As a student of neuroanatomy, I was provided with a human brain in a half-gallon tub. Our lab manual depicted a brain in situ, half-exposed in the head of an aged Irishman cut open along the midline, where his part might have run. My lab partner and I spent a semester peeling away layers of our stranger’s accumulated experience. We sketched coarse outlines to label in Latin and Greek. In an exam, we might find pins in the pons and medulla, in their minor partitions. We might be asked to diagram the flow of information as a child touches a hot stove then withdraws her hand in a thin sliver of a second. This is the allure of neuroscience: it offers an atlas of experience, one whose pages can be laid out for view with a scalpel and steady hand. At 21, I was overwhelmed and enthralled.

Roughly a year later, I joined several graduate students for an afternoon spent kicking our way through ankle- and waist-deep waters, seining for tiny varieties of fishes. We were led by an ichthyology professor who was opinionated and clever. He taught me how to hold the seine, placing my hands on the posts in proper position, tilting them so the net could billow behind me. He showed me how to move through the water to drive fish into our net. And despite my ignorance, he addressed me with deference. ‘You’re a neurobiologist,’ he began, as I watched the Vermillion River work its way across a flat Illinois acre. ‘Why is water so mesmerising?’

Maybe it was the way light and sound leapt from the stream, at once constant and unpredictable. I kept this thought to myself. We could not have anticipated that we would discuss his strange question and our awkward silence for the next 20 years.

Perhaps we have become too easily ashamed of our wonder. Neuroscientists want more than ever to chart the brain’s navigable waters, its every tributary and purling riffle. We have performed meta-analyses of brains lit with love and desire. And when we have these maps, these intimate geographies, what then? As Walt Whitman has written, ‘Your facts are useful, and yet they are not my dwelling.’ Can we learn how a fleeting touch drives a frenzied heart, or why the delay between contact and withdrawal can span a decade? An answer worthy of our effort should begin at the skin’s surface, yet somehow end in poetry.

While walking on a Japanese beach at the end of the 19th century, the Scottish doctor Henry Faulds found pottery fragments that bore impressions from the fingertips of prehistoric craftsmen. Contemporary pots made by similar methods revealed finer details and alerted him to the minute variations of the human hand. Naturalists of the time often documented the delicate forms of exotic ferns by transferring a thin layer of printer’s ink from frond to paper. Faulds made similar records of the intricate ridges of fingers and palms, noting the variety of patterns he observed among the digits of his friends and colleagues.

Faulds published his observations in 1880, in an article that proposes the use of handprints in criminology. He suggested printing furrow patterns onto glass in different colours of ink, so the superposition could be projected by magic lantern. Impressions recovered from soot or blood could be used to incriminate or absolve a suspect. A mutilated, headless body could be identified.

In response to his publication, Faulds soon learned that Sir William Herschel had used fingerprints for the identification of Bengalese prisoners and pensioners. Herschel’s large collection of prints was passed on to Sir Francis Galton, a younger cousin of Charles Darwin and a pioneer in statistics. In 1892, Galton compared the arches, loops and whorls that define the central, bulbous part of the fingertip, the triangular spaces where ridges converge, their infinite permutations. Galton estimated the probability of two fingerprints being identical at approximately one in 64 billion. Apparently, it matters so little exactly how the ridges of our palms and fingers are arranged that there are more ways to make a fingerprint than there are fingers. Fingerprints seem to have become metonyms for identity by evolutionary accident.

For every spike in voltage there was a small but predictable increase in pleasure

With so much variety, it is telling when something remains constant. Try an experiment: lick your fingers as though you were about to turn a page. Instinctively, you’ve licked the spot where fingers grip light objects, and at its centre are the concentric ridges and grooves that define your fingerprint. If you move your finger over an object in most directions, the object will run roughly perpendicular to these ridges, allowing friction to tug on each ridge as though toppling a wall. This central, bulbous part of your fingertip also contains the finest, densest set of ridges. You can see this if you follow your finger a short distance toward your palm, where the ridges become progressively wider. It is no coincidence that the ridges are finest, most centred on the part of your finger that first makes contact with an object. It is also where the nerve endings that sense touch are most dense. If you’re the caressing sort, recall how you have touched a lover, your fingertips scanning as they glide slowly over skin. Perhaps your palm lay flat, presenting the largest possible surface for contact.

