"A wild exodus has begun,” writes Sonia Shah early on in The Next Great Migration. “It is happening on every continent and in every ocean.” In response to the climate crisis, plants and animals that until recently scientists thought were fixed to a particular habitat have been seeking out different surroundings. Butterflies and birds have been edging their way towards the Earth’s poles; frogs and fungi are slowly climbing mountain ranges – while in the oceans, even some coral reefs are moving at the rate of a few kilometres per year. And where wild species go, humans may follow, Shah suggests, noting that more people already live outside their countries of birth than ever before, some of them pushed by war, floods, rising seas and creeping deserts. It sounds apocalyptic. But are we wrong to think so? Shah, a US science journalist, argues in a deeply researched and counterintuitive history that much received wisdom about migration – human or otherwise – rests on a series of misconceptions. We tend to see migration as unwelcome and rare, a flight from hardship or a burden for the place of arrival. But techniques including genetic history, navigational mapping and climatology have revealed that migration and mixing are far more central to life on Earth than previously thought. They may, in fact, “be our best shot at preserving biodiversity and resilient human societies”. Shah takes on the dominant view of displacement as an overwhelming tide of human misery that threatens global stability In making this argument, Shah takes on the dominant view of displacement as an overwhelming tide of human misery that will threaten global stability. Over the past few decades, this perspective has shaped national security policy in powerful countries such as the US, as well as the approach of international institutions such as the UN. Migration, crudely put, equals disorder. If we are to escape this way of thinking, we will have to dispense with the deep-seated assumption that peoples and species are fixed to specific parts of the planet; a “stillness”, as Shah puts it, that sits “at the centre of our ideas about the past”. To make her point, she takes the reader through a history of modern biology, beginning with the 18th-century European naturalists whose effort to categorise the world went hand in hand with the exploration and colonisation of large parts of the globe. From the start, the idea that humans could be divided into a distinct set of biologically different types, each linked to one of the world’s continents, came with value judgments. White Europeans were generally seen as superior, and mixing as undesirable. Shah traces the development of 19th century race science, and how that in turn came to inform political arguments in favour of immigration control. (Arguments in America, at least: while Shah tells a global story in terms of biology, the political narrative is constructed mainly with a US readership in mind.) Ideas about wild species also informed attitudes to humans. Chief among them, in Shah’s account, is the relationship between population and habitat. A mistaken belief among some 20th-century zoologists that nature was essentially “filled up” – each species occupying its own ecological niche, vulnerable to disruption from invasive outsiders, or to self-destruction through overbreeding – led some to imagine migration as “a vector of death”. In the 1960s, a number of ecologists and other scientists began to voice concerns about the supposed destructive effects of human overpopulation, an argument that gained mass appeal through the 1968 bestseller The Population Bomb, by Paul and Anne Ehrlich, which predicted widespread famine and social unrest unless drastic action was taken to limit the world’s population. Although the authors didn’t link this to an explicit call for immigration control, others who followed them did, giving weight to criticism that this was a form of Malthusianism, where “population control really meant controlling certain populations and not others”. Inferior foreigners and the poor, in other words. Yet although Shah is firm in pointing out where scientists have got it wrong, she has faith in science. Animal populations fluctuate, move and adapt their bodies according to the availability of resources. When human population numbers rise, people often work together to find innovative solutions: Shah gives the example of Mexico, where a rising population prompted farmers to improve the efficiency of wheat production, and of cross-border cooperation over water shortages. Perhaps the most groundbreaking discoveries of recent years have been in genetic history. It has already been several decades since the study of DNA revealed how little substance there was to claims of racial difference. Study of genetic material found in ancient bones also suggests that, rather than a single migration out of Africa, humans populated the globe in waves that intermingled, coming back as well as going forwards. “We weren’t migrants once in the distant past and then again in the most recent era,” Shah writes. “We’ve been migrants all along.” That general truth might not be enough to defuse conflicts over migration by itself. But Shah’s tone is neither smug nor triumphalist. She is clear about the power and the danger of xenophobic politics, tracing the anti-refugee backlash that has been mobilised by the right, as well as the threat to our lives posed by the climate emergency. Hers is an optimistic book nonetheless, because it tells us that this is just the latest chapter in a long story of survival and adaptation. Climate change will cause people to move, she argues, but it won’t necessarily be in the way we think: extreme weather, for instance, generally prompts people to move short distances; longer migrations are the result of more gradual change. Migration is disruptive, “but the next great migration will not unfold as an unstoppable physical phenomenon, like a cold front sweeping in from the north”. This is a vivid and engaging story that weaves in the accounts of refugees Shah has met to illustrate the harm done by today’s border controls. As a writer, she has an eye for the visual metaphor, likening anti-immigration arguments to puffball mushrooms, which grow huge, then burst to release their spores, leaving “an empty, crinkled shell”. Those arguments may indeed be hollow but they spread their spores nonetheless: we need books such as this to expose them.