12/25/2020

How Did Dogs Evolve To Be Domesticated?

16:06 minutes

a dog with floppy ears looks up at the camera with big eyes
Credit: Lauren Young

Human DNA ancestry kits have become very popular in the last few years—and now, the trend has arrived for canines. A group of scientists recently mapped out the genomes of twenty-seven ancient dog genomes, looking back as far as 11,000 years ago to trace the evolution of the domesticated dog. Their findings were published in the journal Science

Producer Alexa Lim talks to two of the study’s authors, evolutionary biologists Anders Bergstrom and Greger Larson, about what this tells us about the origins of the domesticated dog, and how they evolved to be pets.

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Segment Guests

Anders Bergstrom

Anders Bergstrom is a postdoctoral researcher in the Ancient Genomics Lab at the Francis Crick Institute in London, England.

Greger Larson

Greger Larson is director of the Palaeogenomics and Bio-Archaeology Research Network and a professor of Archaeology at the University of Oxford in Oxford, England.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. DNA ancestry kits have become very popular in the last few years, even for one of the most beloved members of the household– your doggy. We’ve talked on the show about what these tests can tell you about your pooch’s genetic history, and now a group of scientists went way back. They’ve mapped out the genome of dogs going as far back as 11,000 years to map the evolution of our furry friends.

Producer Alexa Lim has more.

ALEXA LIM: When you look at your labradoodle, you might think to yourself, where did you come from? The family pet napping on the end of the couch can seem very different from their wild wolfy counterpart. A group of researchers wanted to map out this relationship. They sequenced 27 genomes of ancient dogs to make a family tree of domesticated dogs and their prehistoric lineages. So what can this tell us about the evolution of the dog and our own human association with these pets?

Their findings were published in the journal Science. My next guests are here to talk about that. They’re both authors on that study. Anders Bergstrom is a postdoc researcher in the ancient genomics lab at the Francis Crick Institute in London, England. And Greger Larson is the director of the Palaeogenomics and Bio-Archaeology Research Network and a professor of archaeology at the University of Oxford in Oxford, England. Welcome to Science Friday.

GREGER LARSON: Thanks very much. It’s nice to be here.

ANDERS BERGSTROM: Thank you. It’s good to be here

ALEXA LIM: Anders, you study ancient genomes to understand human populations. So why look at dog genomes? Why is that interesting to you?

ANDERS BERGSTROM: Well, I mean, I’m broadly interested in biological evolution and diversity. So in my past research, I have mainly studied humans and trying to understand human history by looking at genomes and genetic variation. And, of course, domesticated animals have a particularly close link to humans. And so understanding their history also allows us to understand more about our own history.

Then, of course, dogs and other animals, they are interesting in their own right as well. If we think about dogs starting out with this wild carnivore and transforming it into all the diverse dogs that we see today, that’s an interesting instance of biological evolution in itself.

ALEXA LIM: When do we think that dogs split off from their wolf ancestors, then?

GREGER LARSON: Well, I’m sure Anders would agree with me on this, is that the short answer is that we’re not entirely certain. The longer answer is that we have some pretty good balance that we’re establishing. So we know it was unlikely to be any earlier than, say, 40,000 years ago or any more recent than 10,000 to 15,000 years ago. So somewhere within that gap is when this process– and we’re not even sure how long the process took, but we do know that it was an interaction between wolves and humans that led to this kind of emergent product, which is the domestic dog.

ALEXA LIM: And that’s kind of what your study is trying to pin down and get to. So Anders, let’s talk about the study. The entire dog genome has been sequenced, and you sequenced 27 genomes of ancient dogs. Can you talk about that and what you found?

ANDERS BERGSTROM: So we have dogs in this study from a number of places– from Europe, from the Near East, from Siberia. And we can look at these early dogs and how they relate to each other and try to reach an understanding of how they diversified and spread across the world. So what we found was that our oldest dog is 11,000 years ago. It’s from northeastern Europe. In Europe, in the Near East, and Siberia, they had already diversified at this point and likely spread across much of at least the northern hemisphere, including North America, such that we’re looking at a quite early process. Of course, as Greger mentioned, diversification probably was even earlier than this.

It’s interesting to compare this to other animals, where they were mostly domesticated in the last 10,000 years after the advent of agriculture. But with dogs, we have a situation where, already before any other animal had been domesticated, we had multiple different types of dogs or genetic lineages that were spreading across the world. Much of our study is about kind of tracking those lineages over time, see how they mix and move and combine in complex ways.

But it’s worth noting that all of these early lineages– and we detect at least five. There might very well have been more. But all of these five lineages are in one way or another still represented in present-day dogs.

ALEXA LIM: What was happening during human evolution at this time? Do the timelines kind of overlap?

ANDERS BERGSTROM: One thing that is still kind of a mystery is how after dogs were domesticated, how were they able to spread so widely across the world? Because at this time period, what we call the Late Pleistocene, we aren’t really aware of any human migrations that are large scale enough to spread dogs to all these places. But somehow they do, and they very quickly spread to a range of prehistoric human societies, perhaps because they were useful to a broad range of humans. But we still don’t really know how this happened.

