Ancient Skull Holds Clues to Dog Domestication

A 33,000-year-old skull of a "wolf on the way to becoming a dog" was found in a Siberian cave. Evolutionary Biologist Susan Crockford, co-author of a study about the skull in PLoS ONE, discusses why the discovery challenges common beliefs about dog domestication.

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Ever wonder where your pet poodle or your Labrador came from, who her ancestors were and when she evolved from wolf to dog? A 33,000-year-old fossil, discovered in a Siberian cave, may help shed some light on those questions. Researchers report that features of the ancient skull, including its overall size and teeth, indicate it might have belonged to a wolf on the way to becoming a dog. They say the skull tells a story that may make us question everything we thought we knew about dog ancestry and domestication.

Dr. Susan Crockford is an evolutionary biologist and adjunct professor at the University of Victoria. She co-authored the study on the skull published in the Public Library of Science. She joins us from British Columbia in Canada. Welcome to SCIENCE FRIDAY.

DR. SUSAN CROCKFORD: Hello.

FLATOW: That's a pretty big claim to question everything we know about dog ancestry and domestication.

CROCKFORD: I'm not sure that that's exactly what we said...

(SOUNDBITE OF LAUGHTER)

CROCKFORD: ...but it certainly does shed some new light on some of what we've been thinking about what's been going on.

FLATOW: Tell us about - give us a little nutshell description of what you found.

CROCKFORD: Well, it's a complete skull with both of the lower jaws. Most of the teeth are there. The skull was found in a cave in Siberia, in the Altai Mountains, in about 1975. The Altai Mountains are in, sort of, southern Siberia. If you look on a map, it's where the confluence of Mongolia, Siberia and Afghanistan, just to sort of place it.

And the original archaeologist who found the specimen thought that it really didn't look like a typical wolf, and so he pulled it out for some re-analysis. And that's what we've been doing now, is going back and doing - did some radiocarbon dating to put it in proper chronological context, and also looked at measuring it and that sort of thing to see how it compared to both more recent dogs and also to wolves of that time period.

FLATOW: Well, what would be the key differences and the key similarities to make it look like it was transitioning from a wolf to a dog?

CROCKFORD: Well, the really key outstanding feature is its size.

FLATOW: Yeah.

CROCKFORD: It's about the size of a large Samoyed dog, so really quite small for a wolf. However, the teeth are still wolf size. So it's this combination of dog and wolf features that makes it different.

FLATOW: So you've pushed this fossil back over 30,000 years. Where would the - what was the last fossil estimate of that timeline?

CROCKFORD: Well, in around 12 to 14,000.

FLATOW: Wow, that's a big difference.

CROCKFORD: Yeah, it is a big difference. And one of the things that really is critical, we think, is that one of the things that happened in between that, of course, was the ice age.

FLATOW: Oh.

CROCKFORD: And so a lot of places where some of the other fossils have been found in Europe actually were covered in glaciers during that ice age period.

FLATOW: Mm-hmm.

CROCKFORD: So it's a really important blockage...

FLATOW: Yeah.

CROCKFORD: ...of things going on in Europe. And one of the things that we noticed was that while this specimen was found before the ice age, there are no specimens similar to it that continue on through the ice age period or after in that same area of Siberia. That area of Siberia actually wasn't covered by glaciers, and so people still live there. And even still, there are no similar specimens that carry on through time.

FLATOW: Mm-hmm. And so is there still this big gap between 12,000 and 33,000 years of knowledge of what happened there?

CROCKFORD: Yes. There are several more specimens similar to this one that we found that were found in Belgium, the Czech Republic and around the Caucasus in Southern Europe and - that are similar to this and about the same - from the same time period. But still this big gap.

FLATOW: Yeah. You know, we all - we've been told over the years that the common belief about how dogs were domesticated is that ancient people tamed wolf pups. They tamed them and created them, brought them in, and they morphed into dogs, so to speak. Does this back that up, or is there a better theory?

