Since the Human Genome Project popularized the incorrect idea that two individuals from different populations are often more alike than are individuals from the same population, people have tried to remedy the misconception of genetic race by portraying the human genome as a single continuum where all groups are like beads on a string. But that’s not a useful metaphor either: You can’t take Elhaik’s hundreds of groups and place them in a single line. While the human genome can be described as a continuum, it is one that branches and changes through time. You can think of it as a tree with a definite and irreducible shape. The end of any healthy, growing branch is a population that exists today. The base of the trunk is the single population from which everyone alive today has emerged. The branches themselves may form a tangled thicket too, different twigs and branches often fusing together to form one.
If you took the genomic tree and made every part of it invisible but the ends of the branches, you would essentially have a map of current human populations. You would see that there were obvious clusters of people but also that there was continuity among the clusters. You might even be able to discern some of the world’s geography in the population map; people are often most like the people who live near them. When scientists like Elhaik analyze the DNA of living populations, they effectively make the whole ancestral tree visible. By measuring ancestry in genomes, they reveal that we are both different—different groups emerge from different branches—and the same—we all emerge from and cluster tightly around the same trunk.
Is it dangerous to contemplate the tree? Despite the resistance to genetic information about history, there has been little research into how people actually use it. Though we are by now well educated about what we fear people will feel, we don’t know much about what they actually do feel.
• • •
When Brian from Texas took a DNA test,* he had always believed that he was a mix of white, Cajun, and French Acadian. But he discovered that a significant amount of DNA on both sides of his family was usually seen in Native Americans. Although he had always identified as French, he no longer does. The results affected how he saw others too. Caucasians now looked different to him.
Brian participated in a survey conducted by Wendy Roth, the professor from the University of British Columbia who found her great-great-great-grandfather’s name in a European cemetery. Roth was fascinated by how people’s identities were affected by information about their DNA, and she felt that there was a “general level of ignorance, a lack of awareness, a lack of interest in this thing that should be worth studying.”
She contacted DNA test takers and found that people’s responses to news about their ancestry were often nuanced and complex and that they changed over time. Most did not experience a significant disruption in their sense of their identity, primarily because the data didn’t contain any big surprises. When people got news they weren’t expecting, for some, like Brian, it changed everything. Yet, Roth recalled, “There are very, very few people I spoke with who will completely change their identification.” Generally the people who said they were changed by news about their ancestry expanded their sense of self to include the new information.
One man whose ancestry was Mexican American discovered that he had Celtic ancestry too. But he had little interest in talking about it because of the stereotype of Celts as physically large and the fact that he was small. People might think it was his fantasy. Others were quite happy to embrace the diversity and complications that their tests revealed, but they found that other members of their families were less open. One woman who considered herself black found that her genome was 39 percent European. While her response was curiosity, her sister did not embrace the news. Another woman who identified as white discovered African ancestry on her father’s side of the family, so she started going to movies and plays exploring the black experience. Yet, she told Roth, she was unable to share the news with her bigoted brother.
Some respondents embraced the new information but became hesitant at the point where it threatened to change significant aspects of their lives. A woman who found out that she had Jewish ancestry was invited to the local synagogue. But the strictness of the religion and the new and different prejudices of the people she met made her feel that she did not belong.
While some felt positive about a newly discovered multiracial history, they were reluctant to announce it in case they were viewed as “wannabes.” A number of Roth’s test takers who discovered Native American ancestry felt the news was complicated by the availability of government money to that minority. They were afraid that people would assume they had produced these lost forebears as a way to access it. Others were concerned that they would be viewed as abandoning their “real” identity and trying to pass as something else.
Sometimes the lack of participation in DNA tests was telling too: Roth was unable to find many Asians for her study and suspects that Asians are less likely to take ancestry tests. When she asked Asians who had taken the tests about this, they would tell her: “Many of us think we know what we are.” Roth observed that “such beliefs that their lineage is completely unmixed is likely no more accurate than for any other groups.” She added, “There is the sense that Asians are very homogeneous in their roots. I think it is related to the national myths and stories that people tell about who they are.”
Overall, Roth found that some people overinterpret DNA and some people don’t, and some people react extremely but most do not; in short, Roth’s responses run the usual human gamut, except for this one ray of light: When many people found out something new, their reflex was to increase their knowledge. One man who discovered that his mother’s line was connected to the Fulani tribe in Africa began to learn the language.
