The sun has nearly set on a moody October evening when we see the bear cross the border. The old logging road we walk on, barred to traffic by a metal gate, seems to have been graveled at one time. Now a soft carpet of moss, shading from light sage to deep puce in the twilight, tapestries its surface, punctuated with spiky clumps of pale gold grasses. The trees of the northern Vermont forest that line the narrow road stand gray and shorn, having long lost their leaves in preparation for winter. But not the beech trees, which are migrants from the tropics and so retain a vestigial clutch on their canopies. Their marigold leaves tremble delicately in the breeze, scattering light over the gray forest. Claw marks scar the tree trunks, evidence of the bears that come from hundreds of miles away, drawn by the migrant trees’ rich fatty nuts that nourish them for their long winter sleeps.
The road descends gently into a wooded valley, surrounded by low, old hills. Somewhere below us, an invisible line separates the woods into the Green Mountains of Vermont on one side and Quebec’s Sutton Mountains on the other. Even as we approach it, there is no telling. The forest stretches over the rolling hills, unbroken.
But because of this invisible line, the forest here is booby-trapped with hidden, battery-powered cameras, painted gray and brown to match the woods. Sometimes the bears destroy them, especially if the border agents who set them up have stopped for a breakfast sandwich on their way to the woods. The bears find the scent left behind on the devices by the agents’ sausage-grease-moistened fingers irresistible. Those that stay intact are monitored by Border Patrol officers stationed in a lonely, overly chilled facility a few miles to the south. Their intent is to detect the illicit movements of cars and drug smugglers. Mostly their cameras capture the movement of wildlife, who trip their shutters some two hundred times a day.
We’ve been hiking for about an hour when a large white Border Patrol van emerges out of the shadows. Two baby-faced agents, their bodies packed into stiff uniforms, politely but sternly inquire about our itinerary. The tension in their bodies almost instantly dissolves when they understand we are just observers and not migrants ourselves. They happily chat with us about the images of wildlife on the move shot by their hidden cameras. They download their favorites to keep in their own personal collections, they tell us. I will see them, later. The portraits—besides those of me and my guide, the wildlife tracker Jeff Parsons, looking rather suspicious in this context—are beautiful and haunting: the large brown eyes of a passing deer, the matted backside of a bear, the lithe torso of a bobcat fill their frames. They even caught one of the rarely seen velvet-coated beauty, the lynx.
The tracks of the animals moving across the land1 here run parallel to those in the sea and the sky, uncaptured by Border Patrol cameras. In the waters a few hundred miles to the east, great white sharks patrol the American coast, gliding from U.S. into Canadian waters and back again. Right whales carve a long watery trail from their calving grounds around the Florida coast to their feeding grounds in Nova Scotia’s Bay of Fundy. Leatherback turtles chase clouds of jellyfish from their breeding grounds in the tropics deep into northern waters. In the sky, monarch butterflies heading south cross paths with ospreys and black-throated blue warblers en route from their winter homes on the island of Hispaniola to their summer homes in the forests of southern Canada.
Finally, the Border Patrol agents leave, and we continue our hike. At the bottom of a hill, we encounter an overgrown three-foot-wide path cut into the forests, running from east to west on either side of the logging road. It’s the international border between the United States and Canada. The two powerful states’ attempt to carve the landscape into separate pieces controlled by their authority has been made manifest in this narrow, dark passage through the forest. We stand there for a while, as the sun sinks below the hills, then head back up to find our parked cars. At the crest of the hill, we turn around to catch another glimpse of the border. In the twilight, I can just barely make out the small black bear ambling across it.
Biologists such as Elton and others who’d dismissed migrants as suicidal zombies and mindless invaders never really examined migrants’ behavior itself nor considered in any depth how it might have evolved. What drives creatures to move2 into new territories, away from where they were born? Leaving behind the known comforts of a home habitat, migrants strike out into the unknown. They forsake the help of kin who stay behind. In return, they may not find anywhere suitable to live at all.
And yet they do it anyway.
Baleen whales migrate thousands of miles from their rich feeding grounds in the far north to the warm waters of the tropics. Zooplankton migrate vertically between the depths and the surface in sync with the fluctuating light. Forests move over thousands of years with the advance and retreat of glaciers. In jungle-dripped Hawaii, tiny goby fish migrate from the open Pacific Ocean back to their birthplaces at the tops of waterfalls. The journey requires swimming against ocean currents into fresh waters and climbing up cliffs. They use suckers on the undersides of their bodies to do it.
In humans, the origin and ecological role of migration continue to be shrouded in controversy and contention. But biologists have forwarded a clear idea about its provenance in animals.
Migration experts such as Hugh Dingle say that migration most likely evolved as an adaptive response to environmental change. Migratory behavior is more common among species that depend on resources exposed to environmental variations than in those whose livelihoods are more buffered from environmental change. Arthropods that live in temporary habitats such as shallow pools and seasonal ponds, for example, are more likely to migrate than those that live in relatively stable environments such as forests and salt marshes. Species that live in places with erratic rainfall or that feed on patchily distributed resources such as fruit and flowers are more likely to migrate than those that live in relatively stable places such as alpine tundra or deep lakes, where a disproportionate number of insect species don’t even have wings. Species that live on the edges of forests or in their canopies are more likely to migrate than those that live in their interiors. Bird species that feed on fruit, which is available only seasonally, tend to migrate more than bird species that feed on insects in the interior of forests, which are not. Bats that roost in trees, where they’re more exposed to the cold and the rain, migrate more than bats that roost in caves, which protect them from the elements.
Even within species, individuals that live in habitats exposed to change3 migrate more than those less exposed. The migratory behavior of white-tailed deer, for example, correlates with the size of their forest patches: deer that live in small patches more exposed to changing conditions migrate more frequently than deer that live in large patches.
