Who let the wolves in?

Dogs were the first domesticated animal in history, emerging from wolves some 20,000 years ago. But how did wolves become dogs? To find the answer, scientists have to play with a lot of puppies. (First published in 2023.) Guest: Kathryn Lord, evolutionary biology researcher at UMass Chan Medical School For show transcripts, go to vox.com/unxtranscripts For more, go to vox.com/unexplainable And please email us! [email protected] We read every email. Support Unexplainable by becoming a Vox Member today: vox.com/members Learn more about your ad choices. Visit podcastchoices.com/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:01Noam Hassenfeld?

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• 0:37Speaker 01

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• 1:08Speaker 02

  Oh, hi.

• 1:11Speaker 03

  Catherine Lord has a job that makes me jealous.

• 1:14Speaker 02

  You're just being silly.

• 1:15Speaker 03

  She plays with puppies for a living.

• 1:16Speaker 02

  Hi, buddy. How are we?

• 1:20Speaker 03

  These are wolf puppies. Catherine sent me some audio she recorded during her research, and you can hear one of the puppies climb into her lap. He's wagging his tail and wriggling around, and he can't sit still.

• 1:34Speaker 02

  I know. Is he taking up the whole lap? Is he taking up the whole lap?

• 1:39Speaker 03

  At eight weeks old, he's small enough to fit in her lap, but he looks like a wolf, with a pointy snout and sharp ears.

• 1:47Speaker 02

  You're okay. You're okay. There you go. Good boy. You can settle down. There you go. There's your good boy.

• 1:58Speaker 03

  Katherine isn't playing with wolf pups just for fun. As an evolutionary biologist, she's fascinated by what separates wolves from dogs. On one hand, they're very closely related. They share 99.9% of their genetics. They share an ancestor, and they can even mate and have fertile offspring together. But they're still very different animals. She told me you can even see that in a wolf puppy.

• 2:21Speaker 04

  They look more like tiny bears than tiny dogs. But they also have a distinctive smell. They have more of a little bit of a musky smell, which is also very pleasant, but very mildly skunky is the closest thing that I could think of, but in a nice way.

• 2:41Speaker 03

  But it's not just the way they look and smell. Wolves have a wildness in them that never really goes away.

• 2:46Speaker 04

  They also still have all of their natural hunting behaviors, which dogs don't have. So if I, for example, had a sore shoulder, I wouldn't go in with the adult wolves, even if I had raised them, even if they knew me really well, because it could trigger their hunting behavior.

• 3:01Speaker 03

  Wolves tend to notice if you're hurt, and they might attack you, which is a kind of behavior that would be pretty concerning to find in a dog.

• 3:08Speaker 04

  And in the wolves, everything you greatly fear seeing in a dog pup is totally normal. And so you're like, oh, my God, oh, no, it's a wolf. This is totally OK.

• 3:21Speaker 03

  Unlike wolves, dogs aren't afraid of strangers or other species like us. And it doesn't take long for them to make friends.

• 3:28Speaker 04

  It's kind of a superpower in the dog. It takes about 90 minutes total between the ages of four and eight weeks for them to start soliciting attention or to be like, hey, look at me, from another species.

• 3:41Speaker 03

  But with wolves, for them to even tolerate being close to us, you have to have 24-hour contact with them for weeks on end when they're really young.

• 3:50Speaker 04

  Basically, as long as you can stand sleeping with them, at some point, they start biting you in the ears. If you don't sit up fast enough, and you hear this wonderful noise, this little...

• 4:04Speaker 04

  And then they chomp you in the ear and you're like, oh! You don't sit up real fast.

• 4:10Speaker 03

  So we've got all these differences between wolves and dogs. But the bigger question isn't the what, it's the how.

• 4:18Speaker 04

  How did those differences evolve in the first place?

• 4:22Speaker 03

  For one, it's just a fun thing to think about.

• 4:25Speaker 04

  I think it gets a lot of attention because we think dogs are cool.

• 4:28Speaker 03

  But there's a bigger potential prize here. Catherine says wolves became dogs because of humans, because of us. So figuring out exactly how this happened could teach us how other animals might be able to survive in a world that we're drastically changing.

• 4:44Speaker 04

  A better understanding how this might have happened long ago, historically, evolutionarily, might give us a better understanding also to how animals and plants and such today might be able to or not able to adapt to us.

• 5:00Speaker 03

  I'm Manning Wint, and this week on Unexplainable, how did we get the friendly and loyal dogs we know and love today from wolves?

• 5:28Speaker 03

  There's a story scientists tell about how wolves became dogs, but it's incomplete.

• 5:33Speaker 04

  So we think dogs evolved from an ancestral wolf. So something very wolf-like that is no longer around today, it's extinct. And it gave rise to modern wolves and dogs.

• 5:45Speaker 03

  We don't know exactly what these ancient wolves looked like or acted like, but we do know that both modern-day wolves and dogs come from them. Because scientists have looked at the genes of all sorts of dog-like animals, including jackals and coyotes, and they've mapped out how they're related.

• 6:01Speaker 04

  We can create these sort of family trees, these phylogenetic trees, based off of their genes to see where they branch off from that ancestor.

• 6:14Speaker 03

  Evolution often happens at the scale of millions of years. But the leap from wolf to dog seems to have happened pretty quickly.

• 6:22Speaker 04

  We're talking maybe thousands, tens of thousands of years, not millions or hundreds of thousands of years.

• 6:29Speaker 03

  This split happened around 20,000 years ago, probably.

