Home Archaeology Finding the First Americans (Part-I)

Finding the First Americans (Part-I)

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Finding the First Americans (Part-I)
A general view of excavations at the White Sands site in New Mexico

Archaeology and genetics can’t yet agree on when humans first arrived in the Americas.

Jennifer Raffis

The debate over how people first arrived in the Western Hemisphere continues to roil archaeology in the United States – and to capture public attention. Today, the scientific community is contending with significant amounts of new genetic and archaeological data, and it can be overwhelming and even contradictory. These data are coming from new archaeological excavations but also from the application of newly developed tools to re-analyze prior sites and artefacts. They’re coming from newly sequenced genomes from ancient peoples and their contemporary descendants, but also from re-analysis of prior sequence data using new modelling tools. The generation of new data at times feels as though it’s outpacing efforts to integrate it into coherent and testable models.

Did humans first populate the Americas 100,000 years ago, 30,000 years ago, 15,000 years ago, or 13,000 years ago? Did they come by boat or by an overland route? Were the ancestors of Native Americans from one population or several? The answers to these questions would help us understand the grand story of human evolution. We know that the Americas were the last continents that anatomically modern Homo sapiens – humans like us – entered, but we don’t know exactly how this happened. These long-ago movements give us hints about the challenges ancient peoples across the world had to contend with during the Last Glacial Maximum (LGM), a prolonged period of coldness and aridity, when animals, plants and humans retreated to environmental ‘refugia’ for several thousand years. How did we survive this Ice Age? What technological and biological adaptations arose as the result of these environmental conditions? These questions capture the popular imagination and challenge the scientists working to uncover the details of individual lives thousands of years in the past.

To their Indigenous descendants, the stories we tell about these First Peoples of the Americas are highly relevant for additional reasons. Their deep ties and claims to the lands have often been ignored or expunged by governments, media and corporations across North and South America in order to make room for narratives that are more palatable, exciting or convenient to certain non-Native groups. The historical exclusion of Indigenous peoples from making decisions about research on their own ancestors and lands has caused significant harms to Native communities and individuals; when Native scientists and community members are full participants in the research process, the stories that emerge are not only more respectful but also more accurate.

Archaeological evidence establishes that Indigenous peoples were present in the Americas at least 15,000 years ago. Scientists don’t agree, however, on when people first arrived. Some archaeologists claim it must have been much, much farther back, citing evidence such as flaked stones in layers dating to ~30,000 years ago at the Chiquihuite Cave site in Mexico, bones with cut marks in layers dating to 34,000 years ago in Uruguay, flaked stones in layers dating to 30,000-50,000 years ago in Brazil, and even broken mastodon bones dating to 130,000 years ago in California. All of these claims are heavily disputed.

As a rule, an archaeological site won’t gain widespread acceptance as legitimate unless there is clear evidence of human activity, that evidence can be securely dated, and it is found in an undisturbed geological context. For example, a hearth containing the remains of charred animal bone fragments and stone tool fragments at the Dry Creek site in Eastern Beringia (near the present-day Denali National Park in Alaska) was dated to 13,485-13,365 years ago from wood charcoal pieces taken from within the hearth. The stone tools – sharpened blades, flakes, end scrapers, and the byproducts of manufacturing them – and repeated controlled fires used to cook animal bones clearly indicate a human presence. The intact stratigraphy and multiple independent radiocarbon dates from the hearth tell us when people were using this particular part of the site. To archaeologists, this is uncontroversial. In contrast to the Dry Creek site, there is no consensus that the very early sites discussed above have met that standard; critics argue that the stone ‘artefacts’ and ‘butchering’ marks could be the result of natural phenomena (or even, in some cases, left by modern construction equipment). There simply hasn’t been any uncontroversial physical evidence of a human presence in the Americas more than 15,500 years ago.

Insert-Tracks-at-site-2
Footprints from Site 2 at White Sands

As they walked along the muddy surface, their feet mushed tiny seeds of ditch grass into the ground

Then, in 2021, a team of archaeologists dropped a bombshell into this debate: they’d found footprints – unquestionable evidence of a human presence – at White Sands National Park in New Mexico, dating to between 23,000-21,000 years ago.

