Origin: A Genetic History of the Americas

by Jennifer Raff

As Adovasio described in his book The First Americans, after finding the point, “we immediately decamped to our favorite bar in town and polished off ten kegs of beer.” He and the students knew they had found something significant in the unpretentious rockshelter. What they perhaps didn’t appreciate at the time is how much of an uproar the find would ultimately cause.

despite my archaeology professor’s insistence to the contrary, one can say broadly that the totality of the archaeological evidence (we will examine recent evidence from genetics in detail in chapters 5–8) indicates that humans were in the Americas by (at the most conservative estimate) 15,000–14,000 years ago, more likely between 17,000 and 16,000 and perhaps even as early as 30,000–20,000 years ago (if you accept the evidence from some of the sites in Central and South America) (19).

the origin of Clovis is the Debra L. Friedkin site in Texas, which contains a series of occupations dating from the late Archaic through pre-Clovis levels dated between 15,500 and 13,500 years ago. These pre-Clovis levels expose a fascinating sequence: stemmed points in the lowest levels, then directly above them in levels dating to about 14,000 years ago somewhat crudely shaped projectile points that were made using similar methods to Clovis points but don’t have their distinctive fluting. Directly above those were Clovis points. And in each level, we can see that people were also making other kinds of tools in very similar ways: scrapers for removing hair from hides, blades for various cutting tasks, and choppers. Archaeologist Michael Waters thinks that these sites contain the clues to the evolution of Clovis: the pre-Clovis tools (which have been classified as the Buttermilk Creek complex) are the technological ancestors of the Clovis tools. “I see broad connections between Clovis and pre-Clovis sites,” he told me. “The early sites show that biface, blade, and osseous [bone tool] technologies were present before Clovis across the Americas. It is from these technologies that Clovis could emerge.”

The new dates for pre-Clovis sites posed new problems. We know that the ice-free corridor between the Laurentide and Cordilleran ice sheets was open at least 13,000 years ago—there’s evidence for gene flow between bison populations north and south of the ice sheets occurring at this time, osteological evidence for elk migration through the corridor by about 12,800 years ago, and direct evidence of humans in the center of the corridor by 12,350 years ago (21). But was the corridor open long enough to allow people to walk through it in time to populate the pre-Clovis sites? This is a controversial question among archaeologists.

there has never been any archaeological evidence whatsoever showing that anyone moved from Beringia through the interior corridor to the Plains or Great Lakes area. We don’t see the kinds of tools—microblades and a kind of spear point called a Chindadn point—that people were making at the northernmost end of the corridor at sites within the corridor or below the ice wall.x The only archaeological evidence in the corridor is of people moving northward: from the Northern Plains to Alaska/Yukon several millennia after Clovis (23).

This molecular time capsule showed them that even if the entirety of the ice-free corridor was open by 13,000 years ago, it wouldn’t have had vegetation until about 12,600 years ago and animals living within it until about 12,500 years ago. The corridor’s “viability” date would have constrained the movement of people through it, as they would have needed things to eat during their trek through the 2,000-kilometer corridor (24). In addition, as paleoecologist Scott Elias notes, the melting of the enormous ice sheets would have littered the corridor with huge amounts of rock, mud debris, chunks of ice, and water everywhere as the billions of gallons of frozen water were released. “As a human migration route, it would have been absolutely awful,” he told me.

The genomes of Native Americans also argue against the ice-free corridor route. We will talk about this more in later chapters, but complete genomes from ancient and contemporary Indigenous peoples show that major population splitting events were almost certainly associated with the initial peopling of the continents. These population splits occurred extremely rapidly—so rapidly that they have been described as “leap-frogging” southward across large tracts of the American landscape. This is not a pattern consistent with slower, overland diffusion of hunter-gatherer populations. Instead, it matches what one would expect to see if people were traveling by a much faster method: by boat.

While traveling southward through the interior of the continent would have required serially encountering and adapting to new ecosystems (mountains, deserts, plains), people traveling southward via the coast would have had reliable access to food resources with which they were already familiar. Coastal resources are fairly consistent regardless of latitude, and people would have encountered similar ecosystems along the Pacific coast from Southeast Alaska to Tierra del Fuego. In recognition of the significant role that nutritionally valuable and abundant seaweeds might have played in a coastal dispersal, this ecological model has come to be known as the Kelp Highway hypothesis (30). While much more work needs to be done to fully test the ecological hypothesis, it’s supported by the fact that people who lived at Monte Verde ate algae and seaweed over 14,000 years ago and by the dietary evidence associated with Shuká Káa that we discussed at the very beginning of this book.

the Out of Japan model isn’t supported by biological evidence. Biodistance studies of dental traits (see sidebar: “Biological Distance Studies”) show that the Jomon are unlikely to be ancestors of Native Americans, and this is supported further by studies of the genomes of both Jomon and First Peoples. Perhaps cultural diffusion—the spread of ideas and technologies—might be a better explanation for the striking similarities between Jomon and Western stemmed points (32).

could one of these possible early human dispersals have occurred in the direction of Siberia, across the Bering Land Bridge, and down through North America, all the way to California? Or could another kind of human, like a Neanderthal, H. erectus, or Denisovan, have gotten to the Americas? Most archaeologists and geneticists are very skeptical of either possibility. From an archaeological perspective, no skeletal remains that look even remotely like an early human have been found in the Americas, and we have none dating from anywhere near that age. We also don’t see any unambiguous stone tools known to have been made by these earlier peoples: no Acheulian handaxes made by H. erectus, or Mousterian knives and scrapers made by Neanderthals, or Aurignacian blades made by Upper Paleolithic modern humans and Neanderthals.