CROCKFORD: Well, I would say not. One of the things that has come to light is that - that idea that taming actually leads to the changes we see between wolves and dogs really has no evidence to back it up. It doesn't mean that it couldn't have happened. It just means that, really, there are - is no evidence to suppose that that's true. So what anthropologists and biologists have been looking at is what kind of process might be a better explanation for how that transition would have occurred.

And what we're coming up with is the idea that actually the wolves domesticated themselves, that when people settled down into permanent villages that some wolves chose to come and live with those people, and as a result of that colonization of the new habitat, that they naturally became this new species that we call a dog.

FLATOW: Now why would it have to become a new species? Why can't a wolf just, you know, learn to live with people if that's what it did?

CROCKFORD: Well, partly because of its own natural inclination to stay away from dangerous predators. And, of course, you have to remember that that's what people were at that point in time. They were hunters, and they were hunting big game animals like mammoths and horses and bison. And they were actually in direct competition with the wolves of that time period. And wolves, almost certainly, would have challenged people, try to come and claim a carcass of a mammoth for themselves, and people would have - had to drive them off. And they would have killed them, I'm sure, if they could have.

FLATOW: So to live peacefully with people, the wolf would have to become more peaceful is what you're saying.

CROCKFORD: Well, I mean, peaceful is one way to put it. And the other - some of the other aspects that need to change in terms of behavior are the fearfulness, the - and the stress response. They need to not run from people when they get close because it's actually - getting close to people means that they can go in an scavenge some of the waste that is laying around.

FLATOW: Mm-hmm. And as far as the skull getting smaller...

CROCKFORD: Well, what we know from experimental evidence is that some of these kinds of changes are the result of growth rates shifting between the wolf and the dog, and that almost certainly happens while the puppies are still embryos. So while they're still inside the mother, they're being exposed or they're growing at a slightly different rate than the wolf ancestor is.

FLATOW: Mm-hmm. So we find evidence that they have evolved before the last ice age. We have this big ice age, this giant gap in history. And now, do they have to then be domesticated all over again?

CROCKFORD: Well - and that's what it looks like, is that this - whatever the process that's going on, is it - was happening from Western Europe, all the way across to Siberia before the ice age. The ice age put an end to that because people had to move away and start doing different things. After the ice age ended people settled down again, and the process started up. So it means that whatever this specimen was, it was maybe in the first few generations on the way to becoming a dog, it didn't make it all the way, and it certainly is not an ancestor of modern dogs.

FLATOW: It's not.

CROCKFORD: No.

FLATOW: Can we get any DNA samples out of these things?

CROCKFORD: Possibly.

FLATOW: Yeah.

CROCKFORD: That's one of the areas of further research that we're going to try and get into.

FLATOW: And where would you look for more fossils that would be the, you know, those missing ears here?

CROCKFORD: Well, we would need to look in an area of Eurasia that wasn't covered in glaciers during the ice age.

FLATOW: And are there people...

CROCKFORD: And so that pretty much puts us into, say, southern - or most of Siberia, actually, and China. China's a very fertile area actually, possibly specimens showing up there. There hasn't been a lot of work done, and there's a very good chance that there could be some quite old early dogs showing up in China.

FLATOW: Mm-hmm. If wolves were evolving to become more domestic, were there other animal relationships evolving with humans at the same time?

CROCKFORD: At the same time as dogs?

FLATOW: Yes.

CROCKFORD: No, not really. It really seems to have been pretty much sequential. It - especially in those early days. And so, the next one after the dog, a little bit of contention there, but it looks like perhaps the pig.

FLATOW: Uh-huh.

CROCKFORD: And then goats and sheep.

FLATOW: When did the housecat come into?