The ridges of our fingers and hands are densely innervated by sensory neurons, nerve cells that translate pressure into changes in voltage. These sensory neurons come in a variety of forms suited for their tasks, named after neuroscientists like Merkel, Ruffini, Meissner and Pacini. Nerve endings can be capped with structures called disks, capsules or corpuscles – each defined by a distinctive weight or stiffness. These tips make the neurons more or less sensitive to pressure. The nerve endings that sense touch can be buried deep in the skin or can be so near the surface you could find them within the ridge of a fingerprint.

When the pressure and depth of touch are just right, the surface of the sensing neuron is deformed, stretched until the tension opens channels that let electrically charged salt ions flow in and out of the cell. The voltage change caused by the flow of ions zips along a cable-like projection to the spinal cord, where it gets passed on to other nerve cells and eventually to the brain. We can judge how smooth or pliant something is because voltages conveying the complex patterns of pressure arrive quickly enough for our brains to perceive subtle variation in timing. Without this ability, touch would feel like a surveillance tape played at half-speed: blurred and coarse. Like other species, we gain this speed by insulating our cables. Nerve cells are highly specialised, and require companion cells to help them with the daily details of cellular living. Some of these companions have developed means of enveloping the cable-like projections of neurons, becoming flat and wrapping themselves around the exterior of the cable again and again, like a king-sized sheet swaddling an infant. Or like rubber coating wire.

Insulated neurons are responsible for fine touch, but there is a second class of receptors that remain bare. These bare nerve endings are slower, and respond to coarser kinds of stimuli. Science has long known that these unmyelinated neurons respond to temperature, pain, tickle and itch. But we have only recently learned that they also respond to the pleasurable sensation of caress. Researchers in Sweden recorded data from neurons in the skin of human subjects as they exposed them to soft slow touch. For every spike in voltage there was a small but predictable increase in pleasure. While these naked neurons are missing in the hairless skin of our fingers and palms, they are found on the rest of the body, on the places you might touch with affection or consolation. And naked fibres are particularly abundant in the places we like to juxtapose – our lips, nipples, genitals and anus. The clitoris and the glans are enmeshed in the unmyelinated ends of sensory neurons. Inexplicably, we have often assumed these naked fibres were there for the sensation of pain, as though we had never known the joy of sexual touch.

Every Friday I joined a group of ichthyologists for happy hour at a neighbourhood pub. I loved the boozy debates, the elaborate arguments diagrammed on damp napkins, the raised voices and laughter. On one of these evenings I ran into my former neuroanatomy partner. We were both animated and friendly. When we shook hands and said goodbye, I pretended not to notice as his middle finger scratched my palm. This concealed touch within a touch was strange – in the Midwest at least, it’s a childhood code for romantic interest. How peculiar to receive this gesture from an adult man. I dissected its meaning with friends. This was not his only unusual behaviour: He knew I didn’t own a motorcycle, for example, yet had invited me to go riding with him more than once.

In my extended public analysis, I did not mention how his touch seemed to leap from my palm to spine. Privately I wrote of the jolt I felt. Objectively, I noted in a spiral notebook now tucked away in a box, its effect seemed likely to have resulted from the sudden realisation that I was an object of sexual interest, of anyone’s interest. This shock, coupled with the sexualised context, would naturally bear some erotic charge. Add to this my own pent-up energies, and such a charge could easily explain both my rise in heart rate and transient tumescence.

Although I could accept this unlikely explanation for my response to a classmate, I found it increasingly difficult to deny my infatuation with the biologist who had been so mesmerised by running water. It was his company I sought every Friday. I delighted in the accidental intimacy of a crowded table. One happy hour stretched late into the evening, into a drunken discussion on the biology of sexual orientation.

