Responses to some questions about our recently published paper on haplogroup X and North American prehistory

Just a few days ago, my co-author Deborah Bolnick and I published a paper in the journal PaleoAmerica on the subject of haplogroup X and Native American population history. Rather than writing a blog summary of it (which has been my usual approach for publications), we decided to try something different: make the paper itself open access and respond here to a few questions about it that we’ve seen from a variety of sources.  We hope that this approach will be helpful to interested readers!

What is the paper about?

We reviewed existing genetic data to answer the question: Could mitochondrial haplogroup X2a have been brought to the Americas by an ancient trans-Atlantic migration? This is a rather old question from the perspective of anthropological geneticists, but we’ve seen it appear in both academic publications and documentaries rather frequently. We thought it was worth revisiting in light of recent genetic publications.

Quite simply, we found that mitochondrial and genomic data do not support this migration hypothesis as the most plausible explanation for X2a’s presence in North America. Instead, the most parsimonious interpretation of the genetic data continues to be that haplogroup X2a had the same migration history and ancestry as the other founder Native American mitochondrial lineages (i.e., from Siberia). Based on the current evidence, we feel that there is no need to invoke a distinct origin for individuals bearing this lineage.

If’ you’d like another summary of the paper, Andy White wrote a very good blog post about it here.

Where can I get a copy?

We’ve sent a request to the publisher to make the paper open-access, and are waiting for them to process the request. We’ll update this post with a link as soon as the process is finalized. In the meantime, you can email us for a copy or find it on Jennifer Raff’s page

How can you say that this proves once and for all that all Native Americans have exclusively Beringian ancestry when you haven’t sequenced all of them? Isn’t that unscientific?

We don’t say that. This work presents our best interpretation of all the genetic evidence currently available that are relevant to this question. In fact, we end the paper saying:

It is of course possible that genetic evidence of an ancient trans-Atlantic migration event simply has not been found yet. Should credible evidence of direct gene flow from an ancient Solutrean (or Middle Eastern) population be found within ancient Native American genomes, it would require the field to reassess the “Beringian only” model of prehistoric Native American migration. However, no such evidence has been found, and the Beringian migration model remains the best interpretation of the genetic, archaeological, and paleoclimate data to date.

We don’t think it’s likely that new evidence will suddenly crop up showing another source of ancestry for Native Americans, but it remains a formal, albeit remote, possibility. Should such evidence be found, it will require us to reexamine our models. But we can’t incorporate hypothetical results into our interpretations. That would be unscientific.

Doesn’t skeletal data contradict the Beringian hypothesis? What about the very early Paleoindians whose skulls look physically different from later and contemporary Native Americans? Aren’t they proof that Native Americans have European ancestry?

The skeletal data show changes over time in the cranial morphology of ancient Native American populations. It’s true that comparisons of skull shapes were, for a very long time, how anthropologists studied genetic relationships between populations. However, over the last few decades, we’ve developed the technology to assess biological relationships between individuals and populations by comparing genomes. It’s generally acknowledged that this is a more precise, direct means of assessing ancestry than morphology, which can be influenced by environmental, developmental, and cultural factors.

Furthermore, the genomes of several of the Paleoindians with differently shaped crania have been examined, and they show no evidence of different ancestry than later or contemporary Native Americans. For example, Kennewick Man, who we discuss in the paper, exhibited what some have described as “Caucasoid” cranial features. However, his overall genetic affinities group him with Siberians/East Asians, not Europeans. And his mitochondrial haplogroup is the most basal lineage of X2a so far observed. This result shows that X2a—and this Paleoindian cranial morphology—are compatible with Siberian ancestry.

Why the skulls of the earliest inhabitants of the Americas look different from the later indigenous inhabitants is a very interesting question. We suspect it has to do with evolutionary forces like selection or drift acting on morphology over millennia. Current genomic research just doesn’t show evidence that they had different ancestry from later Native American groups.

Isn’t it pretty well proven that Clovis technologies are descended from Solutrean technologies?

