Jennifer Raff: A Genetic History of the Americas

Jennifer Raff, Image courtesy of Jennifer Raff

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Jennifer Raff is an Associate Professor of Anthropology at the University of Kansas with a dual Ph.D. in anthropology and genetics and she recently published a book called Origin: A Genetic History of the Americas summarising what we know so far about this ancient migration.

So to set the scene, I asked Jenny what was happening when humans were beginning to reach the Americas? So how long ago was it? What were humans doing? What was their technology? What was their culture?

Jenny: So these are all very big questions and some of them we don't have all the answers to. We currently believe - those of us who work in archeology and genetics - that the timing of the peopling of the Americas coincided or happened just after the last glacial maximum, which you can think of as the Ice Age; a period when the earth globally was colder and drier. And both of these conditions together made most of the planet very uncomfortable for humans and for animals and plants. The last glacial maximum gradually comes to an end starting around 17,000 years ago. But, prior to that, it started around 25,000 years ago, 24,000 years ago, something like that. 

Sally: Okay. I'm just one of those people when it comes to ancient history, I really struggle to put it into a timeline. I mean, it's one of those weird things where Cleopatra was closer to the space mission than she was to the pyramids or something like that.

Sally: How old do we think Homo sapiens the species is?

Jenny: That also is a very complicated question and I don't mean to dodge it. The major Out-of-Africa migrations were happening roughly around 100,000-80,000 years ago, something in that area. But we actually have fossil evidence that maybe humans like us, or at least humans looking like us and genetically resembling us, had made earlier forays out of Africa and had actually intermingled with Neanderthals and possibly other humans around that time.

Jenny: So again, it's a complex process and thinking about it as like one single major event or one single major migration is kind of the wrong way to think about it. It's probably more likely a process that happened slowly.

Sally: Yeah, it's not just one band of intrepid explorers founding the entirety of non-African humans. 

Clovis point

Stone tool from North America roughly 13,000 years ago. Image courtesy of Virginia Dept. of Historic Resources

Jenny: Exactly. And the same is true of the peopling of the Americas. So for a long time scientists and the public have thought of it as this race across the Bering land bridge as soon as it became available and moved down into North and South America and people the continents very rapidly. And that those people were the bearers of what we call the Clovis technology, which is this really beautiful, elegant, fluted-point projectile points that would have gone on the ends of spears, which they used to hunt the mammoths and mastodons, and the large megafauna. 

Sally: You mentioned this a lot in your book, and so I looked up pictures of it and sometimes when someone's like, "Oh yeah, this is a prehistoric tool," you're like, "Is it? Or is it a rock smashed in two?" These are incredibly shaped geometric objects, clearly non-natural.

Jenny: They're stunningly beautiful, yeah.

Sally: I have no idea how you'd make one.

Jenny: I couldn't. I've tried to make stone tools. I'm terrible at it.

Jenny: So these stone tools appear on the archaeological record very abruptly about 13,000 years ago. And everybody thought for a long time, "Okay, these are the first peoples of the Americas", but it turns out that over time, older and older archaeological sites start to be identified. And there's huge controversies about all of them, which I go into excruciating detail in my book about.

Jenny: And the upshot is the combination of archaeology and genetics shows us that, in fact, the Clovis people were not the first people in the Americas. That people had gotten there earlier. And it was a much more complex process than these older models would have led us to believe.

Sally: How much earlier?

Jenny: That's the million dollar question right now. I think the majority of evidence supports a peopling as soon as the west coast would have been ice-free. So by boat, along the west coast, sometime around 16-17,000 years ago. Genetics matches that; we see increases in population size and genetic diversity expanding about the same period of time.

Jenny: But there are some questions about that because very recently, a number of sites that potentially predate that period have been identified and they all have varying degrees of support from archaeologists. Some are kind of ambiguous. Like you mentioned, we have these broken rocks and are they actually tools shaped by humans? Were they broken by natural processes?

White Sands trackways

Ancient fossilised footprints from White Sand park, USA, estimated to be about 23,000 years old. Image courtesy of National Parks Service.

Jenny: But there's one site in particular that I am really fascinated by that was just discovered, or at least just published, just as I was wrapping up this book, this is the "White Sands locality two" site. And this site is actually in the White Sands Park in New Mexico. It's a really amazing site that actually contains trackways from humans that have been dated by means of these little seeds embedded in the footprints to potentially 23-25,000 years ago.

