Who We Are and How We Got Here

by David Reich

  Origins Stories

  In an origins story of the Suruí tribe of Amazonia, the god Palop first made his brother, Palop Leregu, and then created humans. Palop gave the Native American tribes hammocks and ornaments and told them to tattoo their bodies and pierce their lips, but he did not give any of these things to the whites. Palop created languages, one for each group, and scattered the groups across the earth.1

  This origins story was documented by an anthropologist working to understand Suruí culture, and, like origins stories the world over, it is viewed by scholars as fictional, of interest because of what it reveals about a society. But we scientists too have origins stories. We like to think these are superior because they are tested by the scientific method against a range of evidence. But some humility is in order. In 2012, I led a study that claimed that all Native Americans from Mesoamerica southward—including the Suruí—derived all of their ancestry from a single population, one that moved south of the ice sheets sometime after fifteen thousand years ago.2 I was so confident of this theory, which fit with the consensus derived from archaeology, that I used the term “First American” to signal that the lineage we had highlighted was a founding lineage. Three years later, I found out I was wrong. The Suruí and some of their neighbors in Amazonia harbor some ancestry from a different founding population of the Americas, whose ancestors arrived at a time and along a route we still do not understand.3

  If there is anything that scholars studying the history of humans in the Americas agree on, it is that the span of human occupation of the New World has been the blink of an eye relative to the extraordinary length of the human occupation of Africa and Eurasia. The reason for humans’ late arrival to America lies in the geographical barriers that separate the continent from Eurasia: vast stretches of cold, harsh, and unproductive landscapes in Siberia, and oceans to the east and west. It took until the last ice age for Siberia’s northeastern corner to be visited by people with the skills and technology needed to survive there at a time when sea levels were low enough for a land bridge to emerge in what is now the Bering Strait region, enabling them to walk across to Alaska. Once there, the migrants were able to survive, but they still could not have traveled south, at least by land, as they were blocked by a wall of glacial ice formed by the joining together of kilometer-thick ice sheets that buried Canada.

  How were the Americas first peopled? Until two decades ago, the prevailing hypothesis was that the gates of the American Eden only opened after around thirteen thousand years ago. Evidence from plant and animal remains and the radiocarbon dating of glacial features indicate that by this time, the ice sheets had melted enough to allow a gap to open, and sufficient time had passed to allow the barren rocks, mud, and glacial runoff to give way to vegetation.4 In scientific storytelling, this “ice-free corridor” was an American version of the channel of dry land that the Israelites used to cross the Red Sea in the biblical account of the Exodus from Egypt. The migrants who passed through emerged into North America’s Great Plains. Before them was a land filled with massive game that had never before met human hunters. Within a thousand years, the humans had reached Tierra del Fuego at the foot of South America, feasting on the bison, mammoths, and mastodons that roamed the landscape.

  The notion that humans first reached an empty America from Asia—an idea that today is still the overwhelming consensus among scholars—dates back to the Jesuit naturalist José de Acosta in 1590, who, finding it unlikely that ancient peoples could have navigated across a great ocean, conjectured that the New World was joined to the Old in the then-unmapped Arctic.5 This idea gained plausibility when the narrowness of the Bering Strait was discovered by the circumnavigation of Captain Cook. Scientific evidence for humans in temperate America at the tail end of the last ice age came in the 1920s and 1930s, when archaeologists working at the sites of Folsom and Clovis, New Mexico, discovered artifacts and stone tools—including spear tips mixed in among the bones of extinct mammoths—that were effectively smoking guns proving a human presence. Clovis-style spear tips have since been found over hundreds of sites across North America, sometimes embedded in bison and mammoth skeletons. Their similar style over vast distances—contrasting with the regional variation in stone toolmaking styles of the cultures that followed—is what one might expect for an expansion that occurred fast (as the people were moving into a human vacuum). The available evidence suggests that the Clovis culture appeared in the archaeological record around the time of the geologically attested opening of the ice-free corridor, so everything seemed to fit. It seemed natural to think that people practicing the Clovis culture were the first humans south of the ice sheets, and were also the ancestors of all of today’s Native Americans.

  This “Clovis First” model, in which the makers of the Clovis culture emerged from the ice-free corridor and proceeded to people an empty continent, became the standard model of American prehistory. It fostered skepticism among archaeologists regarding claims of pre-Clovis sites.6 It influenced linguists who claimed to find a common origin for a large number of diverse Native American languages.7 The mitochondrial DNA data available at the time was also consistent with the great majority of the ancestry of present-day Native Americans deriving from a radiation from a single source, although with such data alone it was not possible to determine whether that radiation occurred at the time of Clovis or before.8