CROCKFORD: Oh, it's quite late. It's - it doesn't come in until around the Egyptian period, maybe, you know, four to five thousand years ago. So - and it - again, if you think about even the way cats were used or the way that they were living even 50 years ago in rural areas that they were being attracted by the rodents that were living off the stored grain. And so that brought cats in from the wild.

FLATOW: Were they - did they look different than they would have today?

CROCKFORD: Not very much. Cats actually haven't changed as much from their wild ancestors have - as dogs have, and that has to do with this business of how the growth rate changes and how it changes the length of the snout. A wolf has a much larger snout than a cat does, so there's more room for change.

FLATOW: Very interesting. Let's go to the phones. Elle(ph) in Los Gatos, California. Hi, Elle.

ELLE: Hi, yes. I feel that wolves - it's all through food and need in the dead of winter when food is scarce. The wolves need food, and the people thousands of years ago were more in tune with nature and would put food out for them. And little by little, they became more gentle and docile because they trusted. There was a mutual trust. And I've seen this in today with friends who have put cat food out for wild skunks and wild raccoons. And over the years, with many generations, these animals have become quite domesticated and quite happy in their homes. So it's always a need. It's a mutual need that I feel really is how it comes about.

FLATOW: All right, Elle, and thank you for calling. 1-800-989-8255 is our number. This is SCIENCE FRIDAY from NPR. I'm Ira Flatow, talking with Dr. Susan Crockford. What do you think of that? Do you think that might be a mechanism for...

CROCKFORD: Well, she raises an interesting point, and that is that there really have to be some kind of need on the part of the wolf. And one of the things that, I think, comes into play here is that there may have been instances, especially after the ice age, where wolves may have been stressed. There might have had some food shortages going on, in which case it would've made all the waste products that people just left lying around - I mean, they didn't have to put it out for them especially. It was just there, and that would've been especially attractive. So it really raises a point that it, you know, it was beneficial for both. The wolves got to eat, and the people got to have their campsites and their villages cleaned up.

FLATOW: Let's go to Dustin in San Antonio, Texas. Hi, Dustin.

DUSTIN: Hi. How are you today?

FLATOW: Hi, there.

DUSTIN: Thank you. As I understand it, wolves and dogs can successfully interbreed and produce multigenerational successful offspring. Are they still considered a different species? And if so, it's not a bit arbitrary considering the differences seem to be mostly phenotypic. Thank you.

FLATOW: Uh-huh. Good question. Dr. Crockford?

CROCKFORD: Well, that's an issue that has been contentious for a little while, and part of that comes from the idea that animals cannot interbreed if they are indeed true species. They can't interbreed and produce fertile offspring that go on to breed for another generation. And one of the things that genetics has shown us is that very many more real species can hybridize than we have ever been aware of. And what's important to keep in mind is that while interbreeding is possible, it doesn't happen on a regular basis. And so at that level, it really doesn't challenge or put in jeopardy the species as a whole.

FLATOW: Uh-huh. So we're changing out views then about how this might have...

CROCKFORD: Well, I think so. And it - one of the things that's happening is that - one of the arguments is that wolves and dogs are very close genetically and that for - so perhaps we should actually consider them just the same species but a different form, a different subspecies perhaps. But, in fact, what we're learning is that there are very many species who are just as close genetically, and we would not consider them to be members of the same species. So it really is putting more emphasis, I think, back on the differences in form, the body form and the behavior and the physiology, and those differences between them that keep them separated. And that is raising an importance; the genetics, not so much.

FLATOW: Dr. Crockford, thank you for taking time to be with us today.

CROCKFORD: It's been a pleasure.

FLATOW: Have a good weekend. Dr. Susan...

CROCKFORD: You, too.

FLATOW: Dr. Susan Crockford is an evolutionary biologist and adjunct professor at the University of Victoria in Canada, and she co-authored the study on the skull published in the Public Library of Science.

(SOUNDBITE OF MUSIC AND SHOW CREDITS)

FLATOW: Have a great weekend. I'm Ira Flatow in New York.

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