No. The majority of archaeologists think that the similarities between the Clovis and Solutrean points are either spurious or coincidental. Very, very few archaeologists interpret the data as supporting the Solutrean hypothesis. We don’t see the genetic evidence as supporting the Solutrean hypothesis either.

Archaeologists were wrong about the “Clovis First” hypothesis, so doesn’t that mean that you’re wrong too?

Why? These are two separate models. The model of Beringian genetic ancestry of Native Americans is not dependent on the Clovis First hypothesis; in fact, the same evidence from which the “Beringian Pause” model was developed—early coalescence dates of mitochondrial lineages and ancient DNA data—was an important component in overturning the Clovis First model.

In science, any hypothesis is falsifiable, and any model is provisional pending contradictory data. The overturning of the Clovis First model is a great example of the process working as it should.

Isn’t it unfair to critique the Solutrean hypothesis before it’s been fully “fleshed out?” There’s bound to be more data supporting it soon!

Any hypothesis is open to testing, otherwise it’s not scientific. And there’s no “waiting period” to protect a hypothesis until it’s gathered enough data to make it immune to criticism. This argument is a species of special pleading—no other idea in archaeology is treated this way.

What about the signal of “West Eurasian” ancestry seen in Native American genomes? Does it support a trans-Atlantic migration?

This finding has led to a lot of confusion among non-geneticists, and we address it in some detail in the paper. To summarize: Raghavan et al. (2014) and Rasmussen et al. (2014) studied the genomes of the Siberian Mal’ta individual and the Anzick-1 individual, respectively, and they found that a portion of their ancestry (between 14-38%) was derived from a population that also contributed alleles to the contemporary inhabitants of West Eurasia. Notably, the contemporary European gene pool appears to have emerged quite recently—within the last 8,000 years—as a result of significant migration and admixture events. We don’t know what the genomes of Solutrean peoples looked like, since none have been sequenced yet, but from these findings we predict that they would more closely resemble pre-Neolithic hunter-gatherers than contemporary Europeans [see Allentoft et al. 2015, Haak et al. 2015, and Lazaridis et al. 2014]. Importantly, based on the pre-Neolithic genomes that have been studied, it appears that these early European hunter-gatherers did not exhibit close genetic affinities to Native Americans.

Several studies have also formally tested the evolutionary relationships between Native American genomes and genomes from ancient and contemporary populations worldwide (see Rasmussen et al. 2015, Raghavan et al. 2015, and Lazardis et al. 2014). These studies have consistently showed that the model which best fits the current genetic data did not match the predictions of the Solutrean hypothesis. We discussed this in the paper, noting that the most supported model:

was one in which the population ancestral to Native Americans was derived from ancient North Eurasian and East Asian sources, while contemporary Europeans were derived from ancient North Eurasian and West Eurasian sources. In other words, gene flow was from the ancestral North Eurasian population into both the ancestral Native American and ancestral European populations. Lazaridis et al. (2014) did not find any evidence of Pleistocene gene flow directly from West Eurasians into Native Americans. Their model is also consistent with other studies, which have shown that 62-86% of Native American ancestry derives from East Asia.

We’ll update this FAQ with the answers to more questions as they arise, so do check back. If you  have any questions, feel free to leave them in the comments section and we’ll try to get to them as soon as our schedules allow.

Ancient DNA from two 11,500 year old burials in Alaska

Today I and my collaborators have a new paper published in PNAS!  Justin Tackney, the lead author of the paper, was kind enough to write up a summary of the findings to publish here on Violent Metaphors. Here is his take:

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What an ancient Paleoindian girl tells us about Native American prehistory

Photo by Paul Nicklen/National Geographic from



More than 12,000 years ago, a young teenage girl walking through a deep cave (known today as Hoyo Negro) fell down a massive pit. The fall fractured her pelvis, and she died among the remains of giant ground sloths and saber-toothed cats who had met a similar fate. Over the next few millennia, the pit filled with water and their bones were covered with cave formations. They were left undisturbed until discovered in 2007 by cave divers, who named the girl “Naia” in a reference to Greek mythology.