For creatures that live in habitats subject to environmental change, survival rests on one of two strategies: either go dormant and wait for the altered conditions to recede or migrate. Again and again those creatures capable of movement have opted for migration, despite the costs. Dingle distilled a formula that predicts the emergence of migratory behavior. It lies in the ratio between the time it takes to reproduce a new generation and the stability of the environment. If that ratio is less than one—if, say, it takes a couple of years to reproduce the next generation but the habitat, say a vernal pond, lasts only for a season—migration is likely to emerge.4
And so as the northern hemisphere tilts5 away from the sun, lengthening shadows and shortening days, creatures of all kinds prepare to move. Physiological changes transpire inside their bodies, hormones spiking and nervous systems mobilizing. Sap-sucking rosy apple aphids birth special forms with wings. Salmon on the verge of migration experience spikes of hormones such as prolactin and cortisol. Baby eels metamorphose into transparent forms that prefer fresh water to salt water. In preparation for their travels, migratory birds and insects build up fat stores that can comprise more than 50 percent of their body mass, and plants produce tough coats on their seeds. The proportion of fat they deposit in their seeds correlates to the distance the seeds are likely to travel.
As the time to leave approaches, a restlessness sets in.6 Migratory birds trapped in cages will flutter repeatedly to one side of a cage, jumping off their perches and crashing to the side. Which side depends on the direction it faces: whichever matches that of their migratory path. Scientists named their agitation Zugunruhe, German for “migratory restlessness.” It’s hormonal. Remove the gonads of sparrows in the spring, and they’ll be less restless; castrate a migratory bird, and it will still migrate, but in a different direction.
Migratory journeys are not simple extensions of everyday movements, like flying from one tree to another or moving from one cave to the next. In birds, you can tell as soon as they leave, from their rate of climb and the altitude they achieve, that their migratory flights are something different. While en route, their behavior and bodily functions are fundamentally altered. Unlike during ordinary movements, during migrations their bodies halt their own growth and development. They ignore stimuli they’d ordinarily respond to, passing by appealing foods and breeding spots.
But while migration is driven by physiological changes, it is not necessarily the result of some fixed itinerary instilled into the bones or some blueprint encoded in genes, propelling creatures in fixed directions at standardized times. The physiological states required for migration can be flexible and dynamic, too. The muscles that power the wings of migratory aphids, for example, start to break down after migration, the proteins diverted to reproduction instead. Animals’ sensitivity to the quivers and trembles of our pulsating planet—not predetermined internal programs—drives their behavior and movement.
Wild animals’ sensitivity to environmental perturbances7 is the stuff of legend. Anecdotal stories of animals apparently sensing impending environmental disruptions hours or days before they’re detectable to humans stretch back to ancient times. Pliny the Elder described birds’ restlessness before earthquakes. Geese detected the arrival of an invading army of Celts in 387 B.C.E. Rome before the sleeping residents did, their quacking alerting them to the impending onslaught. In 1975 snakes outside the city of Haicheng in China emerged from their hidden shelters and froze to death in the winter’s cold, in advance of a 7.3 magnitude quake. In 2004 elephants in Sri Lanka fled inland hours before a tsunami reached the shore, saving the lives of those who instinctively followed them away from the wall of water.
The signal that the goats living on the slopes of Mount Etna in Sicily detected on a winter’s day in 2012 remains obscure. So does the perceptual mechanism by which they’d detected the signal. But whatever it was and however they’d sensed it, they did so faster and with more sensitivity than any machine devised by humans could have. For nine months, animal trackers who had fitted the goats with transmitter collars had been watching monitors streaming data recording the goats’ wanderings, as they ate, slept, and roamed across the volcano’s slopes. They were watching at the moment when the goats’ movement patterns changed dramatically. The event that triggered their burst of motion became clear six hours later, when the volcano erupted, spewing lava out of its crater for over twelve hours and shooting ash seven kilometers into the air.
Migration’s ecological function extends beyond the survival of the migrant itself. Wild migrants build the botanical scaffolding of entire ecosystems. They spread pollen and seeds, shaping where plants live and in which proportions, and ensuring that seedlings can reach open habitat rather than withering in the shade cast by their parents. The transport their movements provide is so critical to plant survival that many plants have evolved ingenious methods of enticing animals into ferrying their seeds around. They coat their seeds in sticky mucus or produce hooks, spines, and barbs around themselves to hitchhike rides on passing mammals by clinging to their fur, as anyone whose dog has been covered in burrs knows. They produce seeds with fatty bits to attract ants, which carry the seeds and helpfully bury them underground. They produce fleshy, fragrant fruits around their seeds, to entice birds to ingest the fruit and scatter the seeds across their flyways in drips of poo.
Botanists say that the survival of over 90 percent of the trees in rain forests hinges on birds and other animals on the move scattering their seeds. GPS tracking studies have shown that even species reviled as parasites shower the landscape with seeds. The nineteenth-century naturalist Alexander von Humboldt dismissed cave-dwelling guácharos, or oilbirds, as parasites, noting their habit of fruit-eating inside their dark lairs, which condemned seeds to oblivion. In fact, they spend their nights flying across the forests of Venezuela, dispersing seeds along the way. They’re “probably responsible for much of the diversity8 of the rain forest,” the ornithologist Martin Wikelski says.
