Did Neanderthal Men Prefer Human Women? New DNA Study Reveals Ancient Secrets

In the long stretch of prehistory, humans weren’t alone. One of the groups they shared the world and sometimes their lives with were the Neanderthals. Now, a new DNA study suggests those encounters followed a pattern and most often, it was Neanderthal men and human women who got together and interbred. This left a genetic signature that is still found in then DNA of many of us today. It provides a clue as to how our species evolved, how cultures may have collided, and how even the most intimate moments of the distant past shaped who we are now. I have been speaking to Dr Alexander Platt, a senior research scientist at the University of Pennsylvania and an author of the research.

Link to the study 'Interbreeding between Neanderthals and modern humans was strongly sex biased': https://www.science.org/doi/10.1126/science.aea6774

Also available on YouTube: https://youtu.be/gM-XDB9tDEw

Transcript

SPEAKER_00 0:00

Welcome back to History from a New Perspective, a new history channel and podcast covering the history and historical stories from parts of the world which don't always get as much attention. My name is Joseph Keane, and today we'll be looking at Neanderthals. In the long stretch of pre-history, humans weren't alone. One of the groups they shared the world, and sometimes their life with, were the Neanderthals. Now a new DNA study suggests those encounters followed a pattern, but most often it was the Neanderthal men and human women who got together and interbred. This left a genetic signature that is still found in our DNA of many of us today. It provides a clue as to how our species evolved, how cultures may have collided, and how even the most intimate moments of the distant past shaped who we are now. I've been speaking to Dr. Alexander Platt, who is a senior research scientist at the University of Pennsylvania and an author of the research. I started by asking him who the Neanderthals were.

SPEAKER_01 1:02

Neanderthals is a distinct group, originated, let's say about 500,000 years ago, and they did so in Eurasia. Shortly thereafter, about 300,000 years ago, another human popped up in Africa, and that's the anatomically modern human, that's our ancestor. So if we're talking about a period of time where there were both Neanderthals and anatomically modern humans walking around, that's a period from about 300,000 years ago to about 40,000 years ago when Neanderthals stopped being a distinct group, though for most of that time they existed on different continents.

SPEAKER_00 1:43

How would both humans and Neanderthals have been living? Would they have been sort of hunting for food? Would they have been sort of gathering vegetables and fruits?

SPEAKER_01 1:52

Right. So both groups would have been hunter-gatherers at that time, living in fairly small tribes, sort of small individualistic communities. They were both social creatures, they were both engaged in activities beyond mere survival, they had some forms of art, they had some forms of religion, so they weren't just uh scraping by, but they also didn't have things like agriculture or domesticated animals at that point.

SPEAKER_00 2:24

And of course you mentioned that Neanderthals were in Eurasia. So is it accurate to say that sort of many people living in in Africa today or Sub-Saharan Africa today don't have Neanderthal DNA as opposed to people living outside of Sub-Saharan Africa?

unknown 2:40

Right.

SPEAKER_01 2:40

So the Neanderthals themselves probably never made it down to Africa and didn't directly leave any genetic descendants within Africa. However, there has been a fair amount of migration of anatomically modern humans from outside of Africa back into Africa over the last few thousand years. So it depends on exactly whom you're talking about within Africa. It's a large continent, it's a very diverse continent. People whose ancestry is and has always been sub-Saharan African, those people generally have no Neanderthal ancestry. Other groups, primarily in East Africa, some of their ancestry traces back to non-African modern humans who had acquired Neanderthal DNA in their past in Eurasia and brought it with them back into Africa. So you do see some Neanderthal ancestry in Africa, but you certainly don't always see it. And there's plenty of people within Africa who have none.

SPEAKER_00 3:50

Just going to your study, how did you find from your study that interbreeding between Neanderthals and humans was strongly sex-biased? And just tell us a little bit about the, I guess, this idea that Neanderthals and humans did interbreed as well, please.

SPEAKER_01 4:07

We've known that Neanderthals and humans interbred since we first sequenced a Neanderthal genome back 10, 15 years ago, that we found sort of reasonably long stretches of modern human genomes that look awfully similar to the Neanderthal genome, too similar to have happened just by chance, and indicating that at some point in the relatively recent past, about 45,000 years ago, it turns out, there was interbreeding between Neanderthals and modern humans. And so we have some Neanderthal ancestry, and that's what these tracts of similarity looked like. We also noticed at that time that there were far fewer of these tracts of Neanderthal ancestry on the X chromosome. So as mammals, we have a sex determination system that works with females having two copies of X chromosomes and males having one X chromosome and one Y chromosome. So when we see that the X chromosome has a different pattern, well, there were sort of two hypotheses that were floated very early for what could be causing this. Either we tended to have Neanderthal fathers in the interbreeding that led to our Neanderthal ancestry. And from our fathers, we get fewer X chromosomes than we get from our mothers, right? If you have two parents who have a son and a daughter, the father will have passed down an X chromosome to his daughter. The mother will have passed down X chromosomes to her daughter and her son. So two-thirds of the X chromosomes came from the mother, and only one-third came from the father. So if the fathers were the Neanderthals, we never had that many Neanderthal X chromosomes to start with. And that could be a reason why we don't see much Neanderthal ancestry on the X chromosome today. The other hypothesis that was floated was that the Neanderthal ancestry we had on our X chromosome was deleterious. And when you had people with more Neanderthal ancestry on their X chromosome and people with less Neanderthal ancestry on their X chromosome, the people with less Neanderthal ancestry on their X chromosome just did better. And that over time, even though we started out with just as much Neanderthal ancestry on our X chromosomes, we selected for having less and less throughout our history over the last 40, 45,000 years. So we ran through three different hypotheses in this study. They have a lot of modern human ancestry on their X chromosome, but it's mostly the parts that aren't doing anything. If it's the parts that aren't really doing anything, then the story is not about what the X chromosome is doing, it's about how the X chromosome got there and who has X chromosomes. And that points really strongly to these hypotheses about sex bias.