Every professional archaeologist I’ve ever met has had to deal with countless numbers of people bringing them random rocks that they insist are tools.xi Humans are extremely good at detecting patterns, even where there are none, and the field of pseudoarchaeology rests upon countless “evidence” for its non-mainstream claims from this phenomenon.

there was one region of the Arctic that remained unglaciated throughout the LGM. Eastern Beringia—present-day Alaska—was an ice-free cul-de-sac at the end of the Bering Land Bridge (1). People could have lived there. Whether they did so is a question that fascinates many archaeologists.

Over the last two decades, developments in archaeology and genetics have forced researchers to look for new answers to old questions long thought resolved. At present, it’s difficult to find two archaeologists who agree on exactly how the Americas were peopled; they differ on which kinds of evidence they find most convincing in accepting or rejecting the validity of ancient sites, how different sites relate to one another, and how archaeological evidence should be integrated with genetic data

1. Who were the First Peoples of the Americas? 2. From where did they come? 3. What routes were taken? 4. When did the peopling occur? 5. How did dispersing populations move through the “empty” landscape?

Archaeology and genetics show that the many Native peoples of the Western Hemisphere share something else too. The many threads of their histories converge at a point in the far distant past when the ancestors of the first peoples in the Americas moved from Beringia into new lands in North America. Over just a few thousand years, they explored and adapted to environments across more than 16 million square miles of rocky coasts, deep old-growth forests, high plateaus, endless grasslands, lakeshores, and high arctic tundra. They built mobile camps, small settlements, farming communities, and grand civilizations. They lived as hunter-gatherers, ranging seasonally across vast territories. They lived as farmers, fishers, and livestock herders. They developed sophisticated knowledge of ecology, medicinal plants, and astronomy. They passed down this scientific knowledge to their descendants, along with stories, songs, and languages.

The histories of these peoples are told in the things they left behind: gigantic mounds made of earth, apartment houses perched in caves high above valleys, elaborate stone pyramids, networked roads that linked towns, an isolated hearth out on the high plains, a small sandal left behind in a desolate cave decorated with elaborate rock art, a projectile point embedded in the rib of a mammoth, the skids of an ancient sled. These objects, constructions, and artwork tell us about the countless societies that flourished, diminished, or continued into the present day across the American continents. Each society developed sophisticated ways of living in different environments, from the elaborate irrigation canals created by the Hohokam to water their crops in the Sonoran Desert about 1,100–500 years ago to the igluvijait (snow houses) that allowed the Thule to thrive in the long bitter winters above the Arctic Circle a thousand years ago.

every year new findings show us that the early human history of the Americas was more complicated than we could have possibly imagined.

these mating events were between Neanderthal males and AMHS females.

Beringia should more properly be viewed as a lost continent than as a land bridge. The term land bridge gives the impression that people raced across a narrow isthmus to reach Alaska. The oceanographic data clearly show that during the LGM, the land bridge was twice the size of Texas. If the Out of Beringia model is correct, Beringia wasn’t a crossing point, but a homeland, a place where people lived for many generations, sheltering from an inhospitable climate and slowly evolving the genetic variation unique to their Native American descendants (10).

Despite being on different continents, 6,000 miles apart, the genomes of the Anzick-1 child, an ancient man from Spirit Cave in Nevada (10,700 years ago), and five people from the Lagoa Santa site in Brazil (~10,400 to 9,800 years ago) are very closely related to each other. The story that their DNA tells us is that between 15,000 and 13,000 years ago the ancestors of Central and South Americans diverged from populations in North America.

a small number of contemporary populations in the Amazon region—Surui, Karatiana, Xavante, Piapoco, Guarani—shared a small but significant number of alleles with contemporary Australasian populations, including Indigenous Australians, New Guineans, Papuans, and the Onge from the Andaman Islands (7).

Brutal colonization practices, as well as the diseases introduced by Europeans, were long thought to have completely eradicated the Taíno. However, genetic studies of contemporary peoples of Puerto Rico have shown the persistence of Indigenous mitochondrial DNA lineages, and Indigenous (Ceramic-related) ancestry persists in present-day Puerto Rican and Cuban individuals. This finding has been embraced by Indigenous Caribbeans, who celebrate the resilience of their ancestors (20).

There are currently several basic models for how, when, and where people first entered the Americas. The most conservative model resembles a new version of Clovis First: a migration of people belonging to the Diuktai culture in Siberia across the Bering Land Bridge sometime between 16,000 and 14,000 years ago, and south of the Ice Wall—probably down an ice-free corridor—after the LGM. This model is based predominantly on an emphasis of the early Alaskan archaeological record, but does not account for pre-Clovis sites or match the genetic record very well. The model favored by a small group of archaeologists on the other end of the spectrum is one in which people came to the Americas very early—some have proposed as early as 130,000 years ago. I (and most archaeologists) do not find this model very convincing, as it has little archaeological support and is completely at odds with what genetics shows us. The third model—one that I think best fits the totality of both the archaeological and genetic data—posits an entry into the Americas sometime after the first traces of people in Siberia at sites such as Yana around 30,000 years ago. Exactly how early one thinks this may have happened differs depending on which pre-Clovis sites are accepted; the majority of scholars agree that people were present in the Americas by at least 14,000 years ago. Some favor 18,000 to 15,000 years ago to account for the majority of pre-Clovis sites and genetic evidence, and still others argue for ...