  A major blow to the idea that Clovis groups were the first Americans came in 1997. That year marked the publication of the results of excavations at the site of Monte Verde in Chile, which contains butchered mastodon bones, wooden remains of structures, knotted string, ancient hearths, and stone tools with no stylistic similarities to the Clovis remains from North America.9 The radiocarbon dates of Monte Verde indicated that some of the artifacts there dated to around fourteen thousand years ago, definitively before the ice-free corridor had opened thousands of kilometers to the north. A group of skeptical archaeologists who had previously shot down many pre-Clovis claims visited the site that same year, and though they arrived doubting that the site could be that old, they left convinced. Their acceptance of Monte Verde was followed by the acceptance of finds elsewhere that also pointed to a pre-ice-free corridor and a pre-Clovis human presence in the Americas. Nearly as strong a case for a pre-ice-free corridor occupation has been made at the Paisley Caves in Oregon in the northwestern United States, where ancient feces in undisturbed soil layers have also been dated to around fourteen thousand years ago, and have yielded human mitochondrial DNA sequences.10

  How could humans have gotten south of the ice sheets before the ice-free corridor was open? During the peak of the ice age, glaciers projected right into the sea, creating a barrier more than a thousand kilometers in length along the western seaboard of Canada. But in the 1990s, geologists and archaeologists, reconstructing the timing of the ice retreat, realized that portions of the coast were ice-free after sixteen thousand years ago. There are no known archaeological sites along the coast from this period, as sea levels have risen more than a hundred meters since the ice age, submerging any archaeological sites that might have once hugged the shoreline. The absence of archaeological evidence for human occupation along the coast in this period is therefore not evidence that there was no such occupation in the past. If the coastal route hypothesis is right, humans could have walked at that time or later (but still in time to reach Monte Verde) along ice-free stretches of the coastline, possibly bypassing ice-covered sections with boats or rafts, and arriving south of the ice millennia before the interior ice-free corridor opened.

  Ancient DNA studies have also now made it clear just how wrong the Clovis First idea is—how it misses a whole deep branch of Native American population history. In 2014, Eske Willerslev and his colleagues published whole-genome data from the remains of an infant excavated in Montana whose archaeological context assigned him to the Clovis culture and whose radiocarbon age was a bit after thirteen thousand years ago.11 Their analysis showed that this infant
was definitely from the same ancestral population as many Native Americans, but his genetic data also showed that by the time he lived, a deep split among Native American populations had already developed. The remains from the Clovis infant were on one side of that split: the side that contributed the lion’s share of ancestry to all Native American populations in Mesoamerica and South America today. The other side of the split includes Native American peoples who today live in eastern and central Canada. The only way this could have happened is if there had been a population that lived before Clovis and that gave rise to major Native American lineages.

  Mistrust of Western Science

  Ancient DNA studies such as the one of the Clovis infant have the potential to resolve controversies about Native American population history. But such studies have resonances for present-day descendants of those populations that are not entirely positive. That is because the last five hundred years have witnessed repeated cases in which people of European ancestry have exploited the indigenous peoples of the Americas using the toolkit of Western science. This has engendered distrust between some Native American groups and the scholarly community—a distrust that makes carrying out genetic studies challenging.

  After the arrival of Europeans in the Americas in 1492, Native American populations and cultures collapsed under the pressure of European diseases, military campaigns, and an economic and political regime set on exploiting the riches of the continent and converting its inhabitants to Christianity. History is written by the victors, and the rewriting of the past after the European conquests has been particularly complete in the Americas, as there was no written language except in Mesoamerica prior to the arrival of Europeans. In Mexico, the Spanish burned indigenous books, and so most Native American writing literally went up in flames. The oral traditions suffered too. Language change, religious conversion, and discrimination against indigenous ways led Native American culture to be relegated to a lower status than European culture.

  Modern genomics offers an unexpected way to recover the past. African Americans—another population that has had its history stolen as its ancestors descend from people kidnapped into slavery from Africa—are at the forefront of trying to use genetics to trace roots. But if individual Native Americans often express a great interest in their genetic history, tribal councils have sometimes been hostile. A common concern is that genetic studies of Native American history are yet another example of Europeans trying to “enlighten” them. Past attempts to do so—for example, by conversion to Christianity or education in Western culture—have led to the dissolution of Native American culture. There is also an awareness that some scientists have studied Native Americans to learn about questions of interest primarily to non–Native Americans, without paying attention to the interests of Native Americans themselves.

  One of the first strong responses to genetic studies of Native Americans came from the Karitiana of Amazonia. In 1996, physicians collected blood from the Karitiana, promising participants improved access to health care, which never came. Distressed by this experience, the Karitiana were at the forefront of objections to the inclusion of their samples in an international study of human genetic diversity—the Human Genome Diversity Project—and were instrumental in preventing that entire project from being funded. Ironically, DNA samples from the Karitiana have been used more than those of any other single Native American population in subsequent studies that have analyzed how Native Americans are related to other groups. The Karitiana DNA samples that have been widely studied are not from the disputed set from 1996. Instead, they are from a collection carried out in 1987 in which participants were informed about the goals of the study and told that their involvement was voluntary.12 However, the Karitiana people’s later experience of exploitation has put a cloud over DNA studies in this population.