ALEXA LIM: Do you have any theories of how these dogs were getting around then?

ANDERS BERGSTROM: I guess you could imagine that there is some degree of contact between human groups, even though we don’t see it in their genomes. So perhaps they’re trading and exchanging cultural ideas and items and perhaps even animals. In which case, dogs could spread, even though we don’t see humans affecting each other, at the level of DNA, at least.

ALEXA LIM: Yeah. And Greger, what are your thoughts on that? How were they maybe moving around then?

GREGER LARSON: Humans are evolving in Africa. And we kind of leave Africa at like 60,000, or at least the first time our species extends beyond that. Humans get everywhere in Eurasia and even down to Australia without dogs. But it looks as though people don’t get into the Americas or into the Pacific without them.

So there are big, broad, continental scales where people and dogs seem to be moving in conjunction. And other ranges, like in Australia, where it’s not easy to get there at 60,000 years ago, but lo and behold, there are people. And that’s well before dogs are domesticated. So it’s fun to look at these patterns of where the similarity exists and where the differences exist, and that really helps to us to inform not just dog history, as Anders was saying, but human history as well.

ALEXA LIM: But Greger, does this suggest that dogs came from a single origin, like one wolf species? Or were there multiple domestications of dogs that happened that kind of led to maybe these different lineages?

GREGER LARSON: That’s a really good question. And I think this paper goes some way toward pushing the needle further in the direction of only requiring a single population of wolves to explain the variability that we see in all modern dogs and in the ancient dogs in our study. What we– and though we have 27 in our study, an additional five from previous studies that we looked at, that’s still not a whole hell of a lot of dogs when you consider the number of ancient human genomes, for instance, that have been published or the number of ancient horses that have been published.

And it is still possible– although perhaps unlikely, it’s hard to say– that when we sample dogs from other regions and other times where we have yet to sample, that we may find that there are lineages that as Anders said at the beginning, we just haven’t seen yet. And that might help to push the needle to that question in one direction or another.

ALEXA LIM: And Greger, there was another interesting result about how dogs mixed or didn’t mix with wolf populations. Can you kind of explain what is happening there?

GREGER LARSON: Yeah, and this is really down to Anders’ work. What we were able to see is that at several different locations on the planet, you can see a signature of what we call admixture between local dogs and local wolves. And all that means is that they’re swapping genes. There is some gene flow here; there’s some sex that’s taking place between the local population and a dog population.

Now, those analyses on their own aren’t able to tell you definitively which direction that gene flow is going. It could be one, dogs to wolves, wolves to dogs, or both. So what Anders does is there’s a secondary level analysis where he looks at all dogs and all wolves and finds that there is no one wolf population that is any closer in its overall relatedness to any– to all dogs. And what that suggests, then, is that with all dogs being on one side and all wolves being on the other side, when you look at a more global picture, that all of those local signatures that we are seeing are exclusively the result of gene flow from dogs into wolves and not the other way around.

And that is– for me, that’s absolutely fascinating because that means even as far back as 11,000 years ago, having a hybrid dog/wolf was not OK. If you were somewhere between a dog and a wolf, you simply did not survive long enough to pass down that wolf ancestry into all the subsequent dogs that happened after that. So dogs, the way in which they were being managed, the way they fit into human societies, the way they were being cared for, required them to be extremely doggy. And if you had any wolfiness about you, then you were just not OK, and you were probably hit on the head and sent away, because that was just not acceptable.

ALEXA LIM: All right. Anders, what’s your interpretation of that?

ANDERS BERGSTROM: Yeah, I agree, and I think it’s a kind of fascinating thing about dog genetics. I think a lot of people might have this intuitive idea that there will be a kind of a gray zone, a kind of gradual transition between dog and wolf. But actually, at least for these up to 11,000-year-old dogs that we have studied, there’s basically no question. They are fully dogs already by 11,000 years ago.

Of course, if we go even further back into the past, you would at some point reach a state where you have something that is kind of in between. But all the ancient dogs that we have studied, already by 11,000 years ago they are kind of fully formed the dogs. They are not some kind of intermediate stage.

ALEXA LIM: Or I guess the question is– I mean, this is purely speculation, but why are dogs special, then? Why do we have this special relationship? Why don’t we do this to a cow?

GREGER LARSON: Well, so I think what that question presupposes is that we were doing something with an intention, with a long-sighted view toward achieving a goal of a labradoodle on your couch, you know, however long ago. And I think that that underlies a lot of domestication, or at least the popular conception of domestication. And I think instead, what I prefer to see this as, is much more likely within an evolutionary model, which is where both Anders and I come from, is more of an emergent process that comes out as a result of the interaction between wolves and people.

And it’s not that you– it’s not wolf-led, it’s not people-led, it’s just two species getting together and then becoming increasingly reliant on each other for their survival. And not just a species, a specific population of that species. And we see lots of examples of this that involve humans and other animals, but lots of two other animals as well. And there’s all kinds of words within evolutionary biology to describe these kinds of relationships between things.