The White Sands Locality 2 site was once the shore of an ancient lake. For more than 2,000 years, humans and animals visited it. As they walked along the muddy surface, their feet mushed tiny seeds of ditch grass into the ground, leaving a vital organic trace that archaeologists can use for carbon dating. (Some archaeologists have criticized the dating methods used, but there is general agreement that the presence of human tracks with fauna known to have gone extinct around 11,000 years ago dates these to – at minimum – the end of the Pleistocene.) If the find holds up to scrutiny, physical evidence of a human presence in the Americas during the LGM would be a paradigm-changing event, pushing back the date of the earliest migrations to sometime before 25,000 years ago.

When European settlers and explorers first encountered the Native peoples of the Americas, they sought to force the fact of the Native people’s existence into a Biblical worldview. The First Peoples, who built the impressive earthworks, monuments, temples and pyramids throughout the Americas, were recast as members of a lost tribe of Israelites, Irish sailors, or possibly Vikings, for the ideological convenience of settlers.

The pretense that the first peoples of the Americas were a different race than Native Americans – a view known today as the Myth of the Mound builders – became extremely popular in the 18th and 19th centuries. Andrew Jackson explicitly used it in 1830 to justify the brutal Indian Removal Act:

In the monuments and fortresses of an unknown people, spread over the extensive regions of the West, we behold the memorials of a once powerful race, which was exterminated or has disappeared to make room for the existing savage tribes.

As the field of archaeology matured and incorporated the scientific method, scholars began to reject the Moundbuilder Myth. By the end of the 19th century, the US government funded an investigation of mounds throughout North America to identify their creators. The evidence persuaded researchers that the mounds were built by the ancestors of contemporary Indigenous Americans, not some mysterious, lost race. The resulting Twelfth Annual Report of the Bureau of Ethnology to the Secretary of the Smithsonian Institution (1890-91) marked a new era in archaeology. In time, the archaeological, cultural and biological evidence all pointed to shared ancestry with Asians, suggesting that the ancestors of Native Americans came to the continents via a land bridge between Siberia and Alaska.

The question of when did this migration begin remained. Poorly understood geological and cultural chronologies made it a difficult matter to address. Radiometric dating methods were not invented until 1946, and strong rivalries between scientists promoting their own models confused the issue.

In the Clovis First model, hunter-gatherers travelled from Siberia to North America across the Bering Land Bridge

The 1927 discovery of a human-made projectile (spear point) at the Folsom site in New Mexico proved a turning point. The spear point was associated with the remains of a bison that had gone extinct at the end of the Pleistocene (around 11,700 years ago). Even without radiometric dating methods, researchers knew that a human artefact embedded in an animal that went extinct long ago had pushed back the date at which humans were living in the Americas. It marked another important shift in the study of human origins in the Western Hemisphere. Archaeologists used such artefacts to begin piecing together a bigger historical narrative.

The model that emerged and dominated the field for decades has come to be called ‘Clovis First’. It is named after the technologies first found in 1929 at a Pleistocene kill site near the town of Clovis in New Mexico. Characterized by a thin, lance-shaped projectile point flaked on both sides with a single, long flake removed at its base on each side (known as a ‘flute’), the Clovis point is unique to North America. It appeared widely across the continent beginning about 13,400 years ago, near the end of the LGM.

Because of their sudden appearance and spread, and in the absence of recognized earlier sites, many archaeologists believed that the Clovis tools were evidence of the very earliest inhabitants of the Americas. The Clovis First model held that a small group of hunter-gatherers travelled from Siberia to North America across the Bering Land Bridge. They then followed southward an ice-free corridor that warming temperatures had opened along the eastern Canadian Rocky Mountains. The population dispersed rapidly across the continents, leaving their newly invented projectile points embedded in their prey, or cached in special locations, along the way.