Today, a team of archaeologists and geneticists announced the results from sequencing her mitochondrial genome. She possessed a haplogroup (D1) that evolved in Beringia and is seen in modern Native Americans.

So why is this result so important? The Hoyo Negro girl, like other Paleoindians (the oldest inhabitants of the Americas), had a skull shape that was distinctive from later (younger than 9,000 years before present) ancient Americans, who more closely resembled modern Native Americans. Continue reading

Problematic science journalism: Native American ancestry and the Solutrean hypothesis

This is the second post in a series discussing the recent publication of a 12,500 year old genome from Montana. You can find the first post here.

In the weeks following the publication of the complete genome from a Clovis child, there’s been a lot of press coverage of this study and its possible implications. I want to discuss a bit of the media coverage on this subject, since it raises issues that I think science journalists need to consider more carefully.

First of all, to recap the major findings of the original study (discussed in more detail at the link above):
1. Anzick-1, the 12,500 year old Clovis child whose genome Rasmussen and colleagues sequenced, is very closely related to living and ancient Native Americans.
2. Anzick-1 is more closely related to Siberians than other Eurasian groups.
3. Anzick-1 is more closely related to Central and South American Native American groups than to some North American groups.
4. The results from Anzick-1’s genome fit with the scientific consensus about the peopling of the Americas. This consensus encompasses the results of decades of archaeological, genetic, and paleoclimate research.

Unfortunately, several press reports chose to find controversy in a decidedly non-controversial story by giving undue weight to problematic “alternative” explanations of Native American origins, including the Solutrean hypothesis, and other “European contributions” to Native American ancestry.

Clovis tools from the Anzick site. From Rasmussen et al. 2014.

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The oldest North American genome and what it tells us about the peopling of the Americas

Last Wednesday, Dr. Morton Rasmussen of the Natural History Museum of Denmark and his colleagues announced that they had completely sequenced the genome of an infant boy, buried ~12,600 years ago in Montana. A few weeks earlier, I’d been approached by an editor at Nature, who asked me if I and my mentor Deborah Bolnick would be interested in writing a companion paper that would analyze and contextualize their results. We agreed, and the paper was published in last week’s issue, alongside Rasmussen et al.’s work. Because it’s (unfortunately) behind a paywall, I’d like to summarize what we said in that paper for non-scientists. There are a lot of things to talk about with regard to this study, including a consideration of ethical issues and the media’s response, so I’m likely going to do several posts on it. This first post is mainly a discussion of how we interpret the results.

For a TL;DR version of this post, here’s a link to a short interview I did on the subject last week with the BBC World Service.
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Nature publication

I haven’t been writing as much here recently, because I’ve been working on a “News and Views” article for Nature….and now I can finally talk about it! Here’s a link to my article:, and to the paper that it’s discussing: In the next few days I’ll post something here to discuss the main points of the article (for those of you who can’t access it), and also my reaction to the media coverage that the study is getting.

How to tell if an ancient DNA study is legitimate

It seems that every week there are exciting new findings from ancient DNA research.  This is wonderful news, because we’re learning incredible things about the relationship between humans and Neandertals, the prehistory of ancient populations, and even previously unknown hominins.  But on the flip side, we’re also seeing news reports of extremely questionable results, and I’ve gotten more than one inquiry recently from people excited or confused by them. I though it would be a good idea to write a bit about how regular people can figure out whether a study is legitimate or not.

The first step in distinguishing good ancient DNA studies from bad ones is the same as distinguishing pseudoscience from legitimate science in general: ask where the results are published. Are they in a peer-reviewed journal? Or does the author present it as “science by press release,” stating something like:

“Peer review will of course be considered, but this information belongs to THE WORLD; not a few academics…”

The next steps require you to know a bit about ancient DNA itself, and how research is conducted. What most casual readers may not understand is how difficult recovering DNA from ancient remains is….and how easily it can become contaminated.