SPEAKER_00 7:16

I mean, is this an indication that at that time that human women would have had a preference for Neanderthal males? Or are there other possibilities, like for example, that it could have been against their will? Does this kind of give us any indication of any of these ideas?

SPEAKER_01 7:36

It doesn't directly. And I suspect that there is preference being expressed throughout all of these processes. Whose preference, I don't know. Right? Whose preference is dominant, whose preference is the one overriding it, whether it's male preference, whether it's female preference, whether it's both. Um, I think all of those are viable hypotheses at this point. There are other explanations as well that actually don't involve preference at all. You could also get this kind of phenomenon, not exactly through mating preference, but instead through sex bias in terms of cultural practice in terms of things like migration patterns. If you have a society where women tend to stay home and men tend to go out foraging, you have greater possibility for males to migrate into other populations. And therefore you would see a sex-biased gene flow that way. Even if whenever you have interbreeding between two groups, where does the family end up? Is it near the father's family or is it near the mother's family? Those kinds of phenomena create sex biases in terms of gene flow as well. To explain what we see just through these kinds of migration patterns would be a little difficult. We see such a strong effect that we would need to invoke multiple rounds of migrations, each with a unique pattern of who's migrating and who's staying home, that makes things a little complicated, which is why I favor a model that just says maybe they like it this way, and exactly who is they and who's doing the liking. I'm not commenting on. That ends up being a much more powerful system. Men tend to look for relatively feminine mates, and women tend to look for relatively masculine mates. But they're different goals and they're different things they are looking for and different things that they desire. And I don't think there's any reason that the desirability of Neanderthals for modern humans and the desirability of modern humans to Neanderthals as mates should be equal by gender, and that there's no reason that there shouldn't be a sex bias in terms of attractiveness or perhaps lack of repugnance for one sex as opposed to the other.

SPEAKER_00 10:25

In terms of the the Neanderthal DNA that you did study, I guess how limited it was it was your pool of of DNA to study. And also the Neanderthals you did find, where were they found? And also, I just wanted to ask a bit about actually the kind of state that they were found in. Like, you know, for example, were they in very cold conditions?

SPEAKER_01 10:46

We did most of our study, primarily focusing on a single Neanderthal, who was found in the Altai region of Siberia, in a cave in a mountain. This was a skeleton in a relatively sheltered environment, very cold. And this is one of the first Neanderthals to be fully sequenced. It is also one of the oldest fossils, fossil Neanderthals to be fully sequenced, and for that reason was the uh sort of main focus of our work. We then confirmed our findings in two other slightly younger Neanderthal specimens, one from Chigerskia, also from Siberia not far away, who's maybe 20,000, 30,000 years younger, the other from Vindia, Croatia, also, I believe, a relatively sheltered, relatively cool environment for a fossil to be hanging out for 100,000 years. The technology to sequence Neanderthals has has been getting steadily better over the past 10, 15 years. We've gotten a lot better at figuring out where in the body you can find protected DNA, even in skeletons that have not otherwise been especially well preserved.

SPEAKER_00 12:02

I mean, what are some of the theories that we know of how they came to die out eventually? I mean, what why is it they're not here with us today?

SPEAKER_01 12:11

Actually, I'm going to push back a little on this notion that they died out. Outside of Africa, there's something like 6 billion of us walking around, each with about 2% Neanderthal ancestry. So we're talking about the genetic equivalent of 150,000 Neanderthals alive and well today as modern humans, which is likely more Neanderthals than there ever were when Neanderthals were a distinct creature. Clearly, there are no Neanderthals. We don't have a distinct group of humans with a distinct Neanderthal phenotype. But modern humans, to a very large extent, absorbed Neanderthals. There were more at that interaction time 45 to 40,000 years ago, there were more modern humans around in Eurasia than there were Neanderthals, maybe 10 times as many. And when you have interbreeding between a group that's 10 times as large as another group, the smaller group tends to get absorbed. That the ability to hold on to a distinct phenotype and a distinct gene pool gets lost very quickly. Now, that's not the whole story. We sort of took in 5% of our gene pool as Neanderthals. That's not quite enough to explain all of them. So, in addition, Neanderthals' numbers were dwindling. We don't really know why. It looks to some extent like Neanderthal numbers may have been dwindling before modern humans showed up, that they could well have already been on the way out due to environmental changes and other external phenomena. I can't imagine that it helped to have a new human show up in large numbers to also compete for the resources available in Eurasia at the time.

SPEAKER_00 14:10

That was Dr. Alexander Platt, a senior research scientist at the University of Pennsylvania and an author of this study entitled Interbreeding Between the Andetals of Modern Humans was strongly sex biased. Now please see the description below for a link to the research. Thank you for joining us today on History from a New Perspective, and we'll see you in the next episode.