensively in dance, which takes an immense diversity of forms: tribal dances, ballet, the tango, figure-skating, breakdancing and many more. And if music is a means to communicate individual and collective experience, so is dance. This is why dance therapy is possible: by sharing a series of gestures, the client and the therapist can communicate their emotions and build a relationship.
Flutists and dancers in an unusually animated Egyptian wall painting from a tomb in Thebes, c. 1400 BCE.
Why we do all this is surprisingly hard to explain. Like the earliest music-making, movement to music has not left archaeological traces. Dance has certainly been practiced since at least the early civilisations: there are illustrations of dancers from Egypt dating back to the predynastic period, and from Mesopotamia dating to the eighth millenium BCE. But none of this goes back nearly far enough.
Steven Mithen suggests that a hugely important development in both music and dance was the hominid shift to walking on two legs (bipedalism) approximately four million years ago. Our cousins the great apes struggle to stay upright on two legs, and have an anatomy adapted for both knuckle-walking and tree-climbing. Unlike humans they cannot lock their leg into a straight position, and must walk by shifting their weight from side to side on legs that are relatively wide apart. Mithen observes:
We avoid this waddling gait by placing our feet relatively close together on the ground and using strong hip muscles to counter any side-to-side movement.
A reconstruction of Australopithecus afarensis
Humans developed bipedalism as early as the australopithecines, as evidenced by the skeleton of Lucy and the 3.6 million year-old footprints at Laetoli in Tanzania that have been ascribed to Australopithecus afarensis. These species would not have walked as efficiently on two legs as we do. One of the archaeological clues to this is the morphology of the inner ear, which assists our sense of balance: that of the australopithecines was still ape-like, reflecting ways of moving around that included tree-climbing and hanging.
By the advent of Homo ergaster (which had an inner ear morphology much like ours), hominids had developed a bipedalism much like that of modern Homo sapiens.
Anatomical adaptations for bipedalism continue up through the body: a broader pelvis, a curved lower spine, and a head held vertically. With such anatomy, humans have a striding, fluid gait, in which each leg alternately has a swing phase followed by a stance phase.
As argued by the anthropologist Leslie Aiello , it is likely that the increase in brain size in Homo ergaster was necessary to handle the more complex motor control required by bipedalism, an increase that may have given impetus to more general intelligence and language development (and thereby also to singing). It was also significant that a large proportion of our body, most obviously our hands, was freed from being used for locomotion, allowing us to make not only tools but complex gestures.
There has been plenty of debate about why we adopted bipedalism. It was not to free our hands to make tools — although that was certainly useful subsequently — because fossils show it predates tool-making. No adaptation ever takes place in anticipation of some future development. Nor was it to allow us to see over the grasses of the savannah, because australopithecines lived in wooded landscapes. Most likely is that it was a mixture of things, above all to reduce overheating and energy use. As we spent more and more time on open plains, an upright posture exposed less of our bodies to the sun and was a more efficient method of movement. This increased our range for scavenging — our ability of long-distance running is unique among primates.
Whatever its causes, bipedalism had implications for expressive movement. Mithen lists “new degrees of motor control, independence of torso and arms from legs, and internal and unconscious time-keeping abilities” as contributors to our potential for dance. Even though it did not evolve expressly for the purpose, our anatomy enables us to make an immense range of flexible and athletic movements. This may have “initiated the greatest musical revolution in human history.” Mithen notes approvingly the work of the musicologist John Blacking, who wrote:
Many, if not all, of music’s essential processes can be found in the constitution of the human body and in patterns of interaction of human bodies in society.
There seems to be a close connection between the areas of the brain responsible for complex vocalisations and for complex muscular movements. If these evolved together, it might help explain why rhythm, music and dance are so closely related in human cultures. If musilanguage existed, uttered holistic phrases may have been accompanied by gestures of the body such as the head and hands.
Our sense of rhythm, one of the most important aspects of music, begins with a sense of time, something human beings share in both perception and motor behaviour. Brown, Merker and Wallin observed:
One of the most distinct features of music, with reference to both animal song systems and human speech, is its use of isometric rhythms. The human ability to keep time should be distinguished from the ability of most animals (including humans) to move in a metric, alternating fashion.What is special about humans is not only their capacity to move rhythmically but their ability to entrain their movements to an external timekeeper, such as a beating drum.
This ability to entrain is effectively unique to human beings, as the testing carried out so far tells us that animals do not naturally demonstrate this ability; even in the rare exceptions that have been observed, their ability is severely limited compared to ours. Mithen relates this to bipedalism:
Rhythm... is essential to efficient walking, running and, indeed, any complex coordination of our peculiar bipedal bodies. Without rhythm we couldn’t use these effectively: just as important as the evolution of knee joints and narrow hips, bipedalism required the evolution of mental mechanisms to maintain the rhythmic coordination of muscle groups.