I had an excellent command of neuroendocrinological facts: testosterone surges conditioned by arbitrary but sexually contingent odours, sampling biases of famous studies, the malleability of the human brain. He had 49 years of a life lived. He asked why, if sexuality were so damned fluid, I wasn’t sleeping with men. I countered that, while it was true I had no personal experience with same-sex relations, in actual point of fact I would, under the right circumstances, consider sleeping with a man. The room suddenly seemed noisy and close. We settled our bill. He drove me home. We paused awkwardly in front of my house, the ignition off, talking vaguely about the next day’s work. I left his car.

Before long, we were sneaking out for lunch. We watched an annular eclipse cast bright rings onto dappled shade. He taught me how to snorkel in a few inches of rapid water, lying face down on smooth stones to catch colourful fish in private moments.

In the naked stream, touch can be warm or rapturous or full of hurt

Each touch receptor propagates voltages upward toward the spinal cord and brain, voltages that float like bottles bearing notes along a waterway defined by the spindly extensions of sensory neurons. Each current conveys its own kind of message, and the myriad currents coalesce into two north-bound streams.

Of these streams, the routes of discriminative touch are particularly well mapped. In the 1930s, Canadian neurosurgeon Wilder Penfield electrically stimulated the brains of epileptics, probing the cortex for the origin of seizures. Patients had to be awake for this procedure so that he could ask them what experiences were evoked by the faint electrical current. Electricity alone was enough to elicit the feeling of being touched on an arm, or, when delivered to a nearby region of cortex, the shoulder.

Penfield found that the brain contained precise maps of the body; he charted duplicate maps of both touch and movement, side by side, along adjacent folds of the cortex. The resulting ‘homunculus’ is an iconic image in neuroscience – a strange representation of the body whose distortions, like early maps of the world, reflect how we value the body’s surface. Those areas where touch is most sensitive are inflated. And three-dimensional reconstructions of these maps reveal a grotesque caricature of our evolutionary past. Our fingers, faces, palms, lips, tongues and genitals are all out-sized. The map of our brain’s control of movement is similarly distorted – our hands and mouths in particular are both exquisitely sensitive and extraordinarily precise. Play the piano or fellate a pianist and you will invoke our specialisations of sensation and motion to equal degrees.

Perhaps the most remarkable attribute of discriminative touch is that it reveals just how malleable our brains can be. The brains of patients born with syndactyly, in which two or more fingers are fused, represent that set of fingers as a single unit. Free the fingers and their cortical maps soon follow, new borders arising from their independence. Professional string musicians use the left hand for the precise fingering of an arpeggio or aria. With each note played glissando or staccato, with each shimmering or soulful vibrato, the left-handed cortices slowly swell.

If use inflates neural representations, disuse causes them to shrink, allowing neighbouring neurons to squat on the vacant real estate. Neurons that register facial touch lie adjacent to representations of our arms; amputees who lose an arm find that the brain’s face grows to take over the now idle regions of the map. Genital touch and the control of pelvic muscles lie side by side along a central nook of cortex, just below the cortical territories of feet. In one of the more provocative examples of neural plasticity, the neuroscientist V S Ramachandran at the University of California, San Diego, cites two amputees who, after losing a foot, seem to have gained genital sensitivity. One patient reported that his orgasm spanned from his genitals to his phantom foot.

A student of Ramachandran has gone on to suggest that such brain reorganisation contributed to the millennial prevalence of footbinding in medieval China. The brutal process, illegal since 1912, involved the bending and binding of a young girl’s foot, accomplished over years, until it was folded over like a billfold or, more generously, a lotus blossom. While the hobbling of women must have been a primary motive, Paul McGeoch, a clinician in San Diego, suggests that these women would also have experienced the atrophy of foot cortices and the encroachment of genital maps. English language scholarship from the 1960s cites texts that extol the virtues of footbinding. Some claim that it promoted vaginal tone, or that the foot became unusually sensitive to erotic touch. This literature seems somehow complicit with the practice and its misogyny – and yet it is consistent with our understanding of cortical plasticity.

