Beyond: Our Future in Space (2 page)

Genetic anthropology is the use of DNA and physical evidence to trace human migration. We all contain DNA from our last common ancestor, which is how we know when and where humans originated. DNA mixes due to sexual reproduction but some special sequences of DNA pass unaltered from parent to child. For example, the Y chromosome passes only from father to son and so allows men to trace paternal lineages, while mitochondrial DNA passes only from mother to child and so allows both men and women to trace maternal lineages. Both of these sequences of DNA are subject to occasional harmless mutations that become inheritable genetic “markers.” Within a specific geographic region, any particular genetic marker spreads quickly and after several generations it’s found in almost every member of the local population. When people migrate from a region, they carry that marker with them. By studying different genetic markers in many indigenous populations, scientists map out early human migration.

The Genographic Project has painted a picture of human migration using “brushstrokes” of DNA from more than 70,000 members of carefully selected indigenous tribes around the world. Appropriately, most of the funding comes from the National Geographic Society, which has turned from exploration of the planet to exploration of the inner world. The project is not without controversy, as some indigenous peoples have declared it exploitative and have declined to take part. However, the project has gained a big boost from crowdsourcing. More than 600,000 people have received their genetic histories in return for contributing DNA to an open-source database.
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With such a rich resource and powerful computers to apply to it, more than 100,000 genetic markers have been identified in the past decade. The leader of the project is Spencer Wells, a National Geographic explorer-in-residence. He says: “The greatest history book ever written is the one hidden in our DNA.”

Our DNA tells the story of the profound human urge to explore.

Around 65,000 years ago, we first ventured out of the continent of our origin. The route from the Horn of Africa to the Arabian Peninsula was probably across the Bab el-Mandeb strait. Today that strait is one of the world’s busiest shipping lanes; at that time, after the last ice age had lowered sea levels, it was merely a narrow, shallow channel. The tribe that ventured out of Africa may have been only a few thousand strong. It was not a single expedition but a series of small clans of loosely related family members leaving over a period of centuries. They prospered as they dispersed, starting settlements in Central Asia and then in Europe. By 50,000 years ago, they had spread to southern China and Australia. By 40,000 years ago, they’d spread throughout Europe. Populations prospered thanks to hospitable conditions in southern Europe and Asia.

The last stage of the migration was audacious and dramatic. Despite more favorable climates around the Mediterranean and in the Middle East, some nomads ventured northward. The most recent ice age was sharpening, but these intrepid humans spread in an arc across the Siberian tundra. Vast ice sheets had sucked much of the moisture out of the Earth’s atmosphere and dropped sea levels hundreds of feet. This allowed our ancestors to traverse the land bridge across the Bering Strait about 16,000 years ago. There’s evidence they reached southern California just 3,000 years later. It took them only another few thousand years to travel south most of the way through the Americas. Looking at a map of the journey, where our ancestors moved from the frozen wastes of Alaska to the bleak landscape of Patagonia, it seems astonishing that they traveled so swiftly—the migration couldn’t have been motivated simply by food and shelter.

The timeline just described is affected by the possibility that humans migrated by sea. There is some indication that small groups made the arduous voyage across the Atlantic from Europe to North America 25,000 years ago, clinging to the edge of the ice pack. In Australia, a single lock of Aboriginal hair is rewriting the story of how that continent was populated. The traditional explanation is that some humans who had left Africa moved east and settled in Australia after a sea voyage from Southeast Asia. But in 2011, gene sequencing of hair donated to a British anthropologist in 1923 showed that Aboriginal Australians are more closely related to Africans than they are to Europeans or Asians. So present-day native Australians may be the oldest group of humans living outside Africa.
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After tens of thousands of generations on the African savanna, we spread across the Americas in a few hundred (
Figure 1
). This rapid, purposeful exploration of new worlds is in its way as dramatic in terms of leaving our comfort zone and embracing the unknown as our decision to leave the Earth when we developed the technology to do so.

Genetic material can tell us
how
we spread around the world, but it can’t tell us
why
. For that, we have to look into our natures.

The Urge to Explore

Epic animal migrations are driven by climate, the availability of food, or mating, and almost all animal migrations are seasonal. Humans are the only species that moves systematically and purposefully over very large distances, in multigenerational migrations, for reasons not tied to the availability of resources. The itch that led our ancestors to risk everything to travel in small boats across large bodies of water like the Pacific Ocean is related to the drive that will one day lead us to colonize Mars. Its origins lie in a mixture of culture and genetics.

Behavioral psychologist Alison Gopnik has observed that humans are unique in the way they connect play and imagination. Mammal species can be playful when they’re young, but the play is quickly channeled into practicing skills such as hunting and fighting, which are needed as an adult. Human children spend a proportionally longer time in a world where their development is sheltered and facilitated by adults.
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We play, according to Gopnik, by creating hypothetical scenarios that allow us to test hypotheses—acting in effect like miniature scientists. What happens if I mix these two liquids together? If I go through the woods, will I be able to remember enough landmarks to find my way back? Can I make my Lego bridge span the gap from the sofa to the coffee table? Children are fearless hypothesis machines. After the child develops the necessary motor skills, mental exploration then leads to investigation of the physical environment.