The TL;DR version is that for an ancient DNA study to be considered authentic, at minimum it:

  • Must be conducted in the proper facilities
  • Must be conducted by personnel practicing sterile techniques
  • Must utilize negative controls
  • Must have a subset of results reproduced by an outside laboratory
  • Must yield phylogenetically reasonable results (or produce extraordinary evidence to support unusual results), that match the characteristics of ancient DNA.
  • Must conform to any additional standards necessary, depending on the sample and experimental design

Here’s why:

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The mystery of the 900 year old “flesh-joined twins”: an example of the scientific method at work

In 1941, an archaeologist named Glenn Black excavated a site called Angel Mounds just east of Evansville Indiana. Angel Mounds (AD1050-1400) belonged to the Mississippian culture, which was found throughout the Midwest and Southeast in the centuries just prior to European contact.

When excavating a region of the site dense in children’s graves, Black uncovered a grave which contained two babies buried together in a very unusual manner: heads facing away from each other, legs intertwined, hands joined:

From Marshall et al. 2011, figure 2, showing the burial position of the two children.  Note that this is not a photograph but rather a sketch image. While it’s important to show the disposition of the skeletal elements in order to illustrate the scientific background, out of respect for descendent communities I (and others) feel it is inappropriate to post actual photographs of human remains without permission.
From Marshall et al. 2011, figure 2, showing the burial position of the two children.
Note that this is not a photograph but rather a sketch image. While it’s important to show the disposition of the skeletal elements in order to illustrate the scientific background, out of respect for descendent communities I (and others) feel it is inappropriate to post actual photographs of human remains without permission.

He interpreted this burial as “flesh-joined” twins, as they didn’t have any fused skeletal elements. Conjoined twinning* occurs when a single fertilized egg splits only partially into two fetuses (as opposed to complete splitting in monozygotic twins). The rate of conjoined twinning in the United States is approximately 1/ 33,000-165,000 births, but the frequency of conjoined twinning in ancient societies is unknown.

The children’s remains, along with those of other people excavated from Angel, were taken to be cared for by the Glenn A. Black Laboratory of Archaeology at Indiana University, Bloomington.

Seventy years later, my colleague Dr. Charla Marshall became interested in the children, and in Black’s hypothesis that they were conjoined twins. With permission of the curators at the Glenn Black laboratory, she undertook a comprehensive analysis of the children.**

Dr. Charla Marshall, doing ancient DNA work. Note the protective clothing designed to minimize risk of contamination from modern DNA.
Dr. Charla Marshall, doing ancient DNA work. Note the protective clothing designed to minimize risk of contamination from modern DNA.

She and her colleagues found that the two children (designated W11A60 and W11A61) were approximately 3 months old, and had evidence for poor health, but otherwise saw no skeletal evidence that could either support or reject the hypothesis that they were conjoined twins.

Fortunately, Dr. Marshall happened to be an expert in the one method that would definitively tell whether the children were twins or not: ancient DNA analysis. Because mitochondrial DNA is maternally inherited, siblings (and twins) MUST have the same mitochondrial sequence.

Both children, despite having been dead for nearly a thousand years, had ancient DNA still preserved. By extracting the DNA and sequencing it, Dr. Marshall was able to determine their mitochondrial lineages (haplogroups). [I give a little bit of background into how ancient DNA research is done here and here].

Surprisingly, they were different! In the table below, you can see the mutated DNA base positions for each child listed in the third column (under ‘haplotype’). The particular combination of mutations for each child means that they belonged to two different haplogroups: A and C.

Marshall et al. 2011, Table 3.
Marshall et al. 2011, Table 3.

Therefore, the “conjoined twins” were neither twins nor siblings, nor maternal relatives of any kind. Black’s 70 year old hypothesis was wrong.

Why were they buried in such a peculiar way? Dr. Marshall and her colleagues (Cook et al., 2012)*** presented a paper last year at the Midwest Archaeological Conference in which they discussed possible interpretations for this burial practice.

Perhaps, they suggest, the children were non-maternal relatives (maybe half-siblings who shared a father?), who died at the same time and were buried together to reflect this close relationship. Or perhaps the arrangement of the babies’ bodies was entirely symbolic.