The shifting landscape of discriminative touch reveals just how deeply we are shaped by our experiences. Our brains are sculpted by the accretion and erosion of their innumerable connections; the dendrites and spines of our neurons are altered by the information that flows through them. A friend and professional musician has worked his way across Europe, transcribing rare sheets of music written specifically for the viola, and sleeping in bathhouses along the way. At home, he keeps a map with pins in each country whose citizens he has sampled sexually. There are many pins. I imagine what his cortices must look like. Does he touch new skin with his left fingers? Do his lips tremble as he plays a passionate concerto? The ways in which we are changed by our paths through the world suggest an exquisite variety and specificity of experience.

Over winter break, two eventful months after our drunken debate, I accompanied the biologist on a trip to Venezuela. I spent the night before our flight, the first of my adult life, finishing applications to doctoral programs, editing with scissors and tape the drafts I had produced from a dot matrix printer, then photocopying them in a hotel business office. I mailed my final applications a few hours before taking off. Some hours later we began our descent, my uncorrected myopia lighting up what I assumed were Caribbean islands like stars in daylight. Caracas was a city of some two million people, and it seemed as though that many more had been spilled onto the surrounding mountainside. We passed over a graveyard of buses, their white skeletons sinking in the soil. The airport was chaotic, and our group navigated our way to a hotel, where we passed a short night before setting out for the interior.

Our first morning in the Llanos, we awoke to our host – a greying expat working at a small regional university – singing Patsy Cline as he played the piano. He made coffee by passing boiling milk through a sock of grounds. The next day, we set out in two old Jeeps and a Land Rover into the great plains of Venezuela. The Llanos is a marvellous place to spend a few weeks of the North American winter. During the wet season, the vast plains are flooded with the waters of the Orinoco River. By December, the heat burns off the shallow expanse of river, and the water recedes, leaving condensed pools thick with wildlife: brilliantly coloured fish and the many species there to eat them – river dolphins, storks, anaconda.

We spent the next 10 days driving, camping, and seining. We drove into the Guiana shield, a lunar landscape of volcanic rock. We pulled caiman from our nets, and fried piranha for dinner. A giant anteater walked through our camp. I had more adventure, more companionship and more fevered intimacy in those few days than in the sum of my preceding life. Everywhere there were fish to be caught, photographed and made ready for science. And every night, the two of us clung desperately to one another, as though someone might enter at any moment and tear us apart.

The body’s naked, unmyelinated sensory neurons feed a stream that carries information about the qualitative nature of touch – about what a touch might mean. In the naked stream, touch can be warm or rapturous or full of hurt. Many of its tributaries converge on the waters of discriminative touch, perhaps allowing our subtle experience of texture to be imbued with meaning. But the naked stream also flows upward toward its own unique destinations, through an anatomical pathway called the anterolateral system. The anterolateral system and its Grecian destinations mediate our experience of social and sexual intimacies.

we call our mammalian companions ‘pets’ because it is touch and the oxytocin it releases that binds us

The hypothalamus, for example, is a brain region that sits above your palate and coordinates the release of hormones. Among its many tasks, it regulates ovulation and sperm production. In response to its signals, gonadal cells produce hormones such as testosterone, oestrogen and progesterone, each of which fuels the procreant urge. Among vertebrates, ovulation is preceded by a gradual rise in oestrogen, followed promptly by a spike in progesterone. A female rat that wants to mate arches her back and moves her tail to one side to ease access. Beginning in the 1970s, researchers at Rockefeller University in New York City inked the paws of male rats and noted where along the white haunches of females a mounting male grasped. As females come into oestrus, their sullied flanks reveal that they become more forgiving of a misplaced paw. The sensation that links his clinch to her bending curve ascends through the anterolateral system, the naked stream – a now neglected fact we understood long before we thought to ask whether the touch of mothers, lovers and friends had anything in common.

One particularly famous hormone, oxytocin, is released from the hypothalamus in response to a variety of touch. Oxytocin is released by skin-to-skin contact between newborns and mothers. During nursing, the feel of an infant suckling elicits oxytocin release, which in turn evokes milk letdown. But oxytocin is also released by massage, by hugs, by grooming among members of a baboon troop, by rodent mothers that lick their pups. Children raised in large Romanian orphanages and deprived of touch are emotionally ravaged; they also have low blood levels of oxytocin. Oxytocin is thought to underlie the enduring bonds we form with a parent, friend or lover. Presumably we call our mammalian companions ‘pets’ because it is touch and the oxytocin it releases that binds us. Their soft fur, so different from that of wolves or African wildcats, seems designed for the pleasure of our touch. Look into the eyes of your dog and chances are good you will both release oxytocin.

