The Next Great Migration

Just how people had moved around the planet remained an open question until well into the late twentieth century.

The resurrection of Darwin’s ideas after the Second World War elevated the possibility that at some point in the distant past there’d been some ancient migration from a common source. But while Darwin had argued that all our ancestors had originated in Africa, the migration itself remained shrouded in mystery. Darwin never explained how we’d distributed ourselves to disparate corners of the planet. People presumably could have walked out of Africa into the contiguous land masses of the Old World. But how they had reached the heights of the Himalayas, the depths of the Amazon, the frozen tundras of the Arctic, and the remote islands of the Pacific remained unclear.

If anything, decades of scientific research since Darwin had heightened scientists’ awareness of the impassability of geographic borders in the absence of modern navigational technology. Biologists described how long isolation had differentiated us from each other, recapitulating the geographical borders that separated us into biological distinctions between the bodies of people who lived on different continents. They highlighted the dangers in crossing those borders. They documented the nefarious motives and disruptive impacts of movement, from suicidal zombie lemmings to depraved starving Indians. All of it underlined the rarity of migration in our past.

And yet the puzzling fact of ancient human habitation in places to which they couldn’t possibly have walked persisted. Botanists, anthropologists, and geneticists put forward a series of theories to explain it.

The islands of Polynesia lie scattered across the vast Pacific, lava-spewing tips of volcanoes thousands of miles from the continents, surrounded by tens of thousands of feet of water on all sides.

It had taken centuries of effort for European explorers to make their way there. Only the most masterful European adventurers had had the skills to successfully navigate to these distant islands, thousands of miles from home. The English explorer James Cook, who’d navigated around the islands of the South Pacific in the late eighteenth century, had capitalized on the latest navigational techniques and technology. He used charts and magnetic compasses. He made complex measurements. With newly developed marine chronometers, he calculated the amount of time that lapsed between the sun rising high in the sky back home in England and over his own sails, which he then analyzed using spherical trigonometric methods to figure out how far west they’d sailed.

And yet, when he arrived at the Pacific’s watery outposts, he found that a cacophony of living things had preceded him. From Tahiti to Hawaii, the Pacific islands were fully inhabited and alive with thousands of species of plants, birds, and animals.

The people from one island to the next—even those separated by thousands of miles—appeared to share a kinship. At one point, Cook ferried a high priest from Tahiti with him across the South Pacific. The man had spoken to distant islanders less like a stranger than like a long-lost cousin, their languages mutually intelligible.

For Cook, it was as remarkable as discovering that his dog could speak to his plants. Where had these people come from, and how had they successfully colonized all the Pacific islands, where expanses of sea eclipse land masses by a factor of five hundred to one? The logical conclusion, that prehistoric peoples had navigated from the continents over thousands of miles of open ocean, hopping from one remote island to the next and spreading their cultural and linguistic habits, seemed impossible. Long-distance migration was widely known to be an exceptional feat, requiring uncommon prowess and advanced modern technology. “How shall we account for this Nation spreading it self so far over this Vast ocean?” Cook scribbled in his journal.

In the late nineteenth century, Stephenson Percy Smith, the son of English civil servants who’d settled in New Zealand, tried to solve the mystery. Perhaps the Polynesians’ migration to the Pacific could be explained by their superior racial heritage. According to his “Aryan Polynesian” theory,1 the prehistoric settlers of the Pacific had actually been Westerners, too. He pointed to linguistic evidence purporting to show that Polynesian languages originated in Sanskrit and other Old World languages. He referenced Polynesian peoples’ “charming personalities,” which showed that they must have derived from a “common source with ourselves from the Caucasian branch of humanity.”

Peoples in Polynesia had an alternative theory.² The Maori in New Zealand recounted that their ancestors had arrived in Polynesia from a land to the west that they called Hawaiki. They’d carried with them the crops and animals they’d need to settle the islands, such as pigs, dogs, and fowl. And they’d done it centuries before Cook arrived, traveling not in a modern vessel using advanced technology but in a canoe called the Takitumu, sailing across thousands of miles of open ocean, against the prevailing winds and currents, with no charts or compasses to guide them.

Other canoe-traveling migrations had followed,³ they said, including a massive armada of canoes known as the Great Fleet. European travelers such as the Portuguese explorer Ferdinand Magellan, who landed in the Marianas Islands in 1521, had marveled at the speed and navigability of the locals’ canoes. The French explorer Louis-Antoine de Bougainville had been so impressed by their vessels that he’d called Samoa the Navigator Islands. Cook, too, had noted that the place names, artifacts, and languages of the peoples of the Pacific bore eerie resemblances not just to one another but also to those in Asia.

The Norwegian adventurer Thor Heyerdahl, who arrived on the Polynesian island of Fatu Hiva in 1936 to study botany, did not accept the Great Fleet theory.4 Heyerdahl’s theory of Polynesian migration drew on the legend of Kon-Tiki Viracocha, a chief who, according to legend, floated to Polynesia from Peru on a balsa wood raft.

A floating object could essentially drift from the coast of the Americas westward across the Pacific, with no particular means of navigation, on the prevailing winds and currents, Heyerdahl thought. Trade winds that circle the earth around the equator blow westward at a steady thirteen miles per hour. The cold waters of the Humboldt Current flow north along the western coast of South America, then head due west toward the equator at an average speed of about eleven miles per hour. Heyerdahl envisioned Aryan mariners traveling along the coast of the Americas who got swept up in storms or through navigational error got blown off course. The prevailing winds and currents would have deposited this race of “white gods” amid the islands of the Pacific.

