Archaeology and genetics can’t yet agree on when humans first arrived in the Americas.
Jennifer Raffis
Southern Native American populations dispersed throughout the Americas extremely quickly. We know this from the rapid divergence of lineages in ancient members of these groups. Because the speed and timing of their divergence is earlier than the most likely date for the opening of an interior route through the glacial ice (the Ice-Free Corridor along the Eastern Canadian Rockies), the model suggests that the First Peoples’ migration was mainly by boat along the west coast of Alaska, rather than on foot. In fact, the earliest archaeological evidence of people in the Ice-Free Corridor dates to shortly after 13,000 years ago, and indicates they were moving northward, rather than southward.
Following the initial dispersal sometime after about 21,000 years ago, the next 20,000 years brought a complicated series of population movements throughout South and North America. Glaciers that had been preventing eastward movement across the North American Arctic melted, and two successive migrations then peopled the eastern half of the continent. The first was by Palaeo-Inuit around 5,000 years ago, and the second by the ancestors of contemporary Inuit peoples between about 1,000 and 750 years ago. These two groups were culturally very different, but shared ancestry with Siberian populations, and at least the direct ancestors of living Indigenous Arctic peoples shared gene flow with members of the Northern Native American group. Population geneticists are still trying to sort out the complicated population histories of different Indigenous groups within the Arctic from both contemporary and ancestral genomes.
On a Saturday afternoon in April 2022, I sat in a packed hotel conference room in Chicago at the annual meeting of the Society for American Archaeology. It’s not a conference I usually attend, but I’d been asked to serve as a discussant in a session presenting the results from the White Sands site in New Mexico. I had decided in advance that my presentation would focus on trying to reconcile the genetics and archaeological records.
As I sat squeezed in between Dennis O’Rourke, the anthropological geneticist who was serving as the other discussant, and E James Dixon, an eminent archaeologist, I was excited to hear about the excavation of the White Sands site directly from the archaeologists who had studied the footprints up close. To a rapt audience of archaeologists – which included many sceptics of its purported 23,000-21,000-year dates – Vance Holliday, Thomas Urban, Clare Connelly, David Bustos, Amber Kalush, Matthew Bennett and Daniel Odess discussed their excavation of the site, how they obtained dates, how they worked with tribal partners, the future of the site, and the implications of the dates in the context of current models for the peopling of the Americas.
They had a formidable task; White Sands presents a radical challenge to our understanding of the earliest humans in the continents. If the 23,000-21,000-year dates are accurate, it would mean that people had to have made their way into the continents before the ice sheets fused, by 25,000 years ago or earlier. What could account for the huge time gap between the White Sands site and the archaeological and genetic evidence showing a migration after 17,000 years ago?
The seeds embedded in the footprints were not from plants growing in water but on the lakeshore
I recognized several people in the audience who had been shocked and skeptical when the dates had been published a few months prior. ‘They just can’t be right,’ an archaeologist told me at the time. There are some legitimate concerns about the site, which were raised during the question period of the symposium and in subsequent publications. These objections are not, as at other early sites, that the evidence for humans was ambiguous. There could be no question the footprints were human, and their immediate proximity to mastodons and giant sloths meant that they were unquestionably Late Pleistocene. Rather, the major concern raised primarily by geoarchaeologists is about the dating of the Ruppia cirrhosa seeds found embedded in the footprints.
Ruppia (ditch grass) can be influenced by the so-called hard-water effect in which plants growing in water take up old carbon present in it, making samples appear hundreds to thousands of years older than they actually are. Skeptical archaeologists believe that the hard-water effect may be biasing the carbon-14 dates, and that additional dating – ideally of features with other carbon sources – is needed to confirm the ages of the site.