A second, less widely appreciated hormone released by the hypothalamus is b-endorphin, a small protein known for its ability to promote pleasure and suppress pain. Receptors for endorphins are the intended targets of opiates such as morphine, heroin and oxycontin – each of which provides its own flavour of euphoric warmth. Touch releases endorphins. We primates are tactile and social animals, but provide us with a source of touch-free endorphins, and we lose interest in contact. Rhesus macaques tire of being groomed, and heroin addicts abandon sex. Perhaps our endogenous endorphins are why, at either end of a night’s sleep, a lover’s entangling limbs are so beautifully, so perfectly narcotic. Synthetic opiates offer the distilled experience of embrace, a pure warmth and solace that in our waking, sober lives always seems out of reach.

At the end of a remarkable year, the biologist helped me pack my belongings into a dilapidated Ford Mustang as I left for a doctoral programme across the country. Although we had assumed our time together had come to its end, I spent the winter break of my first year returning to the Llanos with his extended crew of students and scientists. The devastating beauty and smothering heat remained, but the logistics of the work left us with little comfort and less privacy. I chafed at the strictures of our discretion. My petulance exasperated him. And yet there were moments that broke through the tensions of group living.

At one stop, we found water moving through narrow paths set along a broad, flat bed of exposed river stones. We grabbed gear and dispersed in groups of two or four to attack the meandering beds and banks, flushing the fish hiding among rocks and crevasses into our nets. Alone for a moment, I came to a site where the stream had carved a pocket a few feet deep beneath a stone face. In the rapid current, armoured catfish wait for nightfall in dark nooks. I put on a snorkel and mask, and dove in fully clothed, grabbing a large stone along the bed to hold me in place. The river pulled my body taut as I held to the rock, looking upward at the shaded fish. There were a half-dozen or so, each 8 or 10 inches long with a carapace like mahogany, pressing their bellies against the underside of stone. When I could no longer hold my breath, I emerged, blowing water from my snorkel as I stood. I saw that I was being watched at a distance, and felt the fleeting bliss of a shared gaze.

The group camped one night on the overgrown bank of a remote river bed, a site on the Rio Apure that seemed blessed with exotic diversity. We heard for the first time the deep guttural cries of the red howler monkey. A herpetology student from Peoria came running out of the brush with a lizard in one hand. ‘What was that?’ he cried. ‘Hogs?’ And later at the site, as I wandered through a turbid stream I felt a sharp, biting jolt. It was an electric eel, presumably the same one we would later catch in our seine: three feet long, with a red chin and the broad flat head of a catfish. We caught another 44 species of fish. We staged photos, pitched tents and drank rum. That night, our sleep was interrupted by the thunderous sound of horses tearing through the camp, galloping at full speed as our tents trembled with their gait.

The electric eel is an enormous relative of knife fish, a group of blade-shaped fish that use electrical current to read the positions of objects in turbid water. That electricity is the language of nerves and muscle, harnessed first by the knife fish for mapping dark places, then amplified by their large cousins for hunting and intimidation. The electric force generated by the uneven distribution of salt ions across a nerve cell commonly measures less than one-tenth of a volt; the electric eel can generate 600 volts, enough to power, for a moment, several major appliances, and more than enough to storm the paths of pain. This pain seems to be an ancient affliction. If we move out from our leaf of the family tree to other primates, or on to rodents, to mammals – or further still to lizards, birds or frogs – we find these same neuroanatomical paths. Move further still, past the electric eel, past the lowly sea cucumber and starfish, and arrive at the insects. The trail becomes more difficult to follow, but still we arrive at something that resembles pain. Fruit flies, the work horses of genetics, learn to avoid mild electric shocks paired with arbitrary odours. Learn to cause pain to one, it seems, and you can cause pain to all. This is the parsimony of our shared heritage.