Wild migrants ferry genes into isolated populations, introducing life-saving genetic diversity. In small isolated populations, genes with once-diluted effects in the population, such as those that code for life-threatening defects or increase vulnerability to disease, become concentrated. As increasingly related couples mate, genetic homogeneity sets in, reducing the ability of their population to withstand disease and disaster. Ecologists saw the dramatic effects in a population of wolves9 on an island in Lake Michigan called Isle Royale. The wolves all descended from a single breeding pair that had arrived during a particularly frigid winter in 1949, when the channel between the island and the coast froze over, allowing the two to pad over on foot. Since then, the population they founded had been marooned. They became increasingly inbred. By 2012, 58 percent of the wolves of Isle Royale had congenital spine deformities, compared to just 1 percent in wolf populations elsewhere. Many had eye abnormalities, with one eye appearing opaque and possibly unseeing. A female wolf died in her den, with seven dead wolf pups in her womb, a single squealing live pup beside her. Ecologists had never seen anything like it before.
The colony’s sole hope: migrants. In 1997 a single male wolf made his way to the island. The migrant’s jolt of genetic rejuvenation single-handedly transformed the ecosystem. Within a generation, the migrant’s genes lurked in 56 percent of the wolf population. The number of wolves on the island rose. The number of moose, which the wolves hunted, fell. The moose-trampled forests recovered. The lone migrant “saved the population for another ten to fifteen years,” the ecologist Rolf Peterson said.
While isolated habitats deprived of the largesse of animals on the move suffer, those that facilitate animal movements flourish,10 as a handful of large-scale experiments in forests have established. In one, ecologists cleared several fifty-hectare patches of mature pine forest along the Savannah River in South Carolina, removing trees and burning vegetation to the ground. They arranged the patches with one in the center and the others arrayed around it. The dense forest that enclosed each patch formed a kind of border around them, disconnecting them from each other. Then they pierced those borders by building a corridor, clearing a single, twenty-five-meter path between the central patch and one of the peripheral patches. They then tracked how plants, insects, and pollen spread from the central patch to the connected and disconnected peripheral patches. In the central patch, they marked butterflies. They doused fruiting shrubs that birds feed on with fluorescent powder. They planted male holly bushes needed to pollinate the female holly bushes planted in the peripheral patches. Then they visited the connected and disconnected peripheral patches, counting the number of marked butterflies, the amount of bird poo with fluorescent-colored seeds, and the number of flowers on female holly bushes.
Opening borders and clearing a path for butterflies, seeds, and pollen allowed them to spread at least twice as fast to the connected patch as to the disconnected ones. By the end of the study, the connected patch was blanketed with flowers, fruit, and butterflies.
Migrants saved the checkerspots.
At the southern end of the Santa Clara valley, just a few dozen miles from where Paul Ehrlich studied checkerspot butterflies, lies an undeveloped expanse of hills called Coyote Ridge. Coyote Ridge is prime butterfly habitat: thousands of acres of butterfly-friendly patches are home to varying mixes of sun, shade, and soils, plentiful wildflowers, and even fuzzy light brown calves and their doe-eyed mothers munching on the grasses that compete with the butterflies’ preferred host plants. If conditions deteriorate on one hillside, butterflies that live here can easily move to another. No miles of intervening highways and shopping malls block their passage. When conditions are good, they can sharpen their genetic adaptations11 to their locales, adding to the overall genetic diversity of their populations.
Ehrlich and his students hadn’t known about the butterflies living here. Butterfly collectors, on whose historical records butterfly biologists long relied, never ventured into these lovely hills. Once they’d found sufficient specimens to stock their collections, they’d stopped looking. But the checkerspot butterflies that lived in Coyote Ridge—and their migratory connections to other butterfly patches—provided a lifeline to the colonies that Ehrlich and Parmesan studied.
Over the course of thirty-five years, checkerspot butterflies reappeared in about 13 percent of the patches that Parmesan and her colleagues monitored. “It was like, ‘Wow! Cool!’ ” Parmesan says. They reappeared on the ranch in Arizona where their colony had collapsed entirely seven years earlier. Somehow butterfly pioneers had emerged.12 And they’d overcome miles of formidable, uninhabitable territory, located suitable new habitat, and established new colonies.
Their survival seemed “something of a paradox,” as one butterfly biologist noted in an Ecology paper. Ehrlich and others had dismissed the movements of butterflies and other “small, feeble insects” as accidental and ecologically meaningless, as a prominent entomologist who studied migration wrote. Ehrlich figured that just 3 percent of his checkerspot butterflies would move even 50 to 100 meters between patches. But in at least one study, biologists recaptured checkerspot butterflies six miles away from where they’d been released.
Just what might have triggered their movements13 remains obscure. Perhaps it was an inadvertent outcome of abundance. By triggering population booms, especially lush conditions might have increased the likelihood of either a rare wanderlust-struck butterfly emerging, or a less adventurous one being swept up in a breeze and successfully deposited in a new habitat. But the butterflies’ migratory urge might also have been a response to scarcity. Suggestively, caterpillars that experience malnutrition increase their investment in flight muscles, compared to well-fed caterpillars. And signals of diminishing resources trigger the emergence of winged forms in ants and termites, equipped to travel.
The morning I visit Coyote Ridge, the sun struggles to burn through a thin blanket of clouds, and a steady damp breeze whips the pages of my notebook, sending strands of hair flying into my face. Despite the weather, it still looks like a scene out of The Sound of Music. Low grasses and wildflowers blanket the broad hilltop meadows, one after the other, as far as the eye can see, interrupted by scattered outcrops of serpentine rocks covered in bright orange lichen. The hum of the highway that leads to the sprawl of Silicon Valley is barely audible. In the distance, the Diablo mountain range looms, distantly dotted with herds of elk and deer.
Little flocks of checkerspot butterflies flit around my feet. They’re everywhere. Migrants dart among them, connecting them to distant habitats with fragile, silken threads.
Scientists have illuminated the origins and ecological functions of migration in animals by amassing indirect data. They can’t ask a wolf padding through snow-capped mountains where it’s going and why. But the migratory longings of humans can be investigated directly.