Another concern, raised by the archaeologist C Vance Haynes of the University of Arizona, concerns the stratigraphic integrity of the sites. Specifically, he raised the possibility that the Ruppia seeds might not be from the same geological layers as the footprints. Rather, he suggests, the seeds might have been redeposited by wind from older strata elsewhere at the site into younger contexts. The apparent co-occurrence of human and Late Pleistocene animals at the site might be due to humans crossing over extinct animal trackways thousands of years later.
If this scenario is correct, additional dating from other organic sources (besides Ruppia seeds) should eventually demonstrate that the geological layers that contain the footprints date to after the LGM, perhaps to the Clovis period, which begins around 13,000 years ago.
But during the symposium and in a subsequent publication led by Jeffrey S Pigati of the US Geological Survey, the archaeologists who excavated White Sands robustly responded to these concerns. The hard-water effect was not biasing the dates, they explained, because the seeds were not from plants growing in water, but rather on the lakeshore, and because all dates were in good stratigraphic order. There was no possibility that the human footprints postdated the Late Pleistocene animals’ footprints because in many cases they were in layers underneath (and therefore geologically older than) animal trackways.
To each objection or concern, they have given a patient and convincing answer. And yet the debate rages on.
However frustrating this debate may be for all participants – and I imagine it can be very frustrating at times – to me it is an excellent example of the dynamic process of the scientific method. Claims are advanced, they are robustly criticized, and additional evidence is brought forward to refute the critiques. Bit by bit, the field advances in its understanding.
I sincerely hope that the public understands this debate, and indeed the broader debate about details regarding the peopling of the Americas, in this light. Unfortunately, because science is all too often taught in school as a collection of facts, rather than a dynamic process of enquiry, people can be vulnerable to being misled by opportunists such as Graham Hancock. A self-described investigative journalist, Hancock has made a career of weaving metaphysical stories about the past for audiences eager to believe that there is a conspiracy among thousands of archaeologists, geologists, palaeoclimatologists and geneticists like myself to suppress the ‘truth’ about the past. In his Netflix series Ancient Apocalypse (2022), Hancock fantasizes about imaginary ‘alternative’ theories of the past, including the Americas, telling audiences they can acquire suppressed knowledge without the tedium of actually engaging with evidence.
But, as we have seen, debate about population history models is a normal part of the scientific process. Contrary to claims made about us, scientists emphatically don’t want dogma to stifle the consideration of new ideas. In contrast to ‘alternative historians’, scientists demand a rigorous evaluation of evidence in considering these ideas.
In contrast to metaphysical storytelling, we have testable hypotheses
There is certainly robust debate about the peopling of the Americas. White Sands, along with other pre-LGM sites, provides a good example of how this debate progresses. For most sceptical archaeologists, the attitude to the claims of the White Sands findings is not hostility, but a cautious ‘let’s wait and see’. It may be that, just as the Folsom and Clovis sites showed archaeologists where to look for evidence of Late Pleistocene humans associated with the remains of extinct animals, White Sands may show us where to look for even earlier peoples. Or it may be that additional dates will eventually show us that White Sands is younger than the LGM.
The important thing is that, in contrast to metaphysical storytelling, we have testable hypotheses. We know what evidence is needed to test these hypotheses, and archaeologists are out in the field right now gathering that evidence.
Outstanding questions
One of the limitations of our current understanding of biological population histories in the Americas is that they are based on very few ancient genomes. Our sampling of genetic diversity across the Americas is very incomplete. The reasons for this are in part historical. Non-Native scientists inherit a legacy of insensitive and exploitative research from our forebears, one that has left Indigenous peoples with little incentive to trust us with the remains of their ancestors. If we wish this to change, we have a great deal of work to do and many factors to consider. There are ongoing discussions about what constitutes appropriate ethical practices in our field, including who is doing the research, who is interpreting the results, and who has the right to determine what is done with the data that result from it. The histories we infer from archaeology and genetics are not abstract to contemporary Native Americans, who have their own scientific and historical knowledge stretching back for countless generations; how we tell the stories that emerge from our genetics data are of critical importance to them. The acquisition of DNA from ancient individuals usually requires the destruction of small portions of bone or teeth; this may be incompatible with a community’s values for how ancestors should be treated.