How old is the comfort of touch? The anthropologist Robin Dunbar at the University of Oxford has pointed out that the elaboration of grooming and touch is common among the old-world primates – the chimpanzee, the gorilla, the baboon and macaque. Some groups of gelada baboons spend up to 20 per cent of their days grooming. The use of touch to strengthen social bonds seems some 30 million years old. Howler monkeys, like other primates in the Americas, split from our lineage nearly 20 million years before this innovation. They seem not to know the pleasures of non-sexual intimacy.

Where does positive affective touch come from? Perhaps it began 350 million years ago, when vertebrates learned to fuck

While howler monkeys might not derive any joy from embrace, other South American species are demonstrative to excess. Mated pairs of Titi monkeys, for example are constantly huddling, grooming, or twining their tails in a long braid. This proclivity for contact has evolved repeatedly among bonding mammals. The mechanisms of parental care are thought to have been repurposed by natural selection. For example, birth and suckling cause the release of maternal oxytocin, and this prompts bonding to infants; oxytocin also promotes pair-bonding among prairie voles, the family-oriented rodents that populate the Midwest; it is released by orgasm, or by the caress that unites partners and groups. And oxytocin is just one of a number of neuromodulators whose jobs in parenting have shaped our sexual and social lives.

And mammals are not unique. Birds care for their young and commonly form breeding pairs. They might preen and coo, but they do not give birth or suckle. How do their brains tell them whom to love? Was avian attachment a wholly new invention, or are there still deeper, more ancient mechanisms that were turned toward the varieties of intimacy? Where does positive affective touch come from? Perhaps it began 350 million years ago, when vertebrates first learned to fuck.

Internal fertilisation is a feature that defines the group of land vertebrates known as the amniotes – the reptiles, mammals and birds. A 2011 paper described the attributes of caress-sensitive neurons, traced by genetically engineering mice so that these neurons would light up and could be easily counted. The authors noted without comment that these were most abundant in the region of the spinal cord that innervates the genitals. Given that the sensory ends of neurons in erogenous zones resemble caress receptors, and that their functions are so similar – to translate a gliding touch into a spark of joy – it seems plausible that pleasurable touch first emerged from the wild throes of coitus.

There is one more wrinkle. Go further back along this tree and compare us not to other amniotes, but to the frogs and newts. These amphibians diverged from our own lineage before internal fertilisation was first conceived. Like our closer relatives, however, mating often involves clinging to one another: A male mounts and clasps a female with his legs; they then coordinate the release of sperm and eggs. This need to place our gametes side by side is essential to all land vertebrates, to all the four-legged creatures who can no longer broadcast their desires to the oceans and streams. Perhaps we owe our capacity for physical intimacy to the air we breathe and the water we left. It is in our eight entwined limbs.

Somehow we managed to stay together despite the thousand miles between us. For years, we met at professional meetings, over excursions and vacations, for whatever days or weeks we could steal from our work. Our time together was compressed and intense. In public, we obeyed rules of decorum, finding silent excuses for our knees to touch, grasping a hand in the moment after the lights of the theatre dimmed and before eyes adjusted. Near the end of our eight years apart, we became more permissive, discreetly sharing with one another the fleeting intimacies we passed with others, like co-conspirators against the petty tyrannies of convention.

I would complete graduate school and postdoctoral appointments. He would leave his academic post for a position in Washington, DC. Eventually, I was hired as a faculty member, and he took early retirement to come with me to a verdant college town with a muggy climate and flourishing bike lanes. We bought a house and converged on routines. We slept in a choreographed nakedness: first face to face, then one body cupped in another, then reversed, and finally akimbo until morning. But the burden of our differences in temperament, easily borne for a few days or weeks, became more wearing with cohabitation. After a fight, we slept clothed. Our sex, measured against the impossible standard of our frantic past, seemed tepid and spare. That spring, while he was chasing fish in the Ozark Mountains, I took a lover.

By summer, I had fled to the montane cloud-forests of Panama for my own season of fieldwork. There I lost myself in the mysterious bleeps I detected on my radiotracking receiver, as I sought singing mice hidden in the mist and dense grass of a fallow pasture. It was cold and constantly damp, and the house I shared with park guards lacked both heat and electricity. In my discomfort, I drank rum and smoked bill-sized tobacco leaves offered by a man who fought for six years with Nicaraguan rebels. In my solitude, I found my mind drifting toward softly lit fantasies of easy intimacy: a Sunday morning sharing a hammock and newspaper, a weekday evening sharing wine and a warm bath.