Whenever I meet people on the move, I ask them the same question. The men and women camping in an abandoned stadium outside Athens, suffering epidemics of scabies and washing their clothes and children in filthy public bathrooms while standing in an inch of stagnant water, who’d arrived from Pakistan. The woman who’d landed in Baltimore with her son, after leaving her three-year-old daughter and her parents on a farm in Eritrea. My father, who still yearns for the choked tenements of Mumbai where he’d grown up, more than fifty years after he left. My father-in-law, who left postwar Britain with nary a look back. I asked them all: Why did you leave?
“When you first called, I had the anxiety,” one migrant tells me, when we meet in a cramped ground-floor office in a small brick building outside Boston. “I did not want to call you back. There is no way for me to come,” he says he remembers thinking. He’d come anyway, driving forty-five minutes to meet me.
But the man, who’d migrated to Boston from Haiti, says he cannot tell me what I came to hear. The documents that allowed him to enter the United States are now in question. He won’t give me much more than impressions about his life before and after he migrated by way of explanation for his journey.
The boy from Kabul teared up, when I asked him. He wanted to study electrical engineering, he told me, and had planned to apply to study it at a specialized school in Kabul. “Nobody wants to leave their country,” he tells me, but “everyone is in danger there. You walk, and there’s bombs going off.” Fearing he’d be recruited by the Taliban, his family sold everything they had to send him on a long trek not just out of the Taliban’s reach but all the way to Europe, on foot, accompanied by some distant relatives. He was in eighth grade when he set off, leaving his parents and older sister behind. But why him, why then, why not others? I wanted to know. He could not tell me.
One man who had left a farm in Haiti for a chilly flat on the outskirts of Montreal smiled nervously when I asked him. “I know someone beat me,” he says to me. “They make me die.” He cannot answer any follow-up questions. I don’t know what to think. But I know that his future depends on how he answers this one particular question of why. If he can convince the immigration authorities that he left for what they consider the right reason, he may be able to stay. If the authorities feel that his reasons are not correct, he’ll be forced to leave. Opportunistic immigration “consultants” gouge migrants like him for hundreds of dollars that they don’t have to help them mold their messy stories into tales that will allure the authorities, whose demands for reasons behind migrants’ movements change with the political winds.
We can ask migrants why they move, but it’s not necessarily possible for them to answer, at least not in the direct and simple way we’d like. The question assumes that human migration can be explained by some singular reason. That assumption shapes how we talk about people on the move. We describe them as “economic migrants” or “political refugees.” Some characterize them by their suspicions about their legal status as “aliens” or “illegals.” We define them by the directionality of their movement over international borders, as “immigrants” or “emigrants,” submerging their equally if not more complex and lengthy movements within borders.
But all we really know is that they are people on the move.
Among migrants, Homo sapiens is king. And yet we have little consensus on why we move around the way we do. The findings of continuous migrations throughout our deep past have upended the idea that we moved only once in the past, attracted by empty lands, but have left the central question intact: Why? Why venture into the oxygen-starved Tibetan plateau or set off on outrigger canoes into the waves of the Pacific? Why leave the comforting certainties of life in Africa, where food and water and other resources abound to this day?
While the ecological role that wild migrants play is increasingly well documented, the motives and impact of human migration remain shadowy14 and ill-defined. Many popular theories locate the origins of human migration in nonmigratory behaviors, as if migration were essentially an accident, a by-product in our quest for other goals. The archaeologist J. Desmond Clark, for example, theorized that our first migrations began simply because we followed wild animals that moved. Our early ancestors hunted herds of wildebeest and antelope, he pointed out, and they moved seasonally over long distances. We followed, spears in hand, bellies empty, and in the process were drawn steadily farther and farther afield, turning us into accidental migrants.
Indeed, human movements continue to track those of wild creatures in modern times. In the seventeenth century, people from France migrated to North America in search of fur-bearing animals, whose pelts they’d use in their felted hats and the like, establishing the colony of New France in Canada. In the late eighteenth century, people from the Azores Islands migrated to New England, following the whales they hunted, establishing still-extant Portuguese communities in Massachusetts. We moved in sync with the animals15 because our livelihoods depended on their fur and flesh.
Most of our livelihoods no longer depend directly on animals and their movements, but all the same we still move in order to secure the economic sustenance they once provided us. Most every migrant could accurately describe16 their movements as motivated by the desire for work and economic security: for all their travails in their home countries, the man from Haiti hoped to become a nurse, the boy from Kabul an engineer. And their labor is one of their most consequential impacts, adding billions of dollars to the economies of the countries they enter. Because so many send money back to their relatives and friends left behind, migrants’ labor adds billions of dollars to the countries they left as well. International migrants send more than $500 billion to their home countries every year, a flow of money that steadily redistributes wealth across borders. For some countries, these so-called remittances from migrants living overseas form sizable proportions of their GDP. According to data from the World Bank, remittances account for around 20 percent of the GDP of Lebanon, Nepal, and Moldova.
Still, our migration patterns cannot be defined solely as a product of the search for jobs. Economists have tried. In one formula, neoclassical economists calculated the likelihood of migration based on the difference between wages here and wages there: ER(O) ffl [PI (t)P2(t)YOt) − P3(t)Yo(t)]ertd − C(0). It looks more like the way to calculate the rate of nuclear fission than the probability of a messy human endeavor such as migration. And it doesn’t actually work.17
Other popular theories18 about the origins of human migration suppose that some systematic change in the climate triggered our first movements out of Africa. Some, presuming migration to be an act of desperation, imagine it must have been a sudden, catastrophic event. The eruption of Mount Toba in Indonesia 74,000 years ago, for example, blanketed the skies with ash and depressed global temperatures for millennia. Perhaps that long volcanic winter “precipitated a desperate search for new food and land,” as Siddhartha Mukherjee put it in his popular history of genetics.