Twins play a special role in Eastern Native American iconography, and different Native American societies treat twins in different ways; in some cases they are regarded as having special spiritual power, in other ancient societies they were thought to be negative. Perhaps the co-burial of two maternally un-related children of the same age was meant to be symbolic of twinship, rather than having a literal meaning.

In general, co-burial of individuals was a pretty common practice among the ancient Mississippians, and typically archaeologists have interpreted the co-buried individuals as being related to each other. However, those of us doing ancient DNA research in the Midwest have been testing this hypothesis on co-burials and finding that they’re almost never maternally related. Because no ancient Y-chromosome DNA has yet been recovered from Midwestern co-burials, we don’t know if they might be paternally related.

The motivation for Mississippians to bury people together, and these two children at Angel Mounds in particular, continues to be a mystery. However, the approach of Dr. Marshall and colleagues is a very good example of how persistent research can disprove a long-standing, wrong hypothesis. It may be that future generations of students will be able to solve this mystery with additional genetic evidence.
*The more popular term, “Siamese twins”, was introduced by P.T. Barnum to refer to Eng and Chang Bunker (, who were members of his circus. “Siamese twins” has therefore taken on negative connotations associated with this history.

**Marshall C, Tench PA, Cook, DC, Kaestle FA. 2011. Conjoined twins at Angel Mounds? An ancient DNA perspective. American Journal of Physical Anthropology 146: 138-142.

***Della Collins Cook (Indiana U), Charla Marshall (Southern
Illinois U Carbondale), Cheryl Ann Munson (Indiana U), and Frederika A Kaestle (Indiana U). 2012. If Angel Twins Aren’t Twins, What DO They Represent? Paper presented at Midwestern Archaeological Conference, East Lansing Michigan, Oct 17-21, 2012

Science papers for non-scientists: Where do Europeans come from?

Reading and understanding scientific literature can be incredibly frustrating for most people. You may want to understand some cutting-edge finding, but find you can’t wade through the technical jargon and obtuse figures, so you give up and read some crappy summary in the news. This doesn’t mean you’re not smart! I’m want to assure you that this is a learned skill–we actually have to explicitly teach our students how to do it.

I feel very strongly about making science accessible to everyone. One of the ways I’m going to do it here is to walk people through recent and exciting scientific papers. Here’s my first attempt. Please feel free to give me feedback!

Summary of Brotherton et al: Neolithic mitochondrial haplogroup H genomes and the genetic origins of Europeans. 2013. Nature Communications 4:1764.

I talked recently about how you can use genetics to test the idea that cultural changes in the past were the result of migration. A few days ago, this study was published, doing just that. I want to go through their findings, because they’re exciting and important.

Europe has a very complex prehistory, characterized by lots of migrations of different ethnic groups. Understanding this prehistory genetically is a tricky endeavor, requiring the sequencing genetic lineages of both modern and ancient populations in order to try to link them in time and space. Remember how I said that the majority of ancient DNA research targets the mitochondrial (maternal) genome? By comparing the frequency of different groups of closely related lineages (called haplogroups) in different populations, we can see how closely they are related. More distantly related populations will have different proportions of haplogroups. This is pretty intuitive when you think about the story behind the science; women living in these populations were passing down their mitochondrial lineages through their daughters and grand-daughters. When a woman moved into a new place, she would have brought her lineage with her. Populations that shared greater proportions of related women would have similar haplogroup frequencies, and would differ from more distant populations.

In modern European populations the most common haplogroup is H; it comprises something like 40% of the population. In fact, my own mitochondrial genome belongs to H, reflecting my mother’s family’s Celtic origins*. It’s therefore crucial to understand how different lineages of haplogroup H are related to each other, or what their phylogeny is. Think of a phylogeny as being analogous to a family tree, with individual mitochondrial lineages being sisters, cousins, second cousins, etc. differing by the mutations they possess. You need to work out how they’re related to each other in order to start understanding their shared histories.

Now, the phylogeny of ancient haplogroup H lineages was worked out previously, but that was done using only the hypervariable regions of the mitochondrial genome. (Again, see this post for an explanation of what the hypervariable regions are, and why they’re the targets of ancient DNA research). It turns out that there’s a whole bunch of genetic variation in the rest of the genome, and without incorporating it, the phylogeny is inaccurate.