We still don’t quite understand how our auditory system links up with our motor system to produce synchronated movements, although they are very clearly connected. It is likely that the evolution of bipedalism had a profound impact on our association of movement and rhythm. The existence of rhythm awareness in newborns — even blind ones, who cannot be imitating their parent — proves that it is not culturally acquired but hereditary. Infants show strong emotional associations too, being able to recognise a difference for example between approving and scolding tones of voice, and this union of emotion with tone and movement lays the foundations for music and dance.
As we all know, our bodies can be highly expressive of emotional states, even unconsciously. The evolutionary changes in Homo ergaster — greater motor control, a strong sense of time and rhythm, the independence of the arms and hands — meant a dramatic increase in hominids’ command of body language. Together with vocalisation, emotional association and a possible musical proto-language, we were equipped with powerful tools for communication in the complex social life of our species.
Gesture works differently to words because it does not rely on any form of grammar. As Mithen puts it:
the majority of spontaneous gestures used by modern humans are iconic, in the sense that they directly represent whatever is being verbally expressed... So if I were describing something big, my gesture would most likely involve placing my hands together and then gradually moving them apart; whereas when describing something as small I might gradually move the finger and thumb of one hand together until they are almost touching.
Not only do such gestures back up what is communicated verbally, they add additional layers of meaning, i.e. they are complementary rather than merely derivative. But gestures can also be made independently of verbal meaning, or convey information that contradicts it. A measure of the communicative power of gesture to illustrate our inner life is that when we are being dishonest with our words, our body language can sometimes betray our true feelings.
Some of the earliest gestures may have been acts of mimicry. As Merlin Donald has explored , our ancestors probably used acting, song and dance to metaphorically recreate certain emotions and experiences in a narrative form. Donald suggests that such mimetic acts would be an early stage in the evolution of the human mind: miming everyday events like hunting, stripping a carcass, etc required us to understand metaphor and so prepared the way for symbolism. Later, dance may have been a means to recreate and preserve tribal wisdom and stories in an age before written records.
Another aspect of mimicry is that by imitating the posture of another person, we imply an empathy with their emotions. When consoling someone, we adopt a similar sad posture, facial expression and tone of voice. Music and dance provide a culturally defined framework for recreating emotional states through synchronicity and cooperation. Understanding how certain facial expressions, for example, communicate certain emotions, we may then adopt those expressions metaphorically, pretending to be sad, happy, and so on to elicit an emotional response from others. The responses elicited then, in turn, have an impact upon the dancer.
Mithen refers to the work of the dancer, choreographer and theoretician Rudolf Laban, widely considered one of the most important figures in the history of dance:
Laban gives the simple example of the expressive range of gestures that can accompany the word ‘no’. He explains that one can ‘say’ this with movements that are pressing, flicking, wringing, dabbing, thrusting, floating, slashing or gliding, each of which ‘says’ ‘no’ in a quite different manner. Once such gestures are integrated into a sequence of body movements and vocalisations, once some are exaggerated, repeated, embedded within each other, one has both a sophisticated means of self-expression and communication, and a pattern of movements that together can be observed as pure dance alone.
Our capacity for music and dance expresses a need to communicate with other humans through co-ordinated gestures in time, a skill we seem to possess to some degree even before we are born. Like musilanguage, gesture and movement as means of communication predate language. It depends upon our ability to recognise others as intentional and sympathetic human beings like ourselves. Like language and music, dance is a means of symbolic communication — because of our differing personal experiences, symbols are never entirely fixed but have differing meanings for individuals, which may help to explain why a certain piece of music can provoke wildly different responses in its listeners.
In music and dance we act as social beings, using a shared cultural framework to share experience through drama and metaphor.
 W. Tecumseh Fitch, ‘Biology of Music: Another One Bites the Dust’, Current Biology (2009). In this essay Fitch observes that entrainment is not strictly unique to humans, reporting evidence for its appearance in some bird species.
 This cultural elitism persists in classical music because of its history, but no one should let that drive them away. The music may be appreciated by anybody.
 Steven Mithen, The Singing Neanderthals (2005).
 L. C. Aiello, ‘Terrestriality, bipedalism and the origin of language’, Evolution of Social Behaviour Patterns in Primates and Man, ed. J. Maynard-Smith (1996).
 John Blacking, How Musical is Man? (1973).
 Wallin, Merker and Brown, ‘An Introduction to Evolutionary Musicology’ in The Origins of Music (2000).
 Mithen, op. cit.
 Mithen, op. cit.
 See Merlin Donald, Origins of the Modern Mind (1991).