“This kind of testing seems to make people more aware of how much racial mixing has gone on historically,” Roth said. “I think a lot of people start out thinking of themselves as being 100 percent something, and they don’t necessarily go into genealogy because they’re trying to challenge that view, but as more people get immersed in genealogy and, especially, as they do tests like this, they realize that, no, they’re not 100 percent something. . . . There is mixing that has happened, whether it’s a long way back or whether it’s just a couple of hundred years ago or whether it is within the last generation or two.”
• • •
If we want to understand mixing, whether it’s in our own family or in some larger group to which we belong, we have to understand DNA, but we also have to take into account its context. Jennifer Wagner is a lawyer and anthropologist who translates science for the legal world and vice versa. She advocates studying human differences “holistically, integrating the contributing factors of culture, sociology, history, genetics, evolutionary biology, and the like.” With a group of colleagues she is working to develop an innovative curriculum to teach about evolution using genetic genealogy (“a more exciting . . . way of teaching these concepts than is the study of peas or fruit flies”).
Currently in the United States, according to Wagner, “minorities are overrepresented in forensic databases and underrepresented in biomedical research databases. Genetic and genomic technologies could either mitigate or exacerbate racial disparities. We must be mindful of that and do everything we can to ensure that every individual shares in the benefits of scientific knowledge.”
The big human family tree that Wagner will teach could have grown into thousands of different shapes, so why has it taken the particular shape it has? In part it is because of its biological machinery, and in part it is because of the events of history. Human choice, chance occurrence, and unpredictable contingency have all contributed to the tree’s growth. It would be impossible to identify all the factors that have shaped the genome, but we are beginning to have the ability to piece together the events that matter. What were the biggest shapers of the genome we have today?
Chapter 12
The History of the World
Human beings are ultimately nothing but carriers—passageways—for genes. They ride us into the ground like racehorses from generation to generation. Genes don’t think about what constitutes good or evil. They don’t care whether we are happy or unhappy. We’re just means to an end for them. The only thing they think about is what is most efficient for them.
—Haruki Murakami, 1Q84
When you visualize the human tree, picture its trunk firmly planted in African soil. Modern humans emerged there several hundred thousand years ago and lived only there from 250,000 years ago for at least 150,000 years—a much longer span of time than we have lived across the globe.
Working out what life was like when humans were an exclusively African species is probably one of our biggest scientific challenges. There are no written records and few fossils from that time, and handmade artifacts date back to only 70,000 years ago. Which is not to say that humans didn’t use tools or wear jewelry before then—it’s simply that, if there are any that remain, we haven’t yet found them. Still, while scientists have only begun to plumb these depths, with each year that passes our view into the past reaches further back as we find new evidence. While we often think of human history as a kind of reverse dimming, in which the light of our consciousness and intelligence grew ever brighter, the evidence that we were profoundly aware as early as 200,000 years ago is growing. In the last few years 60,000-year-old ostrich eggs with marks that appeared to be intentional engraving were discovered. Dating of beads from Israel and Algeria suggests they are between 100,000 and 130,000 years old. Ancient tools found in Crete suggest that someone sailed there more than 100,000 years ago. It looks as if there was ocher processing in the Blombos Cave in South Africa 100,000 years ago, and in the same region people sharpened the tips of their stone tools using heat—a technique that we used to think dated back only 20,000 years. In the history of science few people have dared to imagine that humans were as intelligent or as technologically adept so early in their history.
• • •
For hundreds of thousands of years much of the African continent was inhabited by different family and tribal groups. Around sixty thousand years ago a small band of them—perhaps not much more than one thousand to two thousand five hundred individuals—went traveling. We don’t know why they left or if they had any sense that they were going somewhere new, but we do know their decision kicked off one of the biggest events in the history of the human genome.
Those who remained are the ancestors of most of the one billion people who live in Africa today. The small band that left are the ancestors of everyone else in the world, and the suite of DNA they carried was only a small sample of the variety of human genomes that existed in Africa at the time of their departure. Indeed, we know the migration occurred because even now we can see that the genomes of everyone in the world outside of Africa is a subset of the genomic variation still found in Africa.
When a small sample of a species’s genome is isolated and then becomes the foundation for another group, it’s called a bottleneck. (In this analogy the neck of the bottle is the small founding group, and the expansion of the neck into the body is what happens when the population grows.) Bottlenecks can be caused by many things and are powerful examples of the role of chance in shaping the human genome. “Things that happened a long time ago can constrain what can happen afterwards,” said Marcus Feldman, a professor of biological sciences at Stanford. “If there is a disaster that kills off 98 percent of the organisms of a certain type, then what happens to the rest of the animals is constrained by the fact that there are only 2 percent of this particular type that are left.”