The development of hypothetical scenarios through play isn’t needed for survival and the tendency for mental exploration is peculiarly human. Restlessness isn’t only in our minds; it’s also in our genes.

We share more than 95 percent of our DNA with monkeys and apes, so we have great commonality with our most recent ancestors. Yet certain developmental genes gave us an edge over apes and other hominids: We have lower bodies built for walking long distances, hands that are better for manipulating objects, and brains with larger language and cognition regions. These genes are regulated by regions of DNA that used to be labeled “junk” but are now recognized as being keys to understanding how a species evolves.
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One particular gene has received a lot of attention because of its central role in controlling one of the most important neurotransmitters. DRD4 is one of the genes that control dopamine, a chemical messenger that influences motivation and behavior. People with one of the variants of this gene, called 7R, are more likely to take risks, explore new places, seek and crave novelty, be extroverts, and be hyperactive. About one person in five carries DRD4 in the 7R form.

Intriguingly, the 7R mutation probably first occurred about 40,000 years ago, soon after the exodus from Africa, when humans began fanning out across Asia and Europe. Other studies explicitly tie 7R to migration. Research by Chuansheng Chen at the University of California Irvine showed that among the largely stationary populations of Asia, only 1 percent currently have 7R, while the prevalence is 60 percent in present-day South Americans, whose populations traveled enormous distances from Asia beginning 16,000 years ago (
Figure 2
).
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So is there an “exploration gene”? No. Genes work in combination with each other and behavior is sculpted by the environment, so genes are not destiny and no single gene can hardwire us for exploration. Also, unknown situations can be fraught with danger, so a gene that spurs exploration doesn’t necessarily offer a selective advantage. Moreover, when this gene is expressed, it can have a downside. People with the 7R variation are two and a half times more likely to suffer from ADHD, 50 percent more likely to be sexually promiscuous (which is culturally frowned upon yet is actually an evolutionary advantage . . .), and they’re prone to alcoholism and drug addiction. The safe functioning of any hunter-gatherer society requires intensive cooperation and stable social relationships; too much thrill-seeking would be dangerous and disruptive.

However, in situations of resource scarcity or stress, this particular mutation shows its advantages. Carriers of 7R not only are comfortable with change, they also startle less easily.
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They use less emotion in making decisions and they’re less impacted by the negative emotions of others. Low emotional reactivity and high emotional endurance are valuable traits for a human in a perilous, new environment, as is the ability to plan and solve complex problems when faced with a threat. This “adventure” genotype may even protect against stress, anxiety, and depression.

Even if they’re only present in a fraction of the population, the traits that favor adventurousness are self-reinforcing. If the 7R mutation has slightly higher frequency in a population that migrates, that frequency will increase in a finite gene pool. Mobility and dexterity are enhanced as they are expressed. The most successful nomads will encounter new sources of food and new possibilities for enhancing their lifestyle. The best users and makers of tools will be spurred to come up with new tools and novel applications of existing tools. The fulcrum of this feedback loop is our one attribute that’s unparalleled: a big brain.

Making Mental Models

What would it be like to be a dog?

Despite the empathetic bond that connects us with our “best friends,” a species gulf prevents us from answering that question. Dog brains resemble ours in structure and they undergo chemical changes associated with a wide range of emotional states. Like us, dogs can dream. There’s also intriguing evidence that dogs can mentally sort objects into categories, a talent for abstract thought only previously demonstrated among certain primates and birds. But a dog’s emotional development stops at a level corresponding to the maturity of a human toddler.

Dogs are unable to make mental models of the sort that humans thrive on. If you were suddenly trapped inside the mind of your dog, you’d be subjected to a cacophony of smells and visual stimuli, adept at molding your behavior according to the external environment and your owner’s wishes. But you’d never make mental models based on your experience to guide you in future decisions.

Humans have a singular ability to reason with language and symbols. It starts early. Between the ages of six and nine months, a baby will move from babbling and mimicry to attaching words to real objects. Around the same time, a baby becomes able to hold the idea of an object even when the object is removed from view. Both transitions involve the creation of a mental scheme as a proxy for the real world.

By the age of two, a child is able to detect statistical patterns and draw inferences about cause and effect from that evidence. In an example reported by Alison Gopnik, two-year-olds were faced with a toy box containing green frogs plus a few yellow ducks.
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The experimenter took a few toys from the box, seemingly at random, and asked the baby for one. The baby showed no color preference if the experimenter drew green frogs from the box of mostly green toys. But the baby specifically gave the experimenter a duck if the experimenter had drawn the relatively rare yellow ducks from the box. The baby knew it was unlikely for the experimenter to draw mostly ducks, so the experimenter’s behavior indicated a preference for ducks. Babies aren’t doing experiments or crunching statistics in the self-conscious way that adults do, but they’re unconsciously processing information in a way that parallels the scientific method.