Restless Genes

The compulsion to see what lies beyond that far ridge or that ocean—or this planet—is a defining part of human identity and success.

This story appears in the January 2013 issue of National Geographic magazine.

In the winter of 1769, the British explorer Captain James Cook, early into his first voyage across the Pacific, received from a Polynesian priest named Tupaia an astonishing gift—a map, the first that any European had ever encountered showing all the major islands of the South Pacific. Some accounts say Tupaia sketched the map on paper; others that he described it in words. What’s certain is that this map instantly gave Cook a far more complete picture of the South Pacific than any other European possessed. It showed every major island group in an area some 3,000 miles across, from the Marquesas west to Fiji. It matched what Cook had already seen, and showed much he hadn’t.

Cook had granted Tupaia a berth on the Endeavour in Tahiti. Soon after that, the Polynesian wowed the crew by navigating to an island unknown to Cook, some 300 miles south, without ever consulting compass, chart, clock, or sextant. In the weeks that followed, as he helped guide the Endeavour from one archipelago to another, Tupaia amazed the sailors by pointing on request, at any time, day or night, cloudy or clear, precisely toward Tahiti.

Cook, uniquely among European explorers, understood what Tupaia’s feats meant. The islanders scattered across the South Pacific were one people, who long ago, probably before Britain was Britain, had explored, settled, and mapped this vast ocean without any of the navigational tools that Cook found essential—and had carried the map solely in their heads ever since.

Two centuries later a global network of geneticists analyzing DNA bread-crumb trails of modern human migration would prove Cook right: Tupaia’s ancestors had colonized the Pacific 2,300 years before. Their improbable migration across the Pacific continued a long eastward march that had begun in Africa 70,000 to 50,000 years earlier. Cook’s journey, meanwhile, continued a westward movement started by his own ancestors, who had left Africa around the same time Tupaia’s ancestors had. In meeting each other, Cook and Tupaia closed the circle, completing a journey their forebears had begun together, so many millennia before.

Cook died in a bloody skirmish with Hawaiians ten years later. (The Hawaiians snatched a boat; Cook lost his temper and fired upon them; although he killed one and his crew killed several others, the Hawaiians caught him in the surf and stabbed him to death.) His death, some say, brought to a close what Western historians call the age of exploration. Yet it hardly ended our exploring. We have remained obsessed with filling in the Earth’s maps; reaching its farthest poles, highest peaks, and deepest trenches; sailing to its every corner and then flying off the planet entirely. With the NASA rover Curiosity now stirring us all as it explores Mars, the United States, along with other countries and several private companies, is preparing to send humans to the red planet as well. Some visionaries even talk of sending a spacecraft to the nearest star. (See “Crazy Far.”)

NASA’s Michael Barratt—a doctor, diver, and jet pilot; a sailor for 40 years; an astronaut for 12—is among those aching to go to Mars. Barratt consciously sees himself extending the journey Cook and Tupaia took in the Pacific.

“We’re doing what they did,” he says. “It works this way at every point in human history. A society develops an enabling technology, whether it’s the ability to preserve and carry food or build a ship or launch a rocket. Then you find people passionate enough about getting out there and finding new stuff to strap a rocket to their butts.”

Not all of us ache to ride a rocket or sail the infinite sea. Yet as a species we’re curious enough, and intrigued enough by the prospect, to help pay for the trip and cheer at the voyagers’ return. Yes, we explore to find a better place to live or acquire a larger territory or make a fortune. But we also explore simply to discover what’s there.

“No other mammal moves around like we do,” says Svante Pääbo, a director of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, where he uses genetics to study human origins. “We jump borders. We push into new territory even when we have resources where we are. Other animals don’t do this. Other humans either. Neanderthals were around hundreds of thousands of years, but they never spread around the world. In just 50,000 years we covered everything. There’s a kind of madness to it. Sailing out into the ocean, you have no idea what’s on the other side. And now we go to Mars. We never stop. Why?”