Such a raft journey would explain how people with what Heyerdahl considered Stone Age technology had accomplished what, for Europeans, was a high-tech feat of migration. It would also explain the curious presence of sweet potatoes in Polynesia, Heyerdahl thought. European explorers had first encountered the sweet potato in the Americas. Perhaps people had floated from Peru to Polynesia, as Kon-Tiki had, and had brought American sweet potatoes with them.

Heyerdahl’s theory maintained the fiction that borders, in which migration is a by-product of modernity, like electricity and telephone service. If the ancestors of the Polynesians had arrived by a Kon-Tiki-style raft, there hadn’t been any purposeful migration at all. They’d arrived by accident.

Heyerdahl’s Kon-Tiki theory presupposed 5 a fantastic journey. The raft would have had to encounter miniscule specks of land after drifting across five thousand miles of open ocean. It would have been like dipping your hand into the sea and inadvertently touching a dolphin swimming by. While the Great Fleet theory and its notion of Asians migrating to Polynesia failed to convince the Western scientific establishment, for many of Heyerdahl’s colleagues, the Kon-Tiki-raft theory appeared equally improbable. In 1946 Heyerdahl approached a group of influential American anthropologists for support for his theory. They scoffed. One said mockingly, “Sure, see how far you get yourself sailing from Peru to the South Pacific on a balsa raft!”

To Heyerdahl, who’d grown up camping in Norwegian snow caves and climbing mountains with his Greenland husky, this sounded like a proposal. He did not know how to swim. He had never sailed, nor spent any significant time on the water. (If he had, he later said, he would have known that “you couldn’t cross the ocean in the Kon-Tiki.”) But he had faith in his theory: it made too much sense to not be true.

He mustered a small crew, procured some balsa wood in Ecuador, and set off for the port of Callao in Peru, where he built a nine-log raft outfitted with radio equipment and a painting of Kon-Tiki on its rudimentary sail. He persuaded the U.S. military to provide sleeping bags, field rations, suntan lotion, canned goods, and navigational and radio equipment.

On April 28, 1947, a tugboat launched Heyerdahl’s little wooden raft into the brisk waters off the Peruvian coast.⁶ The crew aboard the raft, young Norwegian scientists, sent regular dispatches to the Norwegian embassy using a radio transmitter powered by a hand-cranked generator. Stories tracking their progress appeared in newspapers around the world. They “feel safe,” the New York Times assured its readers on July 7, and were no longer fearful that “every twist, groan and gurgle inside the raft meant it was coming apart.” They’d been “caught in a gale,” the Times reported the next day. The day after that the storm died down, but the crew lost their “fine green parrot, Mauri,” and were “desperately fighting sharks, tuna and dolphins.” In just a few hours, the crew pulled seven encircling sharks and two tunas out of the water, and an octopus had washed aboard.

Finally, after 101 days of drifting in the Pacific, the Kon-Tiki raft ran aground on an uninhabited atoll in the Tuamotus, a group of French Polynesian islands. They’d drifted 6,900 kilometers, carried by currents and winds from the western shore of South America to these Pacific islands, just as Heyerdahl suspected the prehistoric settlers of Polynesia had.

Upon his return to Europe, Heyerdahl wrote a book about the Kon-Tiki journey.7 The account was wildly popular. Publishers translated it into fifty-three languages. A couple of years later, Heyerdahl produced a film about the journey, which won the Academy Award for best documentary in 1951. The idea that ancient migration had consisted of a series of mishaps delighted the viewing public. Scores of other explorers followed in Heyerdahl’s footsteps, building their own rafts to re-create the accidental drifts that they believed had populated the Pacific.

The most salient objection to Heyerdahl’s Kon-Tiki theory,8 that a raft set adrift on a random course in the Pacific would be unlikely to intersect with its widely dispersed specks of land, had been proved baseless.

In 1963 the historian Andrew Sharp published a devastating critique of the Great Fleet theory of Stone Age voyages to Polynesia from Asia. Polynesian canoes had had no keels, no metal fastenings, nor any of the other features of European vessels. That excluded them from the ranks of those vessels technologically capable of the journey, he wrote. The Great Fleet was just a legend used to justify insecure locals’ territorial claims. Hawaiki was not a real place but a mythical site, like the Garden of Eden and Atlantis. Local stories about earlier migrations had included some obviously fraudulent content, after all, like people hitching rides on albatrosses and floating on pieces of pumice. The case was clear, he wrote: there’d been no purposeful prehistoric migration into the Pacific from Asia.

If anyone had succeeded in propelling themselves across the globe without modern know-how, they could have done it only by serendipitous misadventure. For the millions of people enthralled by the Kon-Tiki adventure, the case of the peopling of remote Polynesia was closed: they’d gotten there by mistake.

The University of Pennsylvania anthropologist Carleton Coon, president of the American Association of Physical Anthropologists, went further. He claimed that there’d been no prehistoric migrations at all.9

According to his theory, humans hadn’t commonly originated in Africa. Weaving together the fragmentary evidence available in the fossil record, he argued in his 1962 book, The Origin of Races, that each of the human races had emerged and evolved separately.

Populations of now-extinct Homo erectus had dispersed across the planet and slowly evolved into modern Homo sapiens independently on each of the five continents, Coon argued. These disparate evolutionary journeys explained how the bodies of the five continental races had become as biologically distinct as generations of scientists had maintained. “Each major race,” Coon wrote, “has followed a pathway of its own through the labyrinth of time.” Over the course of millennia, they’d been variously “molded in a different fashion to meet the needs of different environments.” If so, there was no reason to suggest that there’d been any migrations at all before the modern era.

