Last month’s release of The Ice Bridge, an episode in the Canadian Broadcasting Corporation series The Nature of Things has once again revived public discussion of a controversial idea about how the Americas were peopled known as the “Solutrean hypothesis”. This idea suggests a European origin for the peoples who made the Clovis tools, the first recognized stone tool tradition in the Americas. As I was one of the experts appearing on the documentary, I want to share my thoughts about it and why I see the ideas portrayed within as unsettling, unwise, and scientifically implausible.
First, in addition to the scientific problems with the Solutrean hypothesis which I’ll discuss shortly, it’s important to note that it has overt political and cultural implications in denying that Native Americans are the only indigenous peoples of the continents. The notion that the ancestors of Native Americans were not the first or only people on the continent has great popularity among white nationalists, who see it as a means of denying Native Americans an ancestral claim on their land. Indeed, although this particular iteration is new, the idea behind the Solutrean hypothesis is part of a long tradition of Europeans trying to insert themselves into American prehistory; justifying colonialism by claiming that Native Americans were not capable of creating the diverse and sophisticated material culture of the Americas. Unfortunately, the producers of the documentary deliberately chose not to address this issue head-on, nor did they include any critical perspectives from indigenous peoples. While supporting the agenda of white nationalists was not the intent of the producers or of the scientists involved, it would have been appropriate for the documentary to take a stand against it, and I and many archaeologists are disappointed that they did not.
Bruce Bradley and Dennis Stanford, proponents of the Solutrean hypothesis, base it on the claim that the North American Clovis stone spear points are the technological descendants of a subset of those made by the Upper Paleolithic southwestern European Solutrean peoples. Specifically they cite fact that both are made by a technique known as “overshot” flaking as evidence for their common origin. From this starting point, Bradley and Stanford propose a hyperdiffusionist scenario in which a group of Solutreans migrated across the Atlantic Ocean to North America via an “ice bridge” approximately 20,000 years before present (YBP). Although they don’t deny that the majority of Native American ancestry comes from a group of Siberians who lived in Beringia during the Last Glacial Maximum (~23,000 YBP-13,000 YBP), they claim that “great numbers” of Solutreans must also have migrated to North America.
Solutrean tools 22000-17000 Crot du Charnier Solutre Pouilly Saone et Loire France Photograph: By World Imaging [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons
Archaeologists have taken a hard, long look at this idea and dismissed it on the basis of insufficient evidence. The mismatch between the archaeological record and the Solutrean hypothesis is so extensive that I can’t cover every problem, but here is a sample:
1. There’s a serious time gap between when the Solutreans could have crossed the Atlantic via the ice bridge (~20,000 YBP) and when Clovis tools begin to show up in the archaeological record (~13,000 YBP). This means that they would have made the points in exactly the same way for 7,000 years. Nowhere else in the Americas do we see technologies and cultures existing unchanging for that length of time.
2. There is no evidence of boat use, or tools used for making boats at Solutrean sites. Although the Ice Bridge documentary makes much of an image of a fish and an auk in a French cave, it is a bit of a stretch (to say the least!) to claim that this is sufficient to demonstrate a sophisticated seafaring culture, capable of crossing the Atlantic. The existence of a year-round “ice bridge” across the Atlantic during the Last Glacial Maximum is not supported by paleoclimate data. Instead, sea ice in the Atlantic would most likely have been seasonal, with a connection between North American and Europe only a few months out of the year.
3. The notion of overshot flaking technique as evidence of a link between Clovis and Solutrean has been challenged by many archaeologists, who think it far more plausible that the two cultures arrived at the same technology independently. As Strauss (2000) puts it, “One or two technical attributes are insufficient to establish a cultural link or long-distance interconnection.”
4. Radiocarbon dates of Clovis sites do not show a pattern one would expect if people diffused into North America from the east coast, as postulated by Stanford and Bradley.
Geneticists, too, have tested the Solutrean hypothesis. If it were true, we would expect to see ancestry from non-Siberian descended populations present in the genomes of ancient Native Americans. We don’t. All contemporary and ancient Native Americans, including the only known ancient individual buried in association with Clovis tools, show descent from an ancestral population with Siberian roots. There is a very clear pattern of evolutionary history recorded in ancient genomes from Siberia, Beringia, and North America, and no evidence for trans-Atlantic gene flow.
This is where the Ice Bridge documentary runs into great problems. It ignores all genomic evidence and instead relies upon an old idea that a particular mitochondrial haplogroup (a group of closely related maternal lineages) known as X shows a connection between North America and Europe. In the documentary, pediatrician/popular science writer Stephen Oppenheimer asserts that the presence of haplogroup X in an ancient North American population is a priori evidence for a European connection. The documentary makes this case persuasively with graphics and maps showing the presence of this haplogroup in both Europe and North America. But look below the surface and the entire argument falls apart. First of all, Standford, Bradley, and Oppenheimer simply assume that Solutreans would have had X because it’s seen in contemporary European populations. But in fact, the contemporary European gene pool was formed only within the last 8,000 years, and it’s unknown whether earlier peoples would have had haplogroup X in the same frequencies (or at all). No genomes from Solutren peoples have ever been sequenced, and you should always be cautious when a case is made for extending present day patterns of genetic variation into the past without direct confirmation from ancient DNA.
Today, lineages of haplogroup X are found widely dispersed throughout Europe, Asia, North Africa, and North America. We can reconstruct their evolutionary relationships – much like you can reconstruct a family tree – by looking at patterns of shared and derived mutations. Lineages found in the Americas, X2a and X2g, are not descended from the lineages (X2b, X2d, and X2c) found in Europe. Instead, they share a very ancient common ancestor from Eurasia, X2. (Here is a detailed discussion of the evolution of these haplogroups for anyone interested).
X2a is of a comparable age to other indigenous American haplogroups (A,B,C,D), which would not be true if it was derived from a separate migration from Europe. Finally, the oldest lineage of X2a found in the Americas was recovered from the Ancient One (also known as Kennewick Man), an ancient individual dating to ~9,000 years ago and from the West Coast (not the East Coast as would be predicted from the Solutrean hypothesis). His entire genome has been sequenced and shows that he has no ancestry from European sources. There is no conceivable scenario under which Kennewick Man could have inherited just his mitochondrial genome from Solutreans but the rest of his genome from Beringians. Thus, without additional evidence, there is nothing to justify the assumption that X2a must have evolved in Europe.
The Ice Bridge unfortunately relied on cherry-picking of data to support the ideas of Bradley and Stanford, and it’s not up to the standards of The Nature of Things. When I write about this issue, I frequently hear an argument along the lines of “Well, it could have happened, so maybe it did”. But science isn’t built on “could haves” and “maybes”. You must build your models based on evidence you have, not evidence you wish you had, and the Solutrean hypothesis is lacking sufficient evidence to be considered seriously.
References and further reading
Raff J, and Bolnick D. (2015) Does Mitochondrial Haplogroup X Indicate Ancient Trans-Atlantic Migration to the Americas? A Critical Re-Evaluation.
O’Brien, Michael J., Matthew T. Boulanger, Mark Collard, Briggs Buchanan, Lia Tarle, Lawrence G. Straus and Metin I. Eren (2014). “On thin ice: problems with Stanford and Bradley’s proposed Solutrean colonisation of North America”. Antiquity. 88: 606–624.
Stanford, Dennis J. & Bruce Bradley (2012). Across Atlantic Ice: The Origin of America’s Clovis Culture. Berkeley: University of California Press.
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