Our need for intimacy follows naturally from our primate heritage. Social psychologists, arguably among the most specialised primatologists, have documented the complex roles touch plays in our species. They find, for example, that clients touched by salespeople rank them more favourably. We tip better to touchy waitstaff, and we once were more likely to return a dime found in a phone booth if the caller had touched us before leaving it behind. And, of course, we are very selective about who touches us and where. Most of us seem to develop some loyalty to the stylists and barbers who cut our hair, for example, and we are not alone. Among the !Kung San, the hunter-gatherers of South Africa and Namibia, women form hairdressing cliques that help define and maintain their social status. Among college students and adolescents, electric clippers and curling irons seem to have played similar roles. While adults in many cultures have largely ceded this role to skilled professionals, we often exhibit a fidelity to these men and women that we don’t display to other service workers. I would never restrict myself to a single restaurant, or buy clothing from a single clerk. We are social beings, and to no small extent, we define ourselves by whom we touch and whom we let touch us.

Touch embeds us in a social network. We choose what contacts we reveal and to whom, and those choices define us to a community

Our response to touch communicates comfort and trust, and the very act of being touched time and again elicits this trust from us. Touch is a tiny tug in the direction of intimacy, and our compliance conveys complicity. Social scientists have watched touch unfold in the context of romance, and their findings are largely familiar. Early in courtship, men tend to initiate touch more than women. The frequency of caress ascends with self-reports of love through the heady days of betrothal. Affectionate touch seems to decline after commitment; while women continue to initiate touch, male touch becomes increasingly responsive, reciprocating. We convey our interest and assurance through neurons tuned to affectionate touch, resuming when a partner needs assurance, withdrawing when we need autonomy, finding our unique paths to tenderness and comfort. Most of us seem to understand the coded signals of intimacy without instruction.

Touch communicates not only between friends or lovers, but also to those around us. The touches we exchange in private, and whom we exchange them with, are very different from the touches we exchange in public. In one study from 1983, the psychologists Frank Willis and Christine Rinck at the University of Missouri asked undergraduates to record the touches they gave and received; 779 of 1,498 touches were deemed personal – lips to face, hand to thigh, genital to genital – and most of those happened in private places, in homes and automobiles. It must be for similar reasons that we are ashamed of our infidelities or, at least, are reluctant to confide them. This is why a lover is owed discretion: touch embeds us in a social network. We choose what contacts we reveal and to whom, and those choices define us to a community.

When Penfield mapped the cortex of touch and movement, there was a landmass conspicuously missing. Pain and warmth lacked a cortical home, a distinct place where they could break the surface of consciousness. Contemporary methods suggest that emotional touch resides in a hidden island of cortex known as the insula: stimulate the insula with an electrode, and you will evoke sensations of pain or warmth; caress an arm, and the insula lights up. A man lying prone in a university hospital’s functional MRI machine, noisy and clinical, nonetheless exhibits insula activation when his girlfriend masturbates him.

It seems that bodily sensations pool in the back end of the insular cortex and then move forward, toward the anterior insula, where they mingle with information on bodily states – hunger, libido, wakefulness – and with sensations of the external world that have been filtered through centres of emotion. Lesions to the insula by stroke or trauma cause peculiar deficits.

Asomatognostic patients are unaware of their bodies; they may not recognise their own arms, or may mistake another’s arm for their own. Anosognosia refers to the heartbreaking disorder of not knowing your own disorder – of being blind, for example, but believing you can see. Or of being paralysed, but believing you can feel. One interpretation is that the anterior insula is responsible for the sense of being here, in one’s own skin, immersed in the luminous stream of experience. Damage to the anterior insula muddies that stream, revealing that our most secure knowledge – the ownership of our own bodies, the integrity of sensation – remains a fragile narrative.