The migratory response to future environmental change is similarly cast as a last-ditch one, forced by catastrophe.19 In white papers and articles, national security and foreign policy experts issue predictions about how the disruptions and dislocations of climate change will affect migration. Food and water shortages will lead to instability, which will force migrants into motion, leading to more instability. Catastrophic floods and expanding deserts will force whole communities to pick up and leave. Rising seas will inundate millions of homes, forcing their residents to flee. By translating each “unit” of climate change into a proportional additional unit of migration, as the geographer Robert McLeman put it, experts such as the environmentalist Norman Myers estimated that by the mid-twenty-first century, climate change will create an army of 200 million environmental refugees, who will scour the planet. Migration will be “one of the gravest effects of climate change,” the Intergovernmental Panel on Climate Change noted, “one of the most dramatic consequences of global warming.” Such climate-driven migrations could even lead to civilizational collapse. According to their assessments, it has happened before.
But perhaps migration takes hold during periods of opportunity, not crisis.20 It’s possible that our restless ancestors, rather than reluctantly escaping from bad conditions, capitalized on good ones. The earth’s orbit wobbles on time scales of tens of thousands of years, the rotations switching from elliptical courses to circular ones. These orbital shifts change the angle and strength at which the sun’s rays hit the planet and so, over time, alter the planet’s climate. Such climate swings might have facilitated human migration, by turning the impenetrable deserts of North Africa, for example, into habitable savannah-like green corridors across which humans might have moved, like butterflies and clouds of pollen across the forests along the Savannah River. Suggestively, computer modelers from the University of Hawaii have found that orbital climate changes correspond with the pulses of human movements out of Africa.
Our fears and confusion about why people move permeate the laws we’ve passed regulating whether we’re allowed to relocate and under which conditions. Migrants’ job-seeking, despite its powerful economic impact both on the societies they enter and on those they leave behind, is only sometimes and in certain places considered a legitimate reason to be granted permission to move across an international border. In places such as the United States, conflicting policies cater both to employers who benefit from free movement of labor and to workers who feel threatened by it. The contradictory result—a flow of newcomers who are simultaneously allowed entry but are stigmatized for doing so—emerges from the political tension.
Authorities’ positions on the legitimacy of moving to escape hardship are equally mercurial. Melting glaciers and rising sea levels have already made scores of towns and villages on low-lying islands such as Kiribati in the Pacific, Isle de Jean Charles in the Gulf of Mexico, and Shishmaref off the coast of Alaska uninhabitable. Even more people have been compelled into motion because their fields have dried up or their crops have failed, one of the many outcomes of climate change that scientists have long predicted. Most would not describe themselves as being displaced by climate change, though they arguably are. Out of all the countries in the world, only New Zealand has considered the idea21 of letting them cross international boundaries for that reason. In the United States, people fleeing natural disasters and armed conflicts can enter under the country’s “temporary protective status” program, but only for limited periods of time, regardless of how permanently their homes and communities are damaged.
Instead, the 144 countries that signed on to the 1951 Refugee Convention offer refuge to only those migrants fleeing a certain kind of abuse and oppression. Refugees, by its definition, are those who flee state persecution of members of their race, or their religious or social group, that is, the kind of abuse doled out by the Nazis, whose crimes motivated the formulation of the convention in the first place. Those who move across borders to escape oppression and abuse doled out through other means—the oppression of poverty or environmental degradation, say, or the abuse of a failed state that refuses to police their communities or educate their kids—do not qualify,22 although they fit perfectly into our colloquial understandings of the term “refugee.”
Some countries have signed treaties with each other so that they can refuse entry even to those fleeing Nazi-style persecution, if they’d passed through any other country where they might have been able to apply for refuge en route. According to these “Safe Third Country” agreements, if migrants’ journeys to, say, Canada wend through the United States, or to the United Kingdom through, say, Greece, authorities can force them to turn back, even if they are fleeing the kind of abuse sanctioned by international law. In the summer of 2019, the Trump administration attempted to force poor and unstable countries such as Guatemala to sign Safe Third Country agreements, too, threatening catastrophic tariffs if they didn’t comply. According to the logic of such agreements, fleeing state persecution is a legitimate reason for migrants to be granted entry only if they are truly desperate, and if they’ve taken their very first opportunity to apply. If they didn’t, it wasn’t. They’d be sent back.23
The migrants I speak to know all this. I don’t think they’re not telling me the truth. But their stories must run through filters, like murky water through sand.
When I was small, like a lot of children I had a habit of asking overly broad “why” questions, like why airplanes fly, then refusing to be satisfied with whatever answer adults provided, continually demanding further explication. In one oft-told tale in my family, my uncle abruptly ended one of our long series of back-and-forths by marching me outside and pointing to the sky. By then I had likely asked him why about a dozen times in a row. “You see the sky?” he asked me, exhausted. “It’s so high. And you can’t fly. That’s why.”
Some phenomena, in other words, are not necessarily amenable to simple explanation. In the case of migration, I’ve come to realize that asking why people move reveals more about us and our expectations and fears than it does about migrants or migration. The idea that there should be a single explanation for migration is “rooted in a sedentarist notion,”24 the geographer Richard Black suggests, according to which “migration is seen as a problem or exception from the norm, which needs explanation.”
While confusion reigns about why we move and which reasons, if any, should be considered legally acceptable, telling evidence suggests that migration is encoded25 in our bodies, just as it is in wild species.
While we may not be able to detect volcanic eruption like the goats of Mount Etna, human bodies are sensitive and responsive to environmental change, too. Our relatively modest number of genes—about the same number as a nematode worm—does not translate into a narrow range of functional, developmental, and morphological differences among us, because our genes function in dynamic interaction with the environment. They’re like letters in an alphabet, capable of expressing a wide variety of meanings, depending on their pattern and context.