And I think a dog is just another example of this. But because it involves us, we want to think of ourselves as the people who are going into the dens and grabbing a puppy and deciding, yes, we’re going to turn this into something cute that our four-year-old can play with without being eaten, which is a whole nother thing. I mean, why would you– you would never do this intentionally if everything is wolf and you have no concept of what a domestic animal is. So I think there’s a lot more accident and by-product in this whole process than there is any sort of top-down intentionality on the part of people.

ALEXA LIM: Right. And Anders, now there are many different breeds and crossbreeds, like a labradoodle. I mean, what is a breed exactly?

ANDERS BERGSTROM: Yeah, I mean, I think a breed isn’t necessarily something very special. I mean, it’s basically just a population of dogs that has been, through human design, genetically isolated from other dogs. And so the mating is controlled to prevent unwanted, so to speak, DNA from entering that population. And then there’s often selection to achieve certain types of traits or characteristics.

But there isn’t really a fundamental difference between breed dogs and non-breed dogs. They have the same roots, the same origins, it’s just that humans sometimes have created these artificially isolated populations to control their evolution in a much more directed way. But that’s basically it. Who knows if all of these breeds will remain genetically isolated from each other in the future? I don’t expect that it will have a particularly large impact on the long-term trajectory of dog evolution.

ALEXA LIM: I’m Alexa Lim, and this is Science Friday from WNYC Studios.

And Anders, even though there are a lot of breeds, the dogs are less diverse than they were in the past. Is that correct?

ANDERS BERGSTROM: In some sense, yes, at least in certain parts of the world– not necessarily on a global scale. A lot of the early diversity that we found, let’s say, 10,000 years ago is actually represented. It’s just it has kind of mixed and transformed itself in different ways. When we look at ancient dogs in Europe, there was actually much more diversity in the past.

So European dogs before, let’s say, 4,000 to 5,000 years ago, actually displayed a much greater diversity on a genetic level. You had some European dogs that were similar to Near Eastern dogs, others that were similar to Siberian dogs, and that whole range in between. But when we look at modern European dogs, they only display a fraction of that early diversity. So at some point, through a process that we don’t understand, this diversity basically collapsed in Europe. And perhaps it was a single population that basically expanded and replaced other European populations.

ALEXA LIM: Greger, if someone is sitting at home listening to this with their husky or their labradoodle or cocker spaniel, I mean, how does that dog fit into this? What should they take away from their– about their pet from this?

GREGER LARSON: Well, I think that they can be– everybody’s got a different relationship with their dog. And by and large– and we can see this for some really great research that’s been done on dog cemeteries recently. And that for a lot of people, a dog is a member of your family, and that you grieve it in the same way that you would grieve the death of a family member. You take care of it in the same way that you take care of a family member.

And it means, you know, if you’re having– if you have kids, like they help to raise the kids and teach them all kinds of things. And there is a real– and a lot– most– in a lot of societies, there is a real attachment and association and importance put on having a family pet that includes a dog. And I think if you’re sitting at home, the thing that you can take away from this is that we know from this study that that– how you’re feeling about your dog right now goes back at least 11,000 years and probably deep much deeper than that.

And that therefore there is this sort of unbroken attachment and emotional connection that you share with your dog, that you know that your ancestors, that whose names you’ll never know, who existed and lots of different parts of the world all share that same sort of emotive, empathetic reaction. And I think that’s certainly true of more than any other domestic animal. No matter how much you care about your guinea pig, it’s never going to have the same depth of history as your dog. So that’s– I think that’s one of the big lessons from this.

ALEXA LIM: Greger, I’m not trying to start something, but how does this all compare to cat domestication and evolution?

GREGER LARSON: Cats are great. You can feel good about your cat. And evolutionarily, there’s some really cool stuff going on with cats where you can buy a Bengal cat or Savannah cat, which are intentionally bred not just with other cats. Like a labradoodle would be a Labrador and a poodle, and that’s two dog lineages, where you have cats that are bred with completely different species. And the way– and to create these kind of weird novel mix-ups of things. And we’re not doing that so much with dogs.

So cats have their own kind of unique weirdness about them as well. But cats are, in terms of living in houses and having that kind of relationship that we associate with dogs, that is a far more recent process. And my understanding is a lot of that has to do with kitty litter, is that you only start to bring in a cat when you allow it to go to the loo inside your own house. And that really only happens around World War II, maybe just a little bit after that.

So when we’re thinking about time depths, again, dogs go way, way back. And I’m not– a lot of dogs are living outside the house, too, but the overall kind of– the style of relationship or the big thing that allowed for people to feel as closely to their cats as they always felt to their dogs really is only about 50 or 60 years old.

ALEXA LIM: Really, it’s kitty litter?

GREGER LARSON: Yeah. Yeah.

ALEXA LIM: Well, we’ve run out of time. I’d like to thank you both. Anders Bergstrom, a postdoc researcher in the Ancient Genomics Lab at the Francis Crick Institute in London, and Greger Larson is the director of the Palaeogenomics and Bio-Archaeology Research Network and a professor of archaeology at the University of Oxford in England.

ANDERS BERGSTROM: Thanks a lot.

GREGER LARSON: Thanks for the opportunity.

ALEXA LIM: For Science Friday, I’m Alexa Lim.

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