Scientific belief in biologically distinct racial groups 10 remained widespread. In 1950, in the wake of revelations of Nazi crimes in the name of biological distinctions between peoples, a top agency of the newly formed United Nations issued a statement officially condemning race as an ideological concept, with no basis in biology. But when the agency asked leading scientists to sign on to the statement, they’d balked. Even those sympathetic to the cause of antiracism were reluctant. “I need but mention the well-known musical attributes of the Negroids and the mathematical ability of some Indian races,” the British primatologist W. C. Osman Hill protested. Dismissing the biology of race was “wishful thinking,” another added. Clearly the mental capacities of the races differed, the evolutionary biologist Julian Huxley said, pointing to the “rhythm-loving Negro temperament” and the “shut-in temperament” of Native Americans. “I fear that I would not like my name to appear on the document,” Huxley added. The evolutionary biologist Theodosius Dobzhansky, whose work in genetics had been instrumental in resurrecting Darwin’s theory of natural selection, felt the statement went too far, too. In the end, 83 of the 106 prominent anthropologists and geneticists asked to sign the statement refused. Those who did sign it, Coon claimed, just gave it “lip service,” then privately “tore it apart.”

Coon’s theory about the human past explained not just racial difference, which scientists accepted as a point of biological fact, but also the still-powerful fantasy of a racial order.11 That some racial groups had accrued far more political, economic, and social capital than others was plain to see. Instead of attributing that result to the political, economic, and social policies that lavished resources on some racial groups and deprived others, Coon’s theory chalked it up to evolutionary history.

Each race’s peculiar history of evolution from Homo erectus to Homo sapiens had occurred at different rates and at different times, Coon said, such that “each had reached its own level on the evolutionary scale.” The Caucasoids—white Europeans—had evolved into Homo sapiens before any of the others, which was why they were “more evolved.” Australian aborigines had only recently become humans, which was why political authorities were correct in treating them as primitive. Black Africans, whom he called Congoids, had “started on the same evolutionary level” as Europeans and Asians but then “stood still for half a million years.” They’d become human so recently that they were essentially two hundred thousand years less evolved than white people, Coon wrote.

Influential scientists lauded the book. Coon’s theory, while “highly speculative,” the anthropologist Frederick Hulse noted, was “really comprehensive.” In the pages of Science magazine, the Harvard evolutionary biologist Ernst Mayr lauded Coon’s book as “bold and imaginative” and of “major scientific importance.”12

Because the idea of a racial order in nature¹³ did not contradict mainstream scientific thought at the time, those scientists who objected to Coon’s theory, such as Dobzhansky, did so on technical grounds. The fossilized remains Coon used as evidence in his argument presupposed the biological distinctions for which he argued, Dobzhansky pointed out. Because they assumed a sedentary prehistoric world, archeologists categorized fossils discovered in far-flung locales as zoologically distinct. They dubbed the archaic human remains found in Indonesia Pithecanthropus erectus, for example, and those found in China Sinanthropus pekinensis, as if they couldn’t possibly be the same species because they’d been found so far from each other. (In fact, both Pithecanthropus erectus, or “Java Man,” and Sinanthropus pekinensis, or “Peking Man,” would turn out to be specimens of a single roaming species, Homo erectus.) Coon used such fossils, with their suggestive nomenclature, as evidence for millennia of isolation between their descendants. It was like proving the distinction between a pretty thing and a beautiful thing because one had been called “pretty” and the other “beautiful.”

Plus, Coon’s theory conflicted 14 with what scientists understood about evolution. If groups of Homo erectus had indeed been marooned on their own continents and evolved separately from each other, they’d have been unlikely to all evolve into the exact same species, as Coon’s theory suggested. More likely they’d diverge into five different ones. Cases of so-called convergent evolution, in which separate lineages evolved in ways that rendered the same result, like marsupials in Australia and birds in Asia both evolving ducklike bills, were relatively rare. Evolution wasn’t a train track leading engines inexorably to the same destination. Coon’s theory required not just one Black Swan event but a handful of them, and all with the same result. The chances of that happening were “vanishingly small,” Dobzhansky noted.

Even if all five marooned populations of Homo erectus had collectively evolved into the exact same species, if Coon’s theory were correct, they would have had to practice strict isolation from one another. If they’d moved around and interacted, the inevitable battles and love affairs between them would have resulted in generations of, say, Mongoloid-Congoid babies and Caucasoid-Aboriginal babies, in whose bodies the biological differences of their ancestors would quickly fade. Despite having evolved into the exact same species, ancient peoples would have to have behaved as if they had not, keeping their distance from their fellow Homo sapiens elsewhere as if they’d been infected with some deadly contagion. Coon must have imagined that newly evolved Homo sapiens “practiced racial segregation during their wanderings,”15 Dobzhansky noted dryly.

Civil rights activists condemned Coon’s theory¹⁶ as a racist fantasy. The Anti-Defamation League published a pamphlet condemning his claims. His colleagues in physical anthropology called a special meeting to censure his work, forcing Coon’s resignation as society president. Segregationists, meanwhile, rejoiced in Coon’s notion of barely human Africans and primitive aboriginals. They disseminated his theory in newspapers and in their own pamphlets. Coon himself maintained an active correspondence with and provided scientific feedback to leading segregationists, such as Carleton Putnam, whose book on the biological backwardness of African people inspired a young Ku Klux Klan enthusiast, David Duke.