Why indeed? Pääbo and other scientists pondering this question are themselves explorers, walking new ground. They know that they might have to backtrack and regroup at any time. They know that any notion about why we explore might soon face revision as their young disciplines—anthropology, genetics, developmental neuropsychology—turn up new fundamentals. Yet for those trying to figure out what makes humans tick, our urge to explore is irresistible terrain. What gives rise to this “madness” to explore? What drove us out from Africa and on to the moon and beyond?

If an urge to explore rises in us innately, perhaps its foundation lies within our genome. In fact there is a mutation that pops up frequently in such discussions: a variant of a gene called DRD4, which helps control dopamine, a chemical brain messenger important in learning and reward. Researchers have repeatedly tied the variant, known as DRD4-7R and carried by roughly 20 percent of all humans, to curiosity and restlessness. Dozens of human studies have found that 7R makes people more likely to take risks; explore new places, ideas, foods, relationships, drugs, or sexual opportunities; and generally embrace movement, change, and adventure. Studies in animals simulating 7R’s actions suggest it increases their taste for both movement and novelty. (Not incidentally, it is also closely associated with ADHD.)

Most provocatively, several studies tie 7R to human migration. The first large genetic study to do so, led by Chuansheng Chen of the University of California, Irvine in 1999, found 7R more common in present-day migratory cultures than in settled ones. A larger, more statistically rigorous 2011 study supported this, finding that 7R, along with another variant named 2R, tends to be found more frequently than you would expect by chance in populations whose ancestors migrated longer distances after they moved out of Africa. Neither study necessarily means that the 7R form of the gene actually made those ancestors especially restless; you’d have to have been around back then to test that premise with certainty. But both studies support the idea that a nomadic lifestyle selects for the 7R variant.

Another recent study backs this up. Among Ariaal tribesmen in Africa, those who carry 7R tend to be stronger and better fed than their non-7R peers if they live in nomadic tribes, possibly reflecting better fitness for a nomadic life and perhaps higher status as well. However, 7R carriers tend to be less well nourished if they live as settled villagers. The variant’s value, then, like that of many genes and traits, may depend on the surroundings. A restless person may thrive in a changeable environment but wither in a stable one; likewise with any genes that help produce the restlessness.

So is 7R the explorer’s gene or adventure gene, as some call it? Yale University evolutionary and population geneticist Kenneth Kidd thinks that overstates its role. Kidd speaks with special authority here, as he was part of the team that discovered the 7R variant 20 years ago. Like other skeptics, he thinks that many of the studies linking 7R to exploratory traits suffer from mushy methods or math. He notes too that the pile of studies supporting 7R’s link with these traits is countered by another stack contradicting it.

“You just can’t reduce something as complex as human exploration to a single gene,” he says, laughing. “Genetics doesn’t work that way.”

Better, Kidd suggests, to consider how groups of genes might lay a foundation for such behavior. On this he and most 7R advocates agree: Whatever we ultimately conclude about 7R’s role in driving restlessness, no one gene or set of genes can hardwire us for exploration. More likely, different groups of genes contribute to multiple traits, some allowing us to explore, and others, 7R quite possibly among them, pressing us to do so. It helps, in short, to think not just of the urge to explore but of the ability, not just the motivation but the means. Before you can act on the urge, you need the tools or traits that make exploration possible.

Fortunately for me, I had to wander only a floor down from Kidd’s office to find someone who studies such tools: developmental and evolutionary geneticist Jim Noonan. His research focuses on the genes that build two key systems: our limbs and our brains. “So I’m biased,” he says, when I press him about what makes us explorers. “But if you want to boil this down, I’d say our ability to explore comes from those two systems.”

The genes that build our human limbs and brains, Noonan says, are pretty much the same as those that build the same parts of other hominids and apes. Each species’ limbs and brains end up different largely because the construction projects directed by these developmental genes start and stop at different times. In humans the result is legs and hips that let us walk long distances; clever, clever hands; and an even cleverer brain that grows far more slowly but much larger than other ape brains. This triad separates us from other apes and, in small but vital developmental details, from other hominids.