Jenny: Now that would have been the height of the last glacial maximum. There would have been a massive ice sheet covering North America; mostly Canada - it wouldn't have extended as far down as New Mexico. But that way should have been blocked. And so if these dates are correct, and not everybody thinks they are, but if they are correct, it means people must have gotten into North America before those ice sheets fuse, so sometime before 25,000 years ago. And that opens up this whole new avenue of exploration and understanding in both the archaeological record and genetics.

Sally: That's doubling the length of time that we originally thought they were in America.

Jenny: Absolutely. It really makes for a whole new story.

Sally: If we're looking anywhere in this huge range between 15-25,000 years ago roughly, that's still a means that for a good 80-90% of the time that humans have been on Planet Earth as modern humans, they haven't been in the Americas. How does this coincide with... so you mentioned they've got tools, you mentioned they were possibly boats.

Sally: So how are we doing with stone tools? How are we doing with clothing? Fire? What does it look like to be a human maybe 20,000 years ago?

Jenny: Great question. So we know they definitely had fire. Fire was harnessed as a human tool much, much earlier than this. They had tailored clothing; archaeologists have found evidence of needles as early as dating to around 30,000 years ago, the Upper Palaeolithic in Siberia.

Jenny: And in fact, some archaeologists have speculated that it is the presence of this sewing needle, the actual ability to make tailored clothing, which allowed humans like us - anatomically modern Homo sapiens - to live above the Arctic circle. Whereas our kin, Neanderthals and Denisovans, don't appear to have ever done so, and we don't see that they have some of these same technologies. Even though physically, Neanderthals were far better adapted to living in cold conditions than our species, our kind of human was at the same time point.

Jenny: By this time, by 30,000 years ago, setting the stage for the peopling of the Americas, anatomically modern Homo sapiens still showed physical adaptations similar for warmer environments, warmer climates.

Jenny: And so we had tailored clothing. We had hunting technologies that included a diverse toolkit made of stone. We had art; we had very sophisticated culture and arts. These are all technological and artistic attributes that we see in the Upper Palaeolithic cultures of Siberia, which is where a lot of the ancestry from Native Americans likely came.

Sally: Have we got bows and arrows? Are we using spears?

Jenny: Spears at this point, yeah. Bows and arrows were invented later.

Sally: And is farming around?

Jenny: Definitely not. Farming is a much more recent development that occurred at various places independently around the world. In the Americas we start to see intensive maize agriculture doesn't emerge in North America until a little bit before a thousand years ago.

Sally: Okay, so quite recent.

Jenny: Much more recently. And even earlier in Mesoamerica and South America.

Sally: And have we got boats? Presumably that's quite important when you're talking about human migration. 

Jenny: Yeah. We don't have direct evidence of boats in the archaeological record at this point for the Americas, but the expectation is that people were using boats. Certainly by 13,000 years ago, there's evidence that people had reached islands that they could not have done so off the west coast unless they had boats. And the strong genetic signal, a very, very rapid diversification and expansion, matches an expansion by boat much faster than a slower overland route would have resulted in.

Jenny: So you see people from North America and South America genetically extremely similar to one another, which could only have happened in this very short period of time if they had been moving rapidly. And in addition to that, we also infer the presence of boats because the west coast of North America became ice-free much earlier than the overland route would have been, and you see the expansion of people reaching sites as far south as South America. And they could only have done that if they travelled by boat along the west coast.

Sally: This is one of the things I was going to ask. In my very basic human evolution lectures, I remember having this map. You've got humans coming out of Africa, some turning left into Europe, some turning right through the Arab peninsula, through India, China following the coast. And then that group splits; half go in the Australia direction and then the other half keep going north and then across the Bering Strait. And so I'm imagining this tiny group of humans, huddling on a pretty flimsy glacier, a little block of ice that's only there temporarily. And then rushing across to get to North America, to Alaska and Canada. Is any of that correct?

Jenny: No, not really, not anymore. Well, I like to invite people to think about the peopling of the Americas as a very dynamic, complex process done by populations that would have probably been a lot larger than you would have been led to the leave from schoolwork.