The Clovis First model faced challenges from growing evidence of an earlier human presence in the Americas. Some of it was archaeological; signs of a human presence at the Monte Verde site in Chile dated to more than 1,000 years before the first appearance of Clovis. As the science of genetics matured, it added a critical source of evidence against the Clovis First model. Sequencing of DNA, and assuming that DNA bases mutate at a known and constant rate, allowed geneticists to estimate when different populations last shared a common ancestor. Depending on which mutation rate was used, the genetic evidence suggested a last common ancestor for Native Americans anywhere from 15,000 to 30,000 years ago, or 2,000 to 17,000 years earlier than accounted for by the Clovis First model.

This is not all that’s revealed by genetic studies of mitochondrial DNA from ancient and contemporary Native Americans. They also showed that the ancestors of the First Peoples had undergone a period of isolation before their lineages diversified. The molecular clock – a dating method based on the rate at which DNA mutates – tied this isolation period to roughly the height of the LGM, beginning about 21,000-20,000 years ago. This is when climate conditions across the globe forced people and animals to retreat into refugia to escape the encroaching ice. When, 17,000 years ago, a potential coastal route in the Americas opened as the ice sheet retreated from the Pacific, lineages began to diversify more rapidly as they spread across geographic space and encountered each other less frequently to spread newly arising genetic variants.

In the last generation, geneticists have integrated their data with archaeological and climatic evidence to produce a three-stage model for the peopling of the Americas. First, approximately 20,000 years ago, a group of Asians migrated into Beringia. They remained isolated there for thousands of years, during which they evolved the founding lineages seen across the Americas. Then about 17,000-16,000 years ago they migrated out of Beringia into the Americas, rapidly peopling the continents. Still later, migrations from Beringian populations then peopled the Arctic. The genetic variation present in Indigenous peoples of the Americas was the result of local evolutionary processes with no gene flow from populations outside the continents. This mitochondrial DNA-based model has come to be known as the ‘Beringian standstill’ or ‘Beringian incubation’ hypothesis.

Native Americans today can trace their genomes to Ancient North Eurasians and an ancient East Asian group

The genetic evidence for the Beringian standstill hypothesis was based on certain types of DNA – like mitochondrial DNA and Y-chromosome DNA – that are inherited from just one parent. These are relatively easy to extract, work on and understand, but they have limitations. The full human genome is massive, with 3 billion base pairs and some 20,000 genes. Each stretch of DNA has its own history of evolution, and each may closely resemble that of the parent population or differ from it in important ways. The most accurate histories that can be reconstructed from DNA come from the study of the entire genome, not single genes.

Getting complete genomes from ancient individuals is difficult. After an organism dies, the cellular processes responsible for repairing damage to DNA cease to work. Over time, an organism’s DNA fragments and accumulates damage until at some point it is no longer possible to recover; exactly how quickly this occurs depends on a combination of environmental conditions and time. In 2010, Danish and Chinese scientists led a study that mapped the first complete genome of an ancient human, from a Palaeo-Inuit Saqqaq man who lived 4,000 years ago in Greenland. This work from Morten Rasmussen, Yingrui Li and Stinus Lindgreen brought the ‘palaeogenomics revolution’ to the Western Hemisphere and transformed our understanding of human history across the continents.

Complete genomes from ancient humans in the Americas allowed researchers to model biological histories on a scale never imagined before. Because each nuclear genome reflects the contribution of thousands of ancestors, entire population histories can be reconstructed from just a few individual genomes. Genomes from the past can often reveal details of human biological history that were obscured by later demographic events (such as population migrations). While models generated from genomic data can’t give insights into all aspects of human history (particularly issues pertaining to cultural identity), they nevertheless provide a powerful way of understanding biological relationships through time.