Together, says Noonan, these differences compose a set of traits uniquely suited for creating explorers. We have great mobility, extraordinary dexterity, “and, the big one, brains that can think imaginatively.” And each amplifies the others: Our conceptual imagination greatly magnifies the effect of our mobility and dexterity, which in turn stirs our imaginations further.

“Think of a tool,” says Noonan. “If you can use it well and have imagination, you think of more applications for it.” As you think of more ways to use the tool, you imagine more goals it can help you accomplish.

This feedback loop, Noonan points out, helped empower the great Anglo-Irish explorer Ernest Shackleton—and saved him when he and his crew were stranded on Elephant Island in 1916. After polar ice crushed their ship, Shackleton, 800 miles from anywhere with 27 exhausted men, little food, and three small open boats, conceived an insanely ambitious sea voyage. Using a handful of basic tools to modify a 22-foot lifeboat, the James Caird (another tool), for a task absurdly beyond its original design, he gathered his navigational instruments and five of his men and executed a trip that few would dare imagine. He reached South Georgia, then returned to Elephant Island to rescue the rest of the crew.

Shackleton’s adventure shows starkly, says Noonan, a dynamic that has driven human progress and exploration from the start: As we leverage dexterity with imagination, we create advantages “that select for both traits.”

Noonan makes a good case that our big brain and clever hands build a capacity for imagination. Alison Gopnik, a child-development psychologist at the University of California, Berkeley, says humans also possess another, less obvious advantage that fosters that imaginative capacity: a long childhood in which we can exercise our urge to explore while we’re still dependent on our parents. We stop nursing roughly a year and a half sooner than gorillas and chimps, and then take a far slower path to puberty—about a decade, compared with the three to five years typical for gorillas and chimps. Dental evidence from Neanderthals suggests they too grew up faster than we do. As a result, we have an unmatched period of protected “play” in which to learn exploration’s rewards.

“I wrote a book called The Scientist in the Crib that looks at this,” says Gopnik. “It could just as well have been titled The Explorer in the Playroom.

Many animals play, says Gopnik. Yet while other animals play mainly by practicing basic skills such as fighting and hunting, human children play by creating hypothetical scenarios with artificial rules that test hypotheses. Can I build a tower of blocks as tall as I am? What’ll happen if we make the bike ramp go even higher? How will this schoolhouse game change if I’m the teacher and my big brother is the student? Such play effectively makes children explorers of landscapes filled with competing possibilities.

We do less of this as we get older, says Gopnik, and become less willing to explore novel alternatives and more conditioned to stick with familiar ones. “It’s the difference,” she says, “between going to your usual, reliable restaurant versus a new place that might be great or awful.” During childhood we build the brain wiring and cognitive machinery to explore; if we stay alert as adults, this early practice allows us to spot situations in which it pays to shift strategies. Might there be a Northwest Passage? Could we get to the Pole easier on dogsleds? Maybe, just maybe, we could land a rover on Mars by lowering it from a hovercraft on a cable.

“We carry this forward,” says Gopnik. And the people who keep this spirit of playful engagement with the possibilities of the moment closest at hand—the Cooks and Tupaias, the Sally Rides and Michael Barratts—are the explorers.

In the 1830s in the deep forests of Quebec, Canada, a restless population of pioneers began a lengthy, risky experiment. Quebec City, built by the French by the St. Lawrence River, was growing fast. To the north, along the Saguenay River, stretched a vast, nearly untouched forest. This rich but brutal country soon attracted loggers and young farming families with a taste for work, risk, and opportunity. Up the valley they went, building one small village after another, creating a wave of settlement moving up the Saguenay. From a biologist’s point of view, such a migratory wave can concentrate not just particular types of people on its frothy front edge; it can also concentrate and aid the expansion of any genes that may encourage those people to migrate.

Sometimes a gene rides such a wave passively, more or less by accident—the gene just happens to be common in the leading migrators, so it becomes common in the communities they establish. For instance, if genes for curly hair had been especially common in the Europeans who first started spreading across North America, curly hair would have become more common in North America as those settlers spread across the continent. The gene doesn’t necessarily bestow an advantage; it just becomes more common because so many people in the front edge have it and then reproduce.