Coon brushed off his critics, characterizing them as “Pavlov’s puppies.” Dobzhansky, he sniffed, was a “stuffed jackass” waging a “campaign of defamation.” Experts who told the “truth about race” such as Coon were being “persecuted,” his supporters said.

Decades would pass before scientists recovered¹⁷ the long-suppressed history of human migrations and shattered the myths of a sedentary past and a racial order. Until then, each new discovery suggesting migratory movements would be crammed into the old paradigm. It finally began to crack the year after Coon’s book came out, when a couple of experimental biologists at Stockholm University, peering at the cells of chick embryos through an electron microscope, spied some strange fibers.

Those fibers, tucked inside the cells’ mitochondria—worm-shaped structures that generate cells’ energy—turned out to be DNA, the same stuff that coiled inside the cell’s nucleus. But unlike the DNA in the cell’s nucleus, which mixed and reassorted with one’s partner’s DNA in unpredictable ways during reproduction, the DNA inside a mitochondrion was kind of a solitary outpost. It contained just a few dozen genes and quietly traveled through the generations solely through the maternal line, from mothers to babies, unaffected by the confused scramble of reassortment. Its order changed solely through a steady drip of random mutations.

That meant, UC Berkeley’s Allan Wilson realized, that differences between DNA sequences could describe the passage of time, like the depth of a sedimentary layer or the number of rings on a tree. Genetic changes that accrued at a predictable rate acted as a kind of stopwatch, recording the number of times they’d passed through the generations, the way a corroded phrase at the end of a game of Telephone might. He started using this insight in the 1970s to compare the sequences of genes and proteins between different species, seeing if he could pinpoint the moment of their divergence.

Until then such questions had been the province of paleontologists and anthropologists, who pieced together the story of the deep past based on shards and scraps of artifacts, fossils, and other clues. Wilson’s “molecular clock” technique fractured their findings. Paleontologists had concluded that chimps, gorillas, and humans had been evolving separately for about 15 million years. Wilson’s research suggested that they’d parted ways only 3 to 5 million years ago.

In the late 1980s, Wilson and his colleagues at Berkeley, Rebecca Cann and Mark Stoneking, persuaded a few hundred pregnant women with recent ancestry rooted in different continents to donate their placentas for a study of their mitochondrial DNA,18 to find out how long their ancestors had been evolving separately. The researchers froze the placentas, ground them up in a blender, then spun them in a centrifuge a few times, extracting a clear liquid containing pure mitochondrial DNA.

Most experts agreed that regardless of whether our ancestors arrived on their separate continents via an ancient migration or emerged there, as Coon argued, the peoples of Africa, Asia, the Americas, and elsewhere had been evolving separately, behind their impregnable geographic borders, for at least a million years.

That’s not what this mitochondrial DNA showed. According to the geneticists’ analysis, the 147 women of different racial and continental backgrounds had shared common ancestors as recently as two hundred thousand years ago. If true, the long period of isolation that scientists had presumed for centuries didn’t exist. The peoples of the world had emerged from a common ancestor so recently that they had had far less time to differentiate than previously believed. And they had migrated during prehistoric times in a far speedier and more extensive fashion than anyone had imagined. In just a few hundred thousand years, people had catapulted themselves into every last corner of the planet.

The scientists poetically dubbed their shared ancestor whose unbroken line of daughters had bequeathed them her mitochondrial DNA “Mitochondrial Eve.” Mitochondrial Eve explained scientists’ decades-long failure to locate clear biological distinctions between us: they didn’t exist. As the evolutionary biologist Richard Lewontin established in the early 1970s, the variation between racial groups accounted for less than 15 percent of the total genetic variation across the entire species. Much more variation existed between individuals 19—whether of the same race or not—than between the races.

After centuries of allegiance to sedentist myths, commentators viewed the notion of a mass migration out of Africa with suspicion.²⁰ Critics complained that Wilson and his colleagues “simply didn’t have the training” to untangle the complexities of the human past. “African migrants” could never have successfully colonized the entire planet, the paleoanthropologists Alan G. Thorne and Milford Wolpoff wrote in a 1992 Scientific American article.

Research by scientists such as the population geneticist Luigi Luca Cavalli-Sforza supported the migratory history suggested by Mitochondrial Eve. Cavalli-Sforza called that ancient exodus the “Recent Out of Africa” migration. He, among others, incorporated the new DNA evidence 21 with the way skulls had changed, how pathogens, languages, and cultures had evolved, and a raft of other archaeological evidence to prove that we had indeed migrated out of Africa just hundreds of thousands of years ago.

Cavalli-Sforza’s work forced the fact of our recently shared African origins into mainstream acceptance. But his theory left other central planks of the sedentist paradigm intact.

The premodern migrations he described occurred under exceptional and short-lived circumstances. The way Cavalli-Sforza imagined it, the journey out of Africa had been a dispersal into empty land, motivated by the allure of unoccupied territory. Our earliest ancestors evolved in Africa in a world of vast, unpopulated spaces, “new, pristine environment[s],” and “virgin territory.” They spilled out of Africa the way a pool of water expands to fill an empty container. Colonizers set out from Africa to settle new places and founded new colonies, which hatched more colonizers to settle more new places, founding more colonies, and so on until all new places were studded with human habitations.

At that point, the historically unique conditions that compelled our prehistoric ancestors into motion vanished, the migratory process came to its natural conclusion, and the potent barriers to migration imposed by geography and culture rose up once more. Cavalli-Sforza’s and his colleagues’ assumption about what happened next was the same as other scientists’ since Linnaeus: thousands of years of stillness, until modern technology artificially lowered nature’s barriers to our movement.