As a group passes through a bottleneck, it becomes particularly vulnerable to drift. DNA diffuses more quickly within a small population, and it may not take too many generations before everyone’s genome starts to look a bit more like everyone else’s. There’s no rhyme or reason to drift; it’s a matter of chance. Bits of DNA may spread throughout a group for no reason other than that the people who carried them ended up with more children and therefore passed them on more often. DNA that has drifted throughout a group may have consequences for the people in it—like red hair or a protruding brow or a certain health issue—or it may not. It is equally possible that some bits of DNA will not come into prominence but instead float off into oblivion.
• • •
The out-of-Africa bottleneck is one of the easiest to identify, but it’s far from the only one in human history. Around seventy thousand years ago one bottleneck may have taken the human race perilously close to extinction. A volcano erupted in Toba, Indonesia, causing abrupt climate change and leaving a layer of ash over a huge segment of the world. Some researchers have proposed that all of us today descend from a small number of Toba survivors. Indeed, if you wanted to summarize the myriad migrations, cataclysms, illnesses, innovations, and acts of love and hate that have changed the human genome, you could say that it’s been shaped by a series of bottlenecks, where a population shrinks, and fusions, where two or more populations come together and blend their genetic material. To be more precise, you’d have to throw in some Darwinian adaptation as well. Marcus Feldman, who has been comparing populations across the world for a long time, says that the differences between populations “are reflective of two processes. One is migration and the distance from Africa, and the other one, most of which happens after the origin of agriculture, is natural selection on some genes.”
Although the first big out-of-Africa migration was enormously significant, as it marked our transition from being a regional animal to being a global one, there have been many significant migrations since then. Indeed, the histories of most of the world’s large populations include a bottleneck of that first out-of-Africa population.
As they traveled, humans made their way through Asia and along the coast into the southeast. They passed through strange climates and terrains, stumbling upon fantastically colorful and unimagined wildlife, much of which tried to harm or eat them. Everywhere they stopped, they left descendants behind, and wherever the descendants stayed, they adapted to the local terrain and available food. Within a few generations these weird new worlds became a familiar landscape that the travelers’ descendants had always known. In the course of time, later generations of their descendants ultimately changed, becoming different colors, shapes, and sizes.
The wanderers and their descendants invented the taming of animals like dogs, goats, sheep, cats, and horses. They invented transport like sailing and skating on ice. Some of the earlier groups of travelers met humanlike creatures, thickset survivors of an earlier exodus. Others who made it all the way to the landmass we think of as Indonesia may have discovered a group of people who were all the size of small children. On the first leg of the trip they traveled as far as possible, arriving in Australia, a land of two-ton wombats, ten-foot-tall kangaroos, and enormous marsupial lions, about fifty thousand years ago. The ancestors of modern humans began to spread through Europe only forty thousand years ago.
Less than 18,000 years ago humans arrived on a landmass now known as North America. (Only Antarctica has been free of humans for longer than the Americas.) It seems that almost the entire indigenous population of the Americas descends from a small group of perhaps eighty people, originally from Siberia, who followed a route that has now been covered over by the ocean. Over 32,000 years ago they took refuge in northwestern Beringia, a land bridge connecting what are now Alaska and Russia. Over the millennia that followed they moved into eastern Beringia, and then sometime before 14,000 years ago they moved into the North American continent, spreading along the Pacific coastline and then eastward. Geneticists have found that Native Americans have only five kinds of mtDNA, and the first four are common in northeastern Asia. With this evidence and other genetic studies, it has become well-established that the small ancestral population of Native Americans can trace its genome back to Asia.
The picture got mor
e complicated when a study by David Reich found an ancient connection between the modern Native American and European genomes, suggesting that there once existed a population in northern Eurasia that was ancestral to both. The study was based on a comparison of modern genomes; no bones from such a population had been found. Yet in 2013 the remains of a young boy who lived 24,000 years ago were found in Mal’ta in south-central Siberia. Analysis of his DNA showed that he was related to modern Europeans and Native Americans. The finding confirmed that at least 14 percent and up to 38 percent of Native American DNA came from a population in western Eurasia. Remarkably, a few months later the scientists who analyzed the Mal’ta boy’s DNA published the genome of another ancient boy, an infant who was buried in Montana more than 12,500 years ago. Anzick-1, as he was called, was covered in red ocher and placed in the earth with stone tools from the Clovis culture. He is the first ancient Native American whose genome has been sequenced. The people who buried Anzick-1 are ancestors of modern Native Americans (although he is more closely related to 44 groups from Central and South America than to others from North America).
The Invisible History of the Human Race Page 29