Our bodies have equipped us with a range of options, producing results that suit a diversity of conditions. Over 180 different genes influence our height. At least eight different gene variants influence the color of our skin, each instructing skin cells to produce variable quantities of pigment. In notable distinction to the principles of eugenics, all those gene variants originated in Africa and are present in the genomes of both dark-skinned and light-skinned people today.
The frequency of genes,26 the presence or absence of other genes, and the environmental microconditions around our genes can dramatically change the way our genes are expressed. Temperature, for example, can change the degree to which a gene is expressed or whether it’s expressed at all. In fruit flies, certain genes are expressed at specific temperatures; at others, they’re not. The color of light that falls on caterpillars will change the way their genes for wing colors are expressed. If reared under red light, they’ll be intensely colored; under green light, dusky; under blue light, pale. Depending on the population density around them, desert locusts will develop into sedentary forms or migratory ones. Depending on whether they detect chemical traces of predators around them, semitransparent crustaceans called Daphnia will give birth to differently formed offspring, either with or without defensive helmet-like structures.
The environment around us shapes how our bodies develop, too. Inside our mother’s wombs, we flail and toss, the pattern of our movements etching on our hands27 the unique furrows and ridges of our fingerprints. Meanwhile signals from the external environment seep through our mother’s bodies into our own, whether it’s chemicals in the air she breathes or the kind and quantity of the local foods she consumes. Our bodies respond to those signals, changing the way our genes instruct our cells to function and shifting the course of our development.
One of the mechanisms by which this occurs involves a process called methylation. Genes have little groups of methyl molecules around them that act like switches, turning genes off and on. This in turn can affect whether other genes get turned off or on, triggering a cascade of interactions. Cues from the external environment, such as our mothers’ experiences of famine or their ingestion of pollutants, shape the process.
People born to women who’d been pregnant during a short-lived famine in the Netherlands during World War II, for example, have different methylation patterns in their genes from those of their same-sex siblings born before or after the famine. Their bodies absorbed signals of famine28 sent via their mothers, and they transformed as a result. Researchers found that people who experienced the “Dutch hunger winter” in utero have increased levels of triglycerides and low-density lipoprotein cholesterol in their blood, suffer higher rates of diabetes and schizophrenia, and have a 10 percent higher risk of mortality than people born before or after the famine.
Even after we’re born, environmental conditions shape the development of our bodies.29 For example, at birth, we all have the same number of sweat glands. But the ambient temperature we experience during the first three years of life dictate how many of these glands will become functional, altering our capacity to withstand heat for the rest of our lives. If the weather is sultry during our first few years, we’ll have more functional sweat glands and be better equipped to withstand heat; if not, we won’t.
When people fanned out of Africa and into novel environments with different climates, foods, and pathogens, our bodies adapted in response. Different genetic variants spread to help us survive the peculiar microbes in the habitats we entered. People who encountered malaria-carrying mosquitoes adapted with genetic variants that protected them from malaria’s appetites. People who lived around the Ganges30 River delta, where cholera lurked, evolved adaptations that reduced the risk of dying from the disease. People from that part of the world have the lowest rates of blood type O, which increases cholera’s deadly effect.
When the weak sunlight of northern climes31 threatened humans with vitamin D deficiencies, narrowing women’s birth canals and ratcheting up the risk that both they and their babies would perish during childbirth, gene variants that increased their ability to absorb vitamin D from the sun’s rays proliferated. That environmental adaptation can be seen in the paler skin tones common among people who live in Europe, North Asia, and elsewhere.
Those who moved into cold regions32 developed higher metabolic rates and stockier bodies that reduced heat loss. People from circumpolar North America and Siberia have higher metabolic rates than other peoples: inland Inuit people’s metabolic rates are up to 19 percent higher than those of non-Inuits, to this day. Genes that helped people digest meat likely spread in people who relied on the flesh of animals for sustenance. Genes that rapidly converted plant lipids spread in those who specialized in vegan diets, such as my own ancestors in India. Genes that helped digest lactose spread in those who relied on milk through adulthood. White nationalists consider their ability to digest milk a point of pride, holding events in which they ostentatiously chug gallons of the stuff, but in fact such genes are present in a range of peoples besides dairy-farming northern Europeans, including cattle-herding peoples in Sudan and the camel-herding nomadic peoples in the Middle East and North Africa known as the Bedouin.
When life in the oxygen-poor heights of the Tibetan plateau felled pregnant women with preeclampsia, genes that allowed us to withstand the perils of high altitude emerged and spread. People from Tibet to this day33 have higher frequencies of genes, such as the oxygen-sensing EGLN and transcription factor EPAS1, which are associated with lower hemoglobin concentrations in the blood that life at height required.
Our bodies’ adaptations34 to the environmental conditions we encountered during our migratory journeys help explain why we carry genes that heighten our risk of disease. Usually, genes that make it more likely we’ll get sick die out over time, as people free of such genes out-reproduce people who carry them. But today a wide variety of genes raise our risk of illness, with nearly all diseases and health conditions having some genetic component.
Some of these genes likely persisted because they helped our ancestors survive the landscapes of the past. A mutation in the GDF5 gene, present in over 50 percent of Europeans and up to 90 percent in some Asian populations, for example, increases the risk of arthritis. When inserted into mice, it exhibits another effect as well: the mutation decreases the length of bones, leading researchers to conclude that the mutation is likely associated with short stature. Because short stature reduces heat loss, researchers hypothesize that the genetic mutation may have arose to protect our ancestors from the cold and frostbite they encountered on migrations into the north. Genes that promote inflammation and increase our risk of chronic inflammatory diseases such as heart disease and arthritis today may have emerged and spread because they helped us survive repeated exposure to food shortages and infections. A genetic variant that puts us at risk of a disease called phenylketonuria—subject of the common newborn screening test performed in hospitals—may also help us survive pathogenic fungi. It’s more common in people who live in damp, fungus-rich Scotland. Gene variants that increase the risk of kidney disease may also help protect people from epidemics of sleeping sickness carried by the tsetse fly. Suggestively, people with higher rates of kidney disease are those with recent ancestry in Africa, where tsetse flies and sleeping sickness lurk.