This assumption had been baked into his own research. Cavalli-Sforza reconstructed the historical relationships between peoples by analyzing their DNA. But to reconstruct the path of prehistoric journeys out of Africa, he didn’t collect DNA from a random cross-section of people across the globe. Instead, he focused on a particular subset—indigenous peoples, in particular those who spoke their own languages and lived within well-defined geographic borders—whom he imagined had remained in the same places where their ancestors had deposited them since time immemorial. He pieced together the story of their ancestors’ movements 22 out of Africa by measuring the relatedness of the long-immobile descendants they’d left behind.

Local communities targeted by his team²³ of scientists had not been pleased. The presumption that the subjects whose blood the scientists sought were no more than “isolates of historical interest” to be pierced, classified, and filed away in gene banks rankled. In Central African Republic, an angry farmer accosted Cavalli-Sforza as he drew blood from a local child. “If you take the blood of the children, I’ll take yours,” he warned, brandishing an ax. The World Council of Indigenous Peoples dubbed Cavalli-Sforza’s work the “Vampire Project.” The Third World Network, an NGO, called it “totally unethical and a moral outrage.”

Some of Cavalli-Sforza’s colleagues objected to his strategy, too, arguing that the groups of people who met his criteria may not have been the isolated and immobile people he presumed. They, too, might be a mix of migrants from different places with a peculiar and checkered history of trade, exchange, conquest, and cultural collision. Perhaps, in other words, the migration out of Africa had been followed not by millennia of stillness but by still more migrations. Cavalli-Sforza’s method of swooping in to extract blood samples as if his subjects’ ancestors had no migratory history to speak of would miss it entirely.

“I am very troubled,”24 one scientist told a reporter from Science magazine. “By sampling that way you bias the results.” Instead of attempting to untangle genetic relationships between groups of people living in different places (or even asking them about their own migratory histories), Cavalli-Sforza’s method simply presupposed them in advance. If those groups of people happened to be as mixed and migratory as their ancestors, his strategy was almost as misleading, the anthropologist Jonathan Marks later wrote, as “asking whether lawyers are more closely related to architects or to accountants.”

But apart from a few grumblings, Cavalli-Sforza’s method stood. Scientific ranks closed around the Recent Out of Africa theory in the first decade of the twenty-first century, a century and a half after Darwin had first proposed²⁵ a common origin in Africa. Documentary films, museum exhibits, and magazine articles popularized the new story of the human past that DNA technology had helped reveal. Many used the metaphor of a tree. The trunk represented ancient peoples of Africa, from whom we’d all evolved. Each population that walked out of Africa into another continent appeared as a branch, reaching out into the distance.

In fact, there was no direct evidence that migration had essentially stopped after the dispersal out of Africa, as the metaphor suggested. The strands of DNA in the ancients’ cells that might have recorded their movements had rotted and decayed, along with their long-buried bodies, millennia ago. But most presumed they’d stayed still. Branches, after all, do not grow back together.

Hints that the past was neither isolated nor sedentary appeared in 2000 with results from the Human Genome Project, a multibillion-dollar program to sequence the human genome.

The sequencers had found barely any difference in any of our genes. According to their results, a paltry 0.1 percent of the sequence of the 3 billion nucleotides strung together on our of DNA differed from any one person to the next. Men and women, the short and the tall, the red-haired and the black-haired, the tongue-curlers and the droopy earlobed and the color-blind, all shared an almost identical sequence of nucleotides in their DNA. Our species had not diverged into separate branches at all. Human beings, President Bill Clinton proclaimed in the White House ceremony announcing the results, were 99.9 percent the same, “regardless of race.”

Relatively speaking, we hardly have any genes at all, the results showed. Ever since the days of Weismannism,26 scientists had believed in the commanding power of biological inheritance. Molecular geneticists had described DNA as a master molecule directing the development and functioning of our bodies as if by dictatorial fiat. The geneticist Richard Dawkins had likened human bodies to “lumbering robots,” manufactured by the sequence of nucleotides in our DNA. Genes played such a central role in our health and behavior, scientists thought, that decoding their sequence would cure cancer and revolutionize the economy. Our gene sequence would tell us “what we ‘really’ are,” recalled Jonathan Marks.

Scientists had expected the human genome to include at least one hundred thousand different genes. They knew that the genome of a millimeter-long nematode worm has around twenty thousand genes. If genes controlled our bodies and behaviors the way many suspected they did, highly complex Homo sapiens would surely have many more, they figured. But as the project proceeded, scientists had had to recalibrate their estimates of the number of genes in the human genome. In 2001 they predicted it might carry not one hundred thousand but perhaps thirty thousand genes. In the end, researchers who analyzed the number of genes revealed by the Human Genome Project sequence found just around twenty thousand—about the same number as the lowly worm.27 Whatever distinctions we noticed among us could not be encoded in our biology in any simplistic fashion, passed down intact from generation to generation. We didn’t have enough genes to spell the difference.

“No one could have imagined,”²⁸ said one, “that such a small number of genes could make something so complex.” Fewer than ten genes, his colleague added, could separate humans from mice.

Studies of the genetics of our fellow primates 29 made the biological borders between us appear even more ethereal. Ernst Mayr had distinguished between species in which biological changes from population to population were abrupt, with each group having character combinations distinct from that of others, and species in which such changes were continuous, shading imperceptibly from one to the next. Chimpanzees and honeybees were of the former type. Our genes revealed us to be of the latter.