The mitochondrial genome, showing the hypervariable control region

The control region, containing two hypervariable segments, makes up only a small proportion of the mitochondrial genome, but is the most frequent target of ancient DNA research. (Image modified from an original source which I’ve unfortunately lost.)

So we (finally!) get to the paper itself! What Brotherton et al. (the authors**) did was first observe that haplogroup H was much less frequent among ancient populations than in modern Europeans; Early Neolithic (~5450 BC) farmers had only a 19% frequency of H, and the older Mesolithic hunter-gatherers basically didn’t have any H. The authors decided to completely sequence the mitochondrial genomes of a sampling of ancient people who were already (through previous research) known to belong to haplogroup H. By expanding sequencing past the hypervariable regions to get at the entire genome, they would be able to “capture” all of the genetic variation, and create more high-resolution phylogenies. This would lead to a better understanding of how individual maternal lineages within H moved into the region.

They chose to sequence DNA from 37 skeletons that spanned ~ 3,500 years of the European Neolithic period (roughly 5450-1575 BC) in the Mittelelbe-Saale region of Saxony-Anhalt (Germany). Without going into the chemistry details, trust me when I say that this is a technically impressive feat!

So what did they find? I’m going to focus only on one of their main results. I’ve excerpted Figure 1A from their paper to show you:

Modified Figure 1a from Brotherton et al., 2013
Modified Figure 1a from Brotherton et al., 2013

I realize this looks like something created by a demented spider. Bear with me, and I’ll explain.

This picture is a network diagram, showing the phylogenetic relationships of all the lineages they obtained from the ancient individuals. The circles are the individuals themselves, colored to represent the different cultures they come from (see the key at top left). The lines are the mutational steps between them, with longer lines indicating more mutations (and thus greater genetic distances). The mutations are listed alongside the lines. Unfilled circles are lineages which aren’t actually present at the sites, but are known about from other places. For fun, I’ve indicated with a purple arrow where I fit in on this network. (Have you ever had your mitochondrial DNA sequenced by one of the commercial genome services? If you belong to haplogroup H, see if you can find yourself on this network, too!)

How do the authors interpret this phylogeny? First, look at the position of the red circles. These are the oldest samples in the study, dating to 5450-4775 BC. Do you see how they’re on shorter lines, closer to the central node? That means they have fewer mutations away from the “basal” H type, and are therefore the oldest lineages! (Remember that lineages accumulate mutations over time, so younger, “more derived” lineages are going to have more mutations). And indeed, we see that the youngest lineages (the ones with the most mutations) tend to correspond to the more recent archaeological sites. It’s a cool pattern, that reinforces the validity of this approach.

This also shows something more subtle, but very important. We’re looking at genetic lineages present throughout time within a single region, remember? So…if that region was continuously occupied by the same group of people and their descendents, we would expect to find the oldest lineages on the same branches as the later lineages. Specifically, we’d expect to see the Early Neolithic individuals (red, orange, yellow, green) to be on the same lines (but closer to the central H node) as the Late Neolithic (light blue and blue), and the Bronze and Iron Age (brown and black) individuals. Instead, they’re all on different lines. This means they’re distinct lineages (not-very-closely-related female ancestors).

And this means that, most likely, there was considerable migration of women (and probably men, though we can’t tell from these data) into Central Europe over time, beginning around 4000 BC. The authors suggest (for various reasons which I won’t get into here) that they were likely immigrants from the West, who interacted with the early Neolithic farmers, and ultimately “superseded” their genetic diversity to shape the patterns of genetic diversity seen in present-day Europeans (including myself!). How cool is that?

Does this explanation make sense? Do you have any questions? Let me know in the comments!

*Specifically, H 5
** We have a convention for referring to a study as “So-and-so et al.” that recognizes the first author (who did most of the work). “Et al.” is short for “Et alii” which means “and the others”. It’s a cool/ pretentious bit of science tradition that reflects the discipline’s historic usage of Latin.