Given the complex and circuitous path that lies between our genes and our behavior, it’s unlikely that any one or even any group of genes could be definitively fingered as the source of our migratory impulse. Genes rarely provide instructions for a single trait, especially not for a complex behavior such as migration. And even when a single gene does code for a single trait, it doesn’t express that trait in a straightforward fashion, but responds to switches and cues from the environment and from other genes. And yet at the same time, given our long history of migrations around the globe, it’s unlikely that there’s no genetic component behind the human propensity to move. So far one potential candidate has been found35: DRD4 7R+. In a 1999 study, geneticists discovered that the frequency of the gene in different human populations correlated with how far from Africa they lived, with higher frequencies among those who’d moved the farthest. It’s more common in nomadic peoples and is associated with openness to new experiences, attention-deficit disorder, and bursts of focused creativity.
Far from being fixed, our bodies are fluid.36 Their shapes, sizes, colors, and ability to withstand climatic variations are not locked in for generations by rigid blueprints. We shed forms and physiologies in favor of others, depending on the changing context in which we find ourselves. Our bodies have evolved, in other words, to “evade substantial ‘genetic commitment’ to local ecological conditions,” write the anthropologists Jay T. Stock and J. C. K. Wells.
Such environmental mutability does not evolve in immobile creatures that live in static, unchanging environments. It evolves in creatures who migrate.
Our bodies are built for it.
Like butterflies and wolves, human migrants change the ecosystems they enter.37 People who decide to migrate are not a random cross-section of the population, like those found perusing the aisles of a grocery store or wandering around a train station. Whether it’s money, skills, connections, or stamina, migration requires capital. Those with no capital, such as the very poor, cannot easily afford to undertake it. Nor can those whose capital derives from land ownership, aristocratic lineage, or titles. They have wealth and status, but they can’t take it with them.
Instead, social scientists have found, migrants tend to be the kind of people who don’t have big bank accounts or landholdings or titles but are rich in good health, skills, education, and social connections with people in other places. Their capital is portable. Demographically, in other words, they’re the kind of people who are “the bedrock of successful communities,” McLeman notes. They’re working people from the middle classes, younger and better educated than their nonmigrant peers, and more likely to hail from societies on the middle rungs of economic development. They’re healthier, too. Public health experts have documented what they call the “healthy migrant effect,” that is, the fact that migrants experience lower mortality rates than the host populations they enter. It’s especially striking considering that the newly arrived live in worse conditions and have less access to health care than the already resident, and most hail from poorer countries to boot. One study found that immigrants to the United States, Canada, the United Kingdom, and Australia had lower rates of chronic disease and obesity than comparable native-born residents. Most smoked less, too.
Their movements create social phenomena with their own self-perpetuating momentum. They move in stepwise fashion, from the countryside to the city, from cities across state borders, from nearby countries to ones farther afield, sometimes within a lifetime, other times, like the monarch butterflies that take four generations to migrate from Canada to Mexico, over the course of generations.
The movements of pioneer migrants pave the way for others to follow. As they arrive, they strengthen migrants’ social network, lowering the cost of migrating for others. Pioneer migrants like my parents, who knew not one soul in the United States before they arrived from Mumbai, helped bring over my cousins and aunts and uncles and even some of their friends, providing them with a place to crash, tips on finding jobs, and rides to the specialty grocery stores where they could buy oily jars of mango pickles and skinny green chilis. The remittances and other support they send tether them to the places they left like fragile silken threads.
Migrants bring new cultural practices, recipes, and ways of living and thinking into the societies they join, injecting novelty into insular populations, just as the migrant wolves of Isle Royale and the butterflies of Coyote Ridge did. And very quickly, unless the host society bars them, they integrate with the locals. Even as fire and fury are directed at newcomers and the economic and cultural disruptions they cause, our mongrel societies can and do rapidly assimilate the migrants among us. Within a generation, social, economic, and health indicators that distinguish immigrants from locals—the number of babies they have, the kinds of jobs they take, their levels of educational attainment, the diseases they suffer—converge. In one study of immigrants in the United States,38 all the immigrant-native differences that economists could discern vanished within a single generation.
The arrival of migrants in coming years will undoubtedly be disruptive to communities,39 even as they replenish them with new minds and bodies. In the past, human migrations have flowed more rapidly from east to west than from south to north, as evidenced in the pattern of genes in populations. In the new era, that circulatory pattern will likely switch, flowing from south to north along the gradient of our warming planet. The pace will be faster. Migrations will unfold over years and decades, not centuries and millennia.
But the next great migration will not unfold as an unstoppable physical phenomenon, like a cold front sweeping in from the north. There is no straightforward equation40 between environmental disruption and migratory effect. Those that occur suddenly and catastrophically, like floods and storms, might be expected to produce the biggest migratory effect, but they don’t. On the contrary, studies of the relationship between migration and sudden flooding and storms have found only a weak correlation. Generally, in those situations people migrate only temporarily and not very far, often moving back to the places they left to rebuild after time passes.