Chimpanzees, primatologists had found,³⁰ live in closed groups that don’t mix with other groups, even when their habitats overlap. That’s reflected in their genetics. Chimpanzees, gorillas, and bonobos all have more genetic diversity within their ranks than we do. The genetic distance between two populations of chimps, geneticists found, is four times greater than the genetic distance between people living on different continents. The isolation of their populations from one another allowed them to differentiate. But the same hadn’t happened to us, despite the fact that we are far more numerous and widely distributed than they are. A history of migration and mixing explains why.

Still, confronted with the new genetic evidence,³¹ many scientists felt compelled to hang on to the myth of Linnaean borders between us. Some felt, like an earlier generation of race scientists, that racial boundaries might yet be found and scientists simply had to look harder for them. In a 2002 study, for example, population geneticists decided to sidestep the subjective bias of people self-reporting their own racial categories. Presuming that biological fault lines between the races existed and that the computer could find them “objectively,” they fed genetic data from 1,052 different people into a computer program called STRUCTURE and asked it to find the genetic borders between them. Since, as genetic evidence suggested, migration had made the pattern of variation between people continuous and graded, this was sort of like asking a computer program to analyze the number of colors in a sunset. The result hinged entirely on what number the program was told to find. If the researchers asked for three groups, STRUCTURE would sort the data into nonsensical, non-race-based groups such as “people from Europe,” “people from Africa,” and “people from East Asia, Oceania, and the Americas.” When they asked for six groups, the software sorted the data into people from each of the continents plus a separate group consisting of people who lived in the mountain valleys of northwestern Pakistan known as the Kalash. It could even sort the data into twenty different groups. Still, when it came up with the five continents after being told to divide the data into five groups, investigators proclaimed victory. In an interview with the New York Times, the study’s lead author, Marcus Feldman, said the study had confirmed the popular conception of race.

Other scientists agreed. “Looked at the right way,” the Imperial College evolutionary developmental biologist Armand Marie Leroi commented in a New York Times op-ed, “genetic data show that races clearly do exist.”32 STRUCTURE had had “no knowledge of the population labels” in common use, added the Harvard geneticist David Reich, but had clustered people into the five groups that “corresponded uncannily well to commonly held intuitions about the deep ancestral divisions among humans.”

Reich’s notion of “race” allowed for more nuance than it did in conventional usage. For him, race referred to a genetically related population group, not the broad conglomerations that Linnaeus had defined by skin color and continental origin. Still, in a 2018 Times op-ed, he mostly elided the distinction. “Differences in genetic ancestry that happen to correlate to many of today’s racial constructs,” he wrote, “are real.”

Myths about Linnaean-style biological differences 33 between human racial groups continued to seduce medical professionals. In a 2016 study, for example, half of white medical students claimed that black people’s skin was thicker than white people’s skin. That false belief, which correlates with medical professionals’ inability to accurately assess black people’s pain, is likely implicated in the pregnancy-related deaths of black women, which occur at a rate three to four times higher than in white women. Other scientists found racial categories to be scientifically convenient, regardless of whether they were biologically relevant. In medical genetics studies, for example, scientists continued to group geographically and genetically diverse populations such as Koreans, Mongolians, and Sri Lankans together as “Asians” and Moroccans, Norwegians, and Greeks as “whites,” just as Linnaeus had recommended centuries ago.

Those colorful distinctions conformed to racial categories more than to the actual relationships between the data sets. The range of genes present in people living in Europe or Asia was not distinct from the range of genes present in people living in Africa, as such maps suggested. Because the peoples who had originally migrated out of Africa to settle the other continents composed a subset of the entire population of Africa, their descendants’ genes were a subset of those present in the peoples of Africa. A more accurate pictorial representation of the range of genes present in different populations might color the African continent using an entire palette of pigments but use just a randomly selected but overlapping fraction of pigments for Europe, Asia, and elsewhere.

People who had faith in the myth of biological race and a racial order found sufficient scientific evidence to back up their beliefs. One popularly cited statistic from the Human Genome Project noted that people are 99.9 percent the same “regardless of race.” That didn’t mean that a consistent 0.1 percent genetic difference defined racial groups. The differences between individuals did not, in fact, fall along racial boundaries. But the locution left open that possibility. Given that we share 98.7 percent of our DNA with chimpanzees, and 90 percent with mice, a 0.1 percent difference between races is not necessarily insignificant. “After all,” one observer noted to the race scholar Dorothy Roberts at a conference, “dogs and wolves are nearly identical at the genetic level,35 but the difference between a dog and a wolf is huge.”

Cavalli-Sforza’s color-coded maps had anti-immigrant and white supremacist commentators crowing 36 with delight, including on the popular white-supremacist website VDARE. “Basically, all his number-crunching has produced a map that looks about like what you’d get if you gave an unreconstructed Strom Thurmond a paper napkin and a box of crayons and had him draw a racial map of the world,” one VDARE writer noted, referring to a notorious pro-segregation U.S. senator from South Carolina. Cavalli-Sforza and his colleagues, he concluded, had “largely confirmed the prejudices … of nineteenth century imperialists.” These color-coded maps, another VDARE advocate wrote in the San Diego Tribune, painted a pretty clear picture. Each race was as clearly distinct from the others as pieces of fruit in a bowl. “What does that sound like to you?” the writer asked, suggestively.