Jenny: One of the prime candidates for the region where people might have waited out the last glacial maximum before dispersing into the Americas is actually the central part of Beringia, the southern coast of the central part of Beringia, which of course today is underwater.

Sally: I was going to say, where's Beringia?

Jenny: Yeah. It's the land connection between Asia and North America or what was the land connection. Now it's the Bering Strait. But during the last glacial maximum, so much water was bound up in these massive ice sheets that covered huge swaths of North America and other continents, that the sea would have been much lower than it is today. And we don't have, unfortunately, direct archaeological evidence of people living in this region where we think they might have lived.

Jenny: We have paleoclimactic reconstructions showing us the southern coast of central Beringia would have been a pretty decent place to live at this time.

Sally: What, without ice covering the land?

Jenny: Yeah, definitely ice-free. It would have had a warmer climate, a wetter climate, thanks to the proximity of the ocean. And also there would have been plants growing there, lots of animals. So this is a really prime location for where people could have been. And we know that they were isolated from all other populations for a period of a few thousand years. The genomes show us this. But unfortunately we can't test this directly, archaeologically, because it's underwater. So that's a bit of a problem.

Jenny: But if people were actually living here as our models suggest, this suggests to us that thinking about the Bering land bridge as this piece of land that you race across to get from Asia to North America is kind of the wrong way of thinking about it. It wasn't a bridge, it was a homeland that people would have lived in and been isolated from.

Sally: And a pleasant one, one where there's food to hunt.

Jenny: Well, pleasant relatively, yes. But certainly they would have been able to adapt to this environment and develop this cultural knowledge and these technological adaptations for a marine environment, which would have, we think hypothetically, allowed them to really thrive as they move along the west coast and move southward down as far as South America. You would have encountered similar environments there.

Jenny: Following that, people could have moved inland and developed adaptations and knowledge and relationships with land and animals all across the Americas in a slower diffusion.

Sally: Now you've mentioned that you're looking at genomic evidence here, and obviously there's the archaeological evidence, the physical evidence of the spearheads. How can you infer genetic information from such old groups? Are you directly getting ancient DNA from ancient bones or are you looking at modern populations and then working backwards? 

Jenny: Great question. And I have to stress, this is not my research. This is the work of many, many people and lab groups. The work that's been done has been a combination of straight population genetics from living peoples, but also, and crucially, genomes from ancient peoples and individuals, mostly dating from various time points throughout this long process.

Jenny: And so some of the earliest genomes that we can look at are from Asia dating to 45,000 years ago. Some really crucial ones were sequenced from teeth that children lost, just shed naturally, at the Yana river site in Siberia. And those dates about 30,000 years ago.

Sally: And one of the lovely things you do in your book is to really set the scene because you can imagine, "Oh yeah, we've got this tooth in this museum." Firstly, there's the amazing fact that the DNA has survived enough for us to sequence, but you're like, "But that was a human that lost that tooth. That was a child whose tooth has become separated from their skulls. So what was happening at that time?" And it really, to use an obvious word here, but it humanizes that human story. 

Jenny: Yeah, that was one of the things I really wanted to do in this book was to get across the fact that these are not just genomes, they're not just samples. I always hate it when people use the word 'specimen' to refer to human ancestors. It drives me crazy.

Sally: They've probably got numbers attached to them. Bone 4521B

Jenny: Oh it's horrible. Absolutely. It's a really gross and embarrassing aspect of the history of our field. This sort of detachment from the humanity of these people.

Jenny: I do imagine what the lives of these people would have been like. And so many of them were children. And so trying to talk about the impact of their genomes, but also who they were or who I imagined them to be because of course we don't know who they really were, it was important to me to do that.

Jenny: Certainly because so many of them were children at the same age as my son, my little toddler who's now a preschooler, when I was writing this book, it was very hard sometimes to write about these children and what their DNA has shown us.

Jenny: And at the same time, it's also tricky because of course, these are the ancestors of people who... you know, I don't belong to that group. I'm not Native American. And so in telling the story of these ancestors, it was really important for me to try to do it respectfully. And I was really lucky to have the help and advice of many colleagues who are indigenous to help me shape these

Jennifer Raff from the University of Kansas, and her book, ORIGIN: A Genetic History of the Americas just came out this month.