Any palaeogenomics project must contend with these issues, and that often takes a great deal of time. The slow nature of the consultation process – building relationships, respecting ‘no’ from communities who refuse research on their ancestors, co-designing a project with community partners from those who wish to do so – is at odds with the fast-paced, intensely competitive research environment characteristic of palaeogenomics. But if we wish to do research in a better way, this process cannot be rushed or circumvented in the pursuit of data.
Our limited sampling of ancient genomes may mean that there is still more genetic variation than accounted for in our models; we may yet have biases in our estimated dates. In addition, there are puzzling findings that need additional genetic data to resolve.
A trans-Pacific migration from Southeast Asia does not in fact comport with the genetic evidence
One major unsolved mystery that whole genomes revealed is the shared ancestry between some South American and Australasian populations (from Australia, Melanesia and Southeast Asia). Geneticists refer to this ancestry as from the ‘Ypikuéra population’ or ‘Population Y’. Population Y is seen scattered inconsistently in genomes throughout the Amazonian and Pacific coastal regions; it has been found in South American genomes as early as 10,000 years ago.
What could account for this pattern? It’s one of the biggest mysteries currently in the field. A trans-Pacific migration from Southeast Asia that seems to offer an easy explanation does not in fact comport with the genetic evidence. Such a migration would leave a very different pattern of genomic footprints; Population Y ancestry is too old, too scarce, and too inconsistent to be explained by this model. Instead, we can trace the Population Y ancestry, tentatively, to East Asia; a 40,000-year-old individual from Tianyuan Cave in China carries its genetic signature. Most likely he represents a population no longer present in the region, which gave rise to both the ancestors of Australasians and Population Y.
So how did Population Y get into the Americas? Given current data, two scenarios suggest themselves. First, Population Y may have been present in the isolated group that gave rise to the different branches of the First Peoples during the LGM. It’s easy to imagine a scenario of a geographically dispersed metapopulation, consisting of multiple groups living in different refugia across Beringia, containing some families with this ancestry that simply didn’t get widely shared due to limited contact. If these different groups entered the Americas separately after 17,000 years ago, Population Y ancestry may have been limited to certain descendant populations.
Another possibility ties together evidence of Population Y and potential archaeological evidence at sites like White Sands. Could there have been multiple migrations into the Americas, with one pre-LGM migration consisting of Population Y individuals (and perhaps Unsampled Population A or other groups we have yet to identify), and one post-LGM migration? This would reconcile the archaeological evidence of early traces of humans in the Americas (if they are indeed legitimate, which has yet to be demonstrated to everyone’s satisfaction) and the genetic data.
Archaeologists skeptical of White Sands’ early dates are dubious of the second scenario, and it is admittedly speculative. We need a great deal more data, both genomic and archaeological, to test it. But, as a field, we are actively engaged in collecting that data, even as I type these words.
We may never know exactly how the White Sands people fit into the biological history of the Americas. But to members of the Pueblo of Acoma, near the White Sands site, the identity of these individuals is not at all mysterious. ‘Thousands and thousands of years ago, our ancestors walked this place,’ said Kim Charlie, a member of the Pueblo of Acoma, in an interview with Lizzie Wade for Science magazine in 2021. Their language even has words for the extinct megafauna seen at the site. In an example of how the discipline of archaeology is evolving for the better, the Pueblo of Acoma is actively involved in the recovery and study of these ancient footprints.
The addition of more ancient genomes from Indigenous populations across time and space will provide fascinating details about the lives, choices and movements of the earliest peoples of the Americas. We already see this occurring, with recently sequenced genomes from Brazil giving insights into migrations through South America. I am excited to see what the future reveals about the past. But I will venture to predict that, whatever my field does reveal, it will ultimately only affirm what Indigenous peoples already know to be true: they have been here since time immemorial. (Concludes)
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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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