Policies that failed to recognize racial biology 37 were “pseudo-scientific,” another white supremacist website, the Daily Stormer, opined. “Science is on our side,” added the founder of a white nationalist group. Some even posed as genetics experts themselves. In early 2019, for example, the Maryland congressman Andy Harris, who held a degree in medicine from Johns Hopkins University, took a meeting with an advocate to discuss the “number of sequenced genomes for research,” as a congressional aide put it. That person had no training in genetics. He was a fund-raiser for white-supremacist outfits.

Just as they had decades earlier when the UN asked them to officially condemn the concept of race in human biology, experts in the field retreated from the political implications of genetic research. “Many geneticists at the top of their field say they do not have the ability to communicate to a general audience on such a complicated and fraught topic” as the biology of human difference, a New York Times article noted. When an organizer with the American Society of Human Genetics attempted to schedule a panel on the political misuse of genetics research, she “found little traction,” the Times noted. David Reich, for example, refused her invitation to lead a public discussion on the issue.

Plus, the hard boundaries of race conformed to a larger view of history that had embedded itself in the public mind. As anyone who’d seen the pictures of the tree representing our population history knew, each continental race had traveled on a separate bough to its destiny, independent of the others. That’s what the DNA revolution had revealed—at least until geneticists got their hands on some petrous bones.

The petrous bone is named after 38 the Latin word petrosus, for “stonelike and hard.” It’s the part of the skull that encases the tissue-lined labyrinth of the inner ear, allowing us to interpret vibrations as sound. It’s the hardest and densest bone in the mammalian body.

It has also protected bits of DNA from the forces of degradation for tens of thousands of years, a fact that geneticists who examine ancient remains happened upon around 2014, when they analyzed a few bony fragments that included petrous bone. Until then, they’d generally stuck to pulverizing femurs and tibias in their search for old DNA, on the theory that weight-bearing bones were the most likely to have retained it intact. As a result, they’d found little ancient DNA in the skeletal remains they examined, besides those that had been preserved under ice or in deep caves.

The discovery of the petrous bone revolutionized paleogenetics. Inside its bony swirls is what one paleogeneticist called the “mother lode” of ancient DNA.39 In 2010 the genomes of five ancient people were published; by 2016, there’d been three hundred; by 2017, more than three thousand had appeared. The work of incorporating the new data rushing out of paleogeneticists’ labs into our understanding of migratory history has only just begun. But already paleogeneticists such as Sweden’s Svante Pääbo and Harvard’s David Reich, among others, have revealed a backstory of ancient migrations that is far more complex than what Cavalli-Sforza and others extrapolated from modern-day DNA.

The Out of Africa journey had been cast as a dispersal into vast empty spaces. But when our ancestors walked out of Africa, new data from ancient DNA revealed, they moved into lands where other peoples already lived. These now-extinct archaic humans had beaten us there, having migrated out of Africa themselves some 1.8 million years ago. When our ancestors encountered them, they did what migrants do everywhere: they had babies with the locals, a process of mixing that allowed bits of their DNA to enter ours. About 2 percent of the DNA in modern-day peoples in Europe and Asia traces back to the migratory collision with Neanderthals; and around that proportion of DNA in people now living in New Guinea and Australia traces back to the Denisovans, a group of ancient humans discovered through genetic analyses. A Denisovan gene that allows people to survive at high altitude now resides in the DNA of people living in Tibet.

Ancient peoples, after their arrival 40 in Eurasia and the Americas, hadn’t stayed put either, ancient DNA reveals. Some migrated back to Africa, endowing their modern-day descendants in eastern and southern Africa with genes from Eurasian peoples. Others migrated to India, joining streams of ancient migrants from Central Asia, the Near East, and the Andaman Islands, all of them leaving their genetic fingerprints behind. Ancient migrants who arrived in Southeast Asia later set off for Madagascar. Those who’d migrated into the Americas picked up and left for Europe.

Geographic barriers—open oceans, mountain ranges—had not barred their wanderings. Nor had a lack of modern navigation technology. Ancient migrants washed over even the remotest regions of earth, and they’d done it successfully more than once. For years, scientists had figured that ancient peoples had migrated into the forbidding Tibetan plateau 15,000 years ago. According to new DNA analyses,⁴¹ they’d also migrated there 62,000 years ago.

No freak accident deposited unsuspecting people⁴² on the remote islands of the Pacific. Ancient peoples had been so determined to settle the Pacific Islands for so long that despite the navigational and technological challenges that the journey entailed, they’d successfully made it there in three distinct waves before Captain Cook arrived, as archaeological, linguistic, and genetic evidence shows.

Patterns revealing genetic relationships among far-flung populations suggest other unexpected journeys. The five-thousand-year-old remains of a farmer buried in southern Sweden turn out to be genetically related to people living in Cyprus and Sardinia today. Modern-day Native Americans turn out to share genes with the Chukchi people of northeastern Siberia, suggesting their ancestors’ migrations from Asia into the Americas and then back again. The ancients roved to and fro to such an extent that even the most seemingly homogenous of their descendants—modern western Europeans, say—could not claim any long period of isolation and differentiation, as much as some might have liked to. The homogenous ancestral population that commentators such as Madison Grant and others imagined never existed. Several genetically distinct groups of people migrated into the region and variously mixed and melded with one another. From what paleogeneticists can piece together, they included dark-skinned hunter-gatherers, farmers with dark eyes and fair complexions, and another group of farmers with light hair. The western Europeans of today are hybrid descendants like the rest of us.