  Another strong response to genetic research on Native Americans came from the Havasupai, who live in the canyonlands of the U.S. Southwest. Blood from the Havasupai was sampled in 1989 by researchers at Arizona State University who were trying to understand the tribe’s high risk for type 2 diabetes. The participants gave written consent to participate in a “study [of] the causes of behavioral/medical disorders,” and the language of the consent forms gave the researchers latitude to take a very broad view of what the consent meant. The researchers then shared the samples with many other scientists who used them to study topics ranging from schizophrenia to the Havasupai’s prehistory. Representatives of the Havasupai argued that the samples were being used for a purpose different from the one to which its members understood they had agreed—that is, even if the fine print of the forms said one thing, it was clear to them when the samples were collected that the study was supposed to focus on diabetes. This dispute led to a lawsuit, the return of the samples, and an agreement by the university to pay $700,000 in compensation.13

  The hostility to genetic research has even entered into tribal law. In 2002, the Navajo—who along with many other Native American tribes are by treaty partly politically independent of the United States—passed a Moratorium on Genetic Research, forbidding participation of Navajo tribal members in genetic studies, whether of disease risk factors or population history. A summary of this moratorium can be found in a document prepared by the Navajo Nation, outlining points for university researchers to take into account when considering a research project. The document reads: “Human genome testing is strictly prohibited by the Tribe. Navajos were created by Changing Woman; therefore they know where they came from.”14

  I became aware of the Navajo moratorium in 2012, while I was in the final stages of preparing a manuscript on genetic variation among diverse Native Americans. After receiving favorable reviews of our manuscript, I asked each researcher who contributed samples to double-check whether the informed consent associated with the samples was consistent with studies of population history and to confirm that they themselves stood behind the inclusion of their samples in our study. This led to withdrawal of three populations from the study, including the Navajo. All three populations were from the United States, reflecting the anxiety that has seized U.S. genetic researchers about genetic studies of Native Americans. At a workshop on genetic studies of Native Americans that I attended in 2013, multiple researchers stood up from the audience to say that the responses of the Karitiana, Havasupai, Navajo, and others had made them too wary to do any research on Native Americans (including disease research).

  Scientists interested in studying genetic variation in Native American populations feel frustrated with this situation. I understand something of the devastation that the coming of Europeans and Africans to the Americas wrought on Native American populations, and its effects are also evident everywhere in the data I and my colleagues analyze. But I am not aware of any cases in which research in molecular biology including genetics—a field that has arisen almost entirely since the end of the Second World War—has caused major harm to historically persecuted groups. Of course, there have been well-documented cases of the use of biological material in ways that may not have been appreciated by the people from whom it was taken, not just in Native Americans. For example, the cervical cancer tumor cells of Henrietta Lacks, an African American woman from Baltimore, were distributed after her death, without her consent and without the knowledge of her family, to thousands of laboratories around the world, where they have become a mainstay of cancer research.15 But overall there is an argument to be made that modern studies of DNA variation—not just in Native Americans, but also in many other groups including the San of southern Africa, Jews, the Roma of Europe, and tribal or caste groups from South Asia—are a force for good, contributing to the understanding and treatment of disease in these populations, and breaking down fixed ideas of race that have been used to justify discrimination. I wonder if the distrust that has emerged among some Native Americans might be, in the balance, doing Native Americans substantial harm. I wonder whether as a geneticist I have a responsibility to do more than just respe
ct the wishes of those who do not wish to participate in genetic research, but instead should make a respectful but strong case for the value of such research.

  The withdrawal of Navajo samples from our study was distressing, since they were among those with the very best documentation of informed consent. The researcher who shared the samples with us had collected them personally in 1993 as part of a “DNA day” that he had organized at Diné College on Navajo lands, so there was no ambiguity involved in the handoff of samples along a human chain. During the workshop, he asked participants if they wished to donate their samples for the explicit purpose of broad studies of population history—specifically for studies that “give prominence to the idea that all peoples of the world are closely related and emphasize the unity of human origins”—and members of the Navajo tribe who wished to participate signed a form indicating that they did. Yet these individuals’ personal decisions to participate in the study were overruled by the tribal council’s moratorium nine years later.

  Should we have respected the wishes of the college students who donated the samples, or the later decision of the tribal council? In the instance, we avoided the issue, acceding to the request of the researcher, who was so concerned that he asked us not to include the samples in the study. I was never comfortable with this decision. I felt that including the samples would best respect the wishes of the individuals who chose to donate their DNA for studies of their history. But I recognize that different cultures have different perspectives. There is a movement among some Native American ethicists and community leaders to argue that any research that has as its subject a tribe should only be considered acceptable if there is community consultation, not just informed individual consent.16 These concerns prompted some international studies of human genetic variation to carry out community consultation in addition to individual informed consent before including samples.17 The very few researchers studying Native American genetic diversity almost all now consult with tribal authorities to obtain feedback on study design—and sometimes to obtain explicit community consent—even if doing so is not legally required.