The insular cortex is active not only during caress, but also during the thought of caress. And it is active not only during pain and the thought of pain, but also during the perception of another’s pain. It is here the body aches. Perhaps we can blame the insular cortex for the devastation wrought by love’s loss, for the drinking to excess in a bath hot with tears and micturition, or for the blackened cigarette butt floating on the water’s surface. Maybe we can blame it for the way a life can throb like a bruised bone. In such fully felt moments, the experience of time dilates. Perhaps insular activity explains why one can remember, a decade later, the positions of two people in a room, one seated, one standing, their conversation strained; a pause; a specific phrase that turned tensions in the direction of resignation and resolution. Perhaps insular activity can explain why memory’s next frames come at lengthening intervals, skipping forward some days to the nettled warmth of a shifting embrace, to a tangled approximation of dance, as Willie Nelson asks not be forgotten; then months more to a dewy bike ride beneath oaks draped with moss; and then to a flat stream haunted by gar; to a drunken evening, to a dim bar: the film’s quickening flicker an accidental record of ardour in decay. Perhaps the insula is the mind’s candid editor, amassing scraps as devotion gives way to nostalgia. Yes, perhaps.

On a recent trip to Washington, DC, I took time to stop at the National Gallery to see Chuck Close’s Fanny (1985), a large painting that details a woman’s face elaborately creased by age, her throat perforated with the open scar of a tracheotomy. The portrait is a study in tenderness, composed entirely of fingerprints, light or firm, as much sculpture as painting. Down the street at the Smithsonian Natural History Museum, I watched as children placed their hands on replicas of ancient artworks: red palm-prints left in Mt Borradaile in Australia; the negative silhouettes of hands left by plumes of red pigment blown in the caves of France, Borneo and Argentina. These children could reach blithely across tens of thousands of years.

Omitted are the prints left in the caves of El Castillo in Spain – so old that anthropologists debate whether they were made by us or by our Neanderthal cousins. Nearby in the museum, a docent paused to describe the footprints of early hominids. He might have explained that an entire discipline, ichnology, seeks to make sense of the traces of touch laid in fossils: before we settled land, a lungfish walked on the shore of Nova Scotia; before we were fully human, a mother and child walked upright through a layer of ash; before his first colour film, Humphrey Bogart pressed his hands into the wet cement outside Grauman’s Chinese Theater. Like Faulds finding a fragment of pottery, we are fascinated by the record of touch.

We are two fishes swimming in the sea together. We are seas mingling

The anthropologist James Frazer might have described our fascination as a variety of sympathetic magic. He recognised a kind of magical thinking in which properties are transmitted through touch like contagion. In his book, The Golden Bough (1922), Frazer writes that: ‘Among the South Slavs, a girl will dig up the earth from the footprints of the man she loves and put it in a flower-pot. Then she plants in the pot a marigold, a flower that is thought to be fadeless. And as its golden blossom grows and blooms and never fades, so shall her sweetheart’s love grow and bloom, and never, never fade.’ It is easy to dismiss magical thinking as provincial folly. I prefer to think of it as a subtle heritage some 400 million years in the making. I confess that I still have a shirt that belonged to this man I have written about, hidden among sundry ragged shirts from my past. He mailed it to me when I first moved away to graduate school. It held his smell for years.

The summer of our dissolution, I passed my idle evenings in Panama translating a few poems by Pablo Neruda, an activity I hoped would improve my Spanish while channelling my turbulence. I learned that un relámpago is a lightning flash. And that, like Whitman, Neruda writes often of water, light and touch. Whitman sings of pent-up aching rivers, of cheerful waves rolling over each other. We are two fishes swimming in the sea together. We are seas mingling. Neruda speaks of water, of dreams, of naked truth. He wonders whether frogs mutter complaints against amphibious indecencies, or whether the bull asks the ox before being seen with the cow. He asks in awe how water enters stars, and what song the rain repeats. He marvels at our ignorance.

And he is right, of course. Our understanding is fragmented and confabulated, a story assembled from colourful shards that catch light in a pleasing way – like a mobile made of beach-glass, its delicate melody at once elusive and familiar. Neruda writes that we will have our answers only in the oblivion, when the wind whispers truths where our ears once were. And yet his asking is no less lustrous.