The sequencing of ancient genomes in Siberia brought one of the most important insights into Native American population history. The first evidence of humans living above the Arctic Circle comes from the Yana Rhinoceros Horn site in northeastern Siberia, where year-round settlements of humans have been dated to 31,600 years ago. From DNA retrieved from the baby teeth of two young children growing up at the site, geneticists were able to piece together a picture of a large Upper Palaeolithic population ancestral to the Ancient North Siberians, a group identified from the genome of a child buried at the Mal’ta site near Lake Baikal 24,000 years ago. The Ancient North Siberians were ancestral to Siberians, Central Asians and Europeans. The genome from the Mal’ta child revealed to palaeogeneticists that this group of Ancient North Siberians contributed some ancestry to the Beringian population who gave rise to the First Peoples. Native Americans today can trace between 14-38 per cent of their genomes to the Ancient North Eurasians, with the rest from an ancient East Asian group, likely from China. This gene-flow event between the First Peoples’ ancestors took place sometime between about 23,000-18,000 years ago.

Population models based on ancient genomes confirmed the Beringian standstill hypothesis. This means that the population’s isolation coincided with the height of the LGM, approximately 23,000-19,000 years ago, when sea levels would have been approximately 100 meters lower than today, and water bound in continental ice sheets. Across Siberia, as in other parts of the Northern Hemisphere, conditions would have been extremely dry and cold. In fact, the archaeological record indicates that Siberia was essentially depopulated. Genomes cannot tell us where the ancestors of Native Americans were isolated but they do tell us that they were isolated. Many geneticists believe that, because they were isolated, it’s unlikely they were living in East Asia during the LGM – there were other groups in the region, and proximity would likely have resulted in gene flow. The southern coast of the Bering Land Bridge in central Beringia is a likely candidate for a refugium, as palaeoclimactic reconstructions show us that it had a relatively mild climate, with abundant plant and animal resources. Since what was the Bering Land Bridge now lies mostly under the Chukchi and Bering Seas, it is difficult to test this hypothesis. Some archaeologists are investigating potential LGM-period sites with hints of human presence at Bluefish Caves in the Yukon, or at Lake E5 in the Brooks Range of Alaska. So far, no definitive evidence of a human presence during the LGM has been found in western Beringia.

Since we cannot tie the population ancestral to the First Peoples to any specific technological or cultural manifestation in the archaeological record, their location during the LGM between 23,000 and about 16,000 years ago remains a mystery for now.

One of the more astonishing insights palaeogenomes have given us is that the population ancestral to Native Americans split into several branches during its isolation between approximately 24,000 and 16,000 years ago. Geneticists refer to one of the branches as the ‘Ancient Beringians’. The Ancient Beringians were identified through the genomes from children buried 11,500 years ago at the Upward Sun River site in the Tanana River Valley in Alaska and from a young girl buried 9,000 years ago at the Trail Creek Cave site on the Seward Peninsula. These genomes suggest that the Ancient Beringian population was widespread across what is now Alaska, but also limited to Alaska. It did not persist (genetically) into the present day; Indigenous Arctic populations do not seem to have Ancient Beringian ancestry. We may eventually find people with Ancient Beringian ancestry as we characterize more genetic variation in present-day and ancient people, or it may be that this population ultimately diminished without leaving any descendants.

Another branch emerged approximately 24,700 years ago. ‘Unsampled Population A’ was identified indirectly from its contribution to the genomes of the ancestors of contemporary Mesoamericans and South Americans.

The First Peoples’ migration was mainly by boat along the west coast of Alaska, rather than on foot

The third known branch that emerged during the isolation period is known as ‘Ancestral Native Americans’. It is ancestral to all populations south of Alaska. Its dispersal southward throughout the Americas, most likely down the Pacific coast, saw a series of population splits. The earliest of these populations, identified by the genome of an individual from the Big Bar Lake site in British Columbia, split from the other Ancestral Native Americans as early as 21,000 years ago. The other two groups, known as ‘Northern Native Americans’ (in Alaska and northwestern Canada) and ‘Southern Native Americans’ (south of Alaska and Canada), diverged from each other sometime after 17,000 years ago as they moved south. (Continues) 

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Jennifer Raffis associate professor in anthropology and affiliate faculty member in the Indigenous Studies Program in the College of Liberal Arts and Sciences at the University of Kansas. She is the author of Origin: A Genetic History of the Americas (2022).

Courtesy: AEON (Received through email) 

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