One kind of environmental change41 that produces a detectable uptick in migration is drought. A study of three decades of data from 36 countries in sub-Saharan Africa, for example, found a correlation between rainfall shortages and increased rural-to-urban migration. Another found that a 10 percent increase in the number of communities experiencing drought correlated with a 10 percent increase in the number of people on the move. Some of the most prominent modern migrations have proceeded in the wake of drought. The Dust Bowl of the 1930s led to the migration of over 2 million people out of the plains states. Hundreds of thousands left their shanties to resettle in California, defying the California sheriffs who blockaded the border with Arizona to repel them. In the corridor of dry forests that wends along the Pacific coast of Central America from western Guatemala to northern Costa Rica, drought has unfolded in tandem with the growing ranks of people migrating from Guatemala, El Salvador, and Honduras, amassing on the United States’ southern border as I write.
It is telling that slow-onset environmental disruptions leave a more detectable signal on migratory flows than do rapid-onset ones. Unlike storms and floods, which level their catastrophic effects all at once, droughts unfold gradually over time. First the rains become unreliable. Then they fail intermittently. Then comes a string of dry years. Like the goats on Mount Etna before the volcanic eruption and the elephants in Sri Lanka before the tsunami, the sons and daughters of farmers and fishers can detect the signs and understand that it’s time to move on. Migration, in that case, is not a last-ditch escape from catastrophe or a zombie march over the ledge envisioned by some alarmists. It’s a much more nuanced and adaptive response to subtle cues in the environment.
It’s also mediated by politics. The mass exodus out of Syria,42 for example, was preceded by one of the worst droughts on record, during which crops collapsed and livestock herds perished. That led to high food prices, which forced rural people to flock to the cities. Between 2002 and 2010, Syria’s urban populace grew by 50 percent, as its cities absorbed 1.5 million newcomers from the countryside. Many crowded into makeshift settlements, where political unrest against the corruption and neglect of the ruling regime built. An explosive civil war followed, which then led to waves of migration.
But the drought alone did not cause43 people to flee Syria. The failure of the political regime to stabilize food prices and provide food aid played as much of a role. So did the lack of adequate housing and jobs in the cities, and the regime’s brutal response to the unrest that triggered the war. Elsewhere, drought may not influence migration at all. In the United States, increasing heat and aridity are unlikely to have much impact on migration patterns, because they have only small effects on agricultural profits, thanks to the resilience of the economic system.
When storms worsen, sea levels rise, and rains fail, picking up and leaving isn’t the sole option.44 Societies may decide instead to ensure that people live in homes that can withstand the weather and that they can grow food under changing conditions. Settlement patterns, building codes, and agricultural practices can all be altered to enhance people’s ability to stay. And the trigger for actions that improve resilience can be environmental disruption itself.
Water scarcity, which is commonly held up as a precursor to conflict and migration, has also led to hundreds of agreements between people across borders, including those whose governments are otherwise in conflict. Inhospitable weather, during the Little Ice Age, a period of several centuries during the Middle Ages, led Europe to abandon feudalism and ushered in the Enlightenment, the historian Philipp Blom writes. In response to desertification, the African Union has launched an effort to build a mosaic of drought-tolerant farms and forests on an eight-thousand-kilometer path across the continent, in an initative dubbed the “Great Green Wall.”
For people like me who’ve lived their lives feeling out of place, the Heraclitan view of nature that has emerged in the past few decades provides a paradoxical sense of belonging. I remember the stab of envy I felt a few years ago, when I passed by two women at a local farmer’s market. One, wearing a head scarf and a long gown, followed tentatively behind the other, a middle-aged woman in jeans and a sweater who confidently strode ahead, explaining the variety of tomatoes and lettuces available in the market. Given my city’s program of resettling refugees, I assumed the first woman was a newcomer, and the second a volunteer helping her acculturate.
The idea of providing this sort of assistance, with its implicit recognition of one’s insider cultural knowledge, immediately appealed to me. (I also get excited when anyone asks me for directions, something that happens only rarely.) But I quickly squelched the idea of volunteering. No foreign newcomer eager to acculturate would want a marginal American like me as his or her guide. Such volunteer work would join any number of other cultural activities I refrained from, like rooting for the home team and boasting about one’s city or town.
But what I’ve come to understand is that in the broad view of human history, we’re all migrants in every place we live, outside parts of Africa. Drawing the line between natives and outsiders based on some number of generations of continuous habitation is, in the end, arbitrary. Even Donald Trump is the child of an immigrant, like me. His mother migrated in 1930, sailing on a British passenger liner from her Gaelic-speaking home in Scotland’s Outer Hebrides islands to New York City, where she took a job as a domestic worker. Her son and others like him shed their migrant histories like outgrown skins, skipping over the border between migrant and native without a look back. But their nativeness was just as provisional as my own.
I recognize that shifting migration from the margins of human experience to the center will not be comforting to many. Those of us who have been taught to expect stability feel entitled to an unchanging nature and our enduring place within it. But scientific findings have made it clear that migration is not an exception to the rule. We’ve been moving all along. And there’s no singular factor that explains why, and that can be isolated and reversed to restore some mythical stasis.
Accepting that allowed me to see myself in a new way: as entitled to my patch of earth as anyone else. If anyone cared to ask, I would now call myself an American, with no extra adjectives to complicate it. And a Baltimorean, too, one with sufficient cultural knowledge to serve as a volunteer for any refugee newcomer.
The past seven thousand years of human migration unfolded under a globally stable climate, with the average global temperature ranging just 0.5 degrees Celsius. That has now changed. Since the industrial revolution, the average global temperature has risen by 0.8 degrees Celsius, bringing longer droughts, stronger storms, and more catastrophic wildfires. More of us than ever before will reach the threshold of the migration ratio, when our generation time eclipses the period of stability we can expect from the places we live in.
But as the next great migration dawns, the relevant question to ask is not why people migrate. Migration is a force of nature, rooted in human biology and history, along with that of the scores of other wild species with whom we share this changing planet. Over the long history of life on earth, its benefits have outweighed its costs.
The relevant question to ask is what we are going to do about it.