The past, in other words, is “no less complicated than the present,” Reich notes. We weren’t migrants once in the distant past and then again in the most recent modern era, with a long defining period of stillness in between. We’ve been migrants all along.43

The image of the tree, with its separate branches representing the continental populations, suggested that continental populations had diverged, each evolving separately from the others as they reached off into the distance. But geneticists have found no evidence of such divergence. The seeming homogeneity within today’s continental populations and races—the similar skin tone of northern Europeans, the straight hair of East Asians—is not the consequence of some long unbroken line of unchanging ancestry but the momentary result of an ongoing process of migration, differentiation, and melding together again.

Sometimes when two separate branches of a tree rub together in the wind, slowly removing a layer of bark, the layers of tissue growing underneath start to fuse together. As the conjoined branches thicken, bark growing around their wounds, they become a normal branch like any other; the immune fighters, microbes, and nutrients that once pulsed through the circulatory system of each branch separately now flow through the fused branches as one physiological entity. Botanists call the process “inosculation,”44 from the Latin for “little mouth.” It can happen between the branches of one tree, or between branches of separate trees.

The result, a braided tree with branches sprouting from its trunk and then merging back together again, is like a river with streams flowing in and out, winding apart and then reuniting.

If our past is a tree, it is this special kind of tree. Our ancestors migrated, met, merged, then migrated again. We continue to do the same today.

Linnaeus named our species Homo sapiens, Latin for “wise man.” A more apt name might have been Homo migratio.

Pius “Mau” Piailug grew up half submerged in water. He’d been born on a single-square-kilometer speck of coconut trees known as the Micronesian island of Satawal, played in tide pools as an infant, and learned to sail at the age of four. Friends said his rippling back muscles recalled the shell of a hawksbill turtle.

He looked out over the low-lying bow of the Hōkūle’a, a sixty-two-foot double-hulled sailing canoe, as it sliced through the deep blue waters of the Pacific. The Hōkūle’a had been crafted to conform to eighteenth-century illustrations of traditional Polynesian vessels, drawn by Captain James Cook’s crew. In it, Piailug would re-create the ancient migrations that had peopled Polynesia.

Linguistic, archaeological, and ancient DNA evidence 45 has shown that prehistoric people migrated from Southeast Asia into the Pacific in at least three distinct waves. First, people crossed from China to Taiwan and the Philippines. Then they covered the open ocean to reach Vanuatu and Samoa. Finally, they reached the remotest islands of Polynesia, such as Hawaii and Easter Island. They hadn’t come from Peru, and they hadn’t arrived by accident.

The anthropologist Ben Finney estimates that over the millennia of prehistoric migrations into Polynesia, upward of a half-million migrants likely lost their lives at sea. But Homo migratio pressed onward regardless. Experts now widely recognize their migration 46 as “arguably the most expansive and ambitious maritime dispersal of humans across any of the world’s seas or oceans,” as a 2016 paper in the Proceedings of the National Academy of Sciences put it.

Hōkūle’a’s passage across 2,700 miles of open ocean between Hawaii and Tahiti would require navigating two different trade wind belts, the windless doldrums, and equatorial currents and countercurrents that steadily push vessels off course. Piailug and his crew would have to dodge hurricanes, typhoons, and blustery squalls with winds that could reach up to thirty knots, and pass by active volcanoes spewing smoke and flames, surrounded by submerged boat-killing reefs.

Most modern mariners that attempt the voyage set off equipped with the latest navigational aids: powerful engines for when the wind dies, GPS devices and chart plotters to keep track of their course in the featureless ocean, satellite phones and other telecommunications to call for help. Even with all that, there are no guarantees. During one attempted crossing in 2017,⁴⁷ two sailors encountered a squall that killed their engine and damaged their mast. They were lost at sea for five months. When they were finally rescued, they had drifted thousands of miles off course.

Piailug would use neither charts nor modern instruments of any kind. He’d rely solely on the traditional navigation techniques that ancient migrants might have used.

“Wayfinding” involved using stars, ocean swells, and behavioral observations to keep track of speed, distance, and position. It allowed mariners to locate their vessels on the open ocean even as winds and currents and waves battered them to and fro. It required making thousands of observations every day, of the position of the sun, moon, and stars, and the subtle changes in the behavior of birds and fish, which shifted depending on their distance from land. Sometimes Piailug would lie down on the canoe’s floor to absorb the feel of the ocean swells, from which he could detect invisible bodies of land in the distance.

Wayfinding could take a lifetime to learn. Piailug had been taught by his grandfather and father. Heyerdahl and the other Europeans who’d intruded into the Pacific hadn’t known about wayfinding, in part because practitioners were forbidden from sharing the quasi-religious practice with outsiders.

Between 1976 and 2009, the Hōkūle’a completed nine voyages using traditional wayfinding.48 It completed its journey from Hawaii to Tahiti in thirty-four days.

Kon-Tiki wasn’t a total bust, though. Heyerdahl was right about the sweet potato. It had come from the Americas. But people hadn’t brought the plant with them on an accidental drift from Peru to Polynesia.

The potato had made it across the Pacific on its own.49 In 2018 a survey of sweet potato DNA, including DNA from sweet potato leaves that had been collected in Polynesia by Captain Cook’s crew and stored in the National History Museum in London, showed that the Polynesian sweet potato had started to evolve separately from American sweet potatoes about 111,000 years ago, tens of thousands of years before humans reached Polynesia. Most likely it made the journey afloat on the water or was carried by birds.

Human migration is not exceptional. Long isolation did not differentiate our species into separate races. Feats of navigation are not the sole province of “white gods” from the West. The oceans can be crossed by canoe.

And humans aren’t the only ones who move across the landscape, leaping over continents and oceans. Plants and animals do, too.