But a migratory wave can also allow genes friendly to migration to drive their own selection. A notable, if noxious, example is the South American cane toad. Introduced to northeastern Australia in the 1930s, it now numbers more than 200 million and is advancing across the continent at 30 miles a year. The leading toads hop on legs that are 10 percent longer than those of their 1930s ancestors—and measurably longer than the legs of toads even a mile behind them. How so? Toads that are both restless and long legged move to the front, bringing any restless, long-legged genes with them. There they meet and mate with other restless, long-legged toads to create restless, leggy offspring that move to the front and repeat the cycle.

Laurent Excoffier, a population geneticist at the University of Bern, thinks something similar occurred with the Quebec loggers. In a 2011 paper Excoffier and some colleagues analyzed centuries of Quebec parish birth, marriage, settlement, and death records and found that the pioneer families behaved and bred in a way that spread both their genes and the traits that drove them to the front. These wave-front couples married and mated sooner than did couples back home, perhaps because they were more impatient folks to begin with and because the frontier gave them access to land and a social atmosphere favorable to starting sooner. This alone produced more children than the “core” families who stayed behind did (9.1 per family versus 7.9, or 15 percent more). And because these children in turn proved likelier to marry early and have more children, each pioneer couple left behind 20 percent more offspring altogether. Twenty percent more offspring produces a huge evolutionary advantage. In this case it rapidly raised the share of these families’ genes and cultures within their own population—and thus within North America’s larger population.

Excoffier believes that if this “gene surfing,” as some call it, happened often as humans scattered around the globe, it would have selected for multiple genes that favor curiosity, restlessness, innovation, and risk taking. This could, he says, “help explain some of our exploratory behavior.” Exploration may thus create a self-reinforcing loop, amplifying and spreading the genes and traits that drive it.

There is another self-reinforcing dynamic operating in exploration—an ongoing conversation between culture and genes, wherein genes shape what sort of culture we create and the culture in turn shapes our genomes.

This is culture in a broad sense—shareable knowledge, practices, or technology that people use to adapt to an environment. These things exist only because our genetic traits evolved to the point where we could create them, and we reshape them constantly. But this changing culture can likewise shape our genetic evolution, sometimes in stunningly quick and direct ways.

The classic culture-gene example is the rapid rise of a gene for digesting lactose. If you lack this gene, you’ll have trouble digesting milk after infancy. If you have it, you’ll easily digest milk all your life. Almost no one carried this gene 15,000 years ago, because it gave no advantage. It was just a mutation floating around. But when early farmers in Europe started raising dairy cattle about 10,000 years ago—a culture utterly novel then, an entirely different way of living—this gene suddenly gave people access to a reliable year-round food source. They could survive food shortages that starved other people. This advantage rapidly spread the gene throughout Europe, even as it remained rare most other places. Culture and genes started selecting for each other: A new culture made a gene more valuable, and as the gene spread through the population, it made dairy-farming culture more important.

You can see signs of this dynamic—genes and cultural bits affecting each other’s value—almost everywhere in complicated human behavior and particularly in exploration. The first time a human ancestor used a rock to smack open a nut, she opened the way to a culture that may have increasingly selected for the genes studied by Jim Noonan that underlie dexterity and imagination. Rising powers of dexterity and imagination in turn accelerated the development of culture. Ernest Shackleton, as Noonan notes, drew heavily on this, exploiting a culture of ships, tools, innovation, and way finding—to say nothing of British stoicism—to explore new ground and get back home. In Gopnik’s explorers in the playroom, an ancient human culture of cooperative child raising—by mother, father, grandparents, other kin—has maximized the value of genes that allow a long period of brain development. And Quebec’s pioneer families leveraged their most restless genes and traits by creating a subculture that placed premiums on curiosity, innovation, toughness, and a willingness to take risks—as well as the crucial physical culture of axes, adzes, pikes, and peaveys they used to build homes and harvest wood, and the sleds, wagons, and canoes they used to travel.

With his ships, his compass and sextant, and his mandate from his king, Cook too leveraged his smarts and curiosity to bring home a map of a world previously uncharted. His return fattened the value of both England’s imperial marine culture and the genetic traits he’d displayed in his relentlessly curious and risky journeys.

But what of Tupaia? His genes and culture, it seems, took a more puzzling path to their meeting with their British counterparts. In fact the Polynesians’ spread across the Pacific represents one of the oddest of the movements that took modern Homo sapiens out of the African homeland and around the globe. It began as the first and one of the fastest, slowed to a halt, and then finished in a record sprint.

Their journey began about 60,000 years ago, when one of the first migratory pulses shot from Africa across the Middle East and along Asia’s southern coast. They reached Australia and New Guinea—more accessible then because of low ocean levels—in only 10,000 years. For another 10,000 years these people spread through this island region, sometimes called Near Oceania, until they reached the curved chains of the Bismarck and Solomon Islands. There they stopped dead.

Up to that point, says Ana Duggan, who is studying this migration at the Max Planck Institute for Evolutionary Anthropology, “the islands they moved among were generally intervisible.” That is, land was always in sight: The island in front of you would rise up from the horizon before the one you’d left sank behind.

Sail beyond the Solomons, however, and you could go weeks without spotting land. Neither the navigation these Near Oceanians used nor their boats—probably fairly crude rafts or dugout canoes—could cope. So they stayed put, limited to their line of sight.

“This next part,” says Duggan, “is a bit controversial,” though it’s supported by most Polynesian scholars and a growing confluence of linguistic, archaeological, and genetic evidence. According to this “out of Taiwan” theory, some 3,500 years ago the Near Oceanians received visitors from the north—a coastal people known as the Austronesians (confusing, since they come from Asia) who had left Taiwan and south coastal China a thousand years earlier and spread slowly through the Philippines and other islands off Southeast Asia before reaching Near Oceania. Once there, they mixed and mated with the native population. Over the next few centuries this meshing of genes and cultures created a new people called the Lapita. Soon after that, the Lapita people started sailing eastward across the Pacific.

What got them started again? It probably wasn’t new genes. None of the incoming Austronesian ones jump out as restless-gene candidates. In fact the 7R and 2Rvariants were less common in the Asians than in the Near Oceanians. But the Asians brought something else that was decidedly new.

“They brought a better boat,” says Duggan.

These boats were ships, really: long canoes with sails, outriggers, and far greater speed and range. They allowed the Austronesians to sail in high winds and higher seas. These craft must have stunned the locals. The excitement the ships created, traceable today in the Polynesian culture and the longevity of its marine vocabulary, would have conferred great status. The motivation to explore increases, as does the reward. Like today’s astronauts, the Pacific Island boatbuilders and sailors would have enjoyed a social cachet that enhanced mating opportunities, commanded social and economic support, and created a motivating force that might coddle any number of restless genes. As anthropologist and National Geographic Explorer-in-Residence Wade Davis puts it, “When you set sail to find new lands, you became mythologized—even if you didn’t come back.” And so Tupaia, riding in the DNA of his ancestors, headed east.

A proper sailing craft like the ship the Polynesians developed makes a near-perfect metaphor for the larger powers we gain through culture. It gives our malleable genomes, imaginative minds, and clever hands the power to transform even the strongest forces in our environment—wind, water, current—from threat to opportunity. Let the wind rise to a howl and raise a great sea; we needn’t stay home or become flotsam, for we can change tack, trim sail, and become what amounts to a different vessel. To the Lapita looking out from the eastern tip of the Solomons, a vast ocean before them, such a boat would offer something like a new set of legs. Tiller in hand and new isles in their minds, they could press on in their journey around the globe.

It’s enough to move even a Max Planck geneticist. Ana Duggan, telling me of these boats in Leipzig, confessed she was by nature not the sailing type. But this bigger boat we were talking about—just the idea of that boat—seemed to rouse her inner mariner.

“If someone pulled up to shore in one of those and said, ‘Look at my big fancy boat. I can go far,’ ” she mused, “I’d get in.”

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