• 6:33Speaker 04

  Every time somebody comes out with a new paper, we have a new date. Some people think it happened more than once. Some people think it only happened once.

• 6:40Speaker 03

  And scientists aren't just confused about when it happened.

• 6:43Speaker 04

  We really don't know where it happened. We have pretty much no idea, except we're pretty sure it wasn't North America.

• 6:50Speaker 03

  But the most important question here isn't where or when, it's how.

• 6:55Speaker 04

  There's a couple of main hypotheses out there for how it happened. They can be split into two big categories. One category is probably the older of the two, is that humans took wolf pups, ancestral wolf pups, out of the den and raised them by hand and purposefully bred them to be what we now think of as dogs.

• 7:23Speaker 03

  If this is true, dogs might have been a domestic invention, something our ancestors did intentionally before agriculture, before writing was invented, which is really impressive. But this would have meant raising wolf pups when they were really young. They would have been barely walking, not eating meat yet, still drinking milk. Because any older than that, and they'd just be uncontrollable.

• 7:47Speaker 04

  So the pups would need to be less than three weeks old when they got them. If they're less than three weeks old, they need to be bottle-fed, but there wouldn't have been any bottles. So people suggest they would have been nursed by human women. This sounds horrible to me, because they get teeth fairly early and they bite when there's not enough milk.

• 8:10Speaker 03

  And milk is just one of the issues here.

• 8:14Speaker 04

  Once they're through that, they would need to feed them meat from their own resources and get them up to adulthood, hopefully without them killing any of their children while they're at it, because they still have all their hunting behaviors. Then they would have to pick the friendliest wolves from that group to breed together, and somehow they'd have to go through the whole process for many, many generations to get this to work.

• 8:43Speaker 03

  Katherine doesn't love this hypothesis. It seems so unlikely. Our hunter-gather ancestors would have had to consistently keep a group of these dangerous wild wolves around them and make the nicer ones breed together. And it's not as if they would have known that they'd get dogs on the other side. She thinks the more likely scenario is that wolves chose to come closer to us because we had something that they wanted. They were lured to us because of our garbage.

• 9:10Speaker 04

  You had wolves hunting in their normal environment. And they came upon a large amount of garbage and refuse and decided to see if there's anything tasty in there. And got some leftover something that they found acceptable to eat. Whenever people showed up, they probably ran away and so they didn't get a lot of resources out of that. But those animals that were able to stick around for a little bit longer did better scavenging out of those garbage piles.

• 9:50Speaker 03

  Something about these wolves made them just a little friendlier and more willing to get closer to us. And these are the ones that would get a little bit more food every day.

• 9:59Speaker 04

  Then, over time, through natural selection, you'd basically get those animals that were better able to stick around and eat that garbage without having to run really far away doing better. And so you'd have selection on surviving in this new environment, which would eventually lead us to dogs over time.

• 10:26Speaker 03

  Katherine says this theory makes sense because there's an obvious benefit to the wolves here. If they found our waste as a source of food, they'd keep eating it. You can see this kind of thing happening today. All sorts of animals see trash as a source of food, including dogs.

• 10:40Speaker 04

  Most of the listeners will probably think about pet dogs, working dogs, dogs that are directly in contact with humans. But we think there's about a billion dogs on the planet, and upwards of 83% of them are living in garbage dumps. If I think about that as the environment they adapted to, they're just like a lion on the Serengeti. Most of them weren't stray. They were born there. They adapted to that and do so well, we can't get rid of them.

• 11:05Speaker 03

  But scientists don't know for sure whether humans domesticated wolves or if wolves did it themselves.

• 11:12Speaker 04

  Right now, we have no way to definitively say one of those two hypotheses is correct because we don't have any way to go back and talk to the people and or look directly at it.

• 11:28Speaker 03

  So Catherine's not just looking at the past, but the present. She wants to figure out exactly what makes wolves and dogs different today, where these differences come from biologically. If she can pinpoint the biology, then maybe scientists can explain how we got the dogs we know today.

• 11:50Speaker 03

  And that's where the wolf puppies come in.

• 11:58Speaker 03

  That's after the break.

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• 15:12Speaker 01

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• 15:44Speaker 06

  You know, one time this wolf I saw- Wolf? What's with all the wolf talk? Can we give it a rest for once?

• 15:56Speaker 03

  We can't go back in time to see how wolves evolved into dogs, but we can get some clues to better understand how it could have happened by comparing dogs to modern-day wolves. This is what Catherine's doing. She's working with wolf puppies to understand their behavior and pinpoint precisely how dogs and wolves differ today.

• 16:15Speaker 04

  If we can't go back in time, what we can look at now is what are the differences we see in dogs that make them special? And then, how did those differences evolve in the first place?

• 16:28Speaker 03

  The biggest clue to understanding how you get really different behavior in such genetically similar animals might have to do with their first few weeks of life. So, wolf puppies.

• 16:43Speaker 03

  When they're really young, dogs and wolves basically act the same. But by about a month, they're completely different.

• 16:49Speaker 04

  So, the brain is developing very rapidly. And what they experienced during that time has a really big effect on their adult behavior.

• 16:58Speaker 03

  This window of time is called the critical period of socialization.

• 17:04Speaker 03

  It's when wolves and dogs aren't afraid of new things yet. And this is the prime time to get puppies used to humans.

• 17:11Speaker 04

  So the beginning of this critical period is the ability to walk and explore. And so in dogs, we think that happens around four weeks of age. At that point, they're pretty much not frightened of new things. They'll just kind of bumble around. If Godzilla showed up, they'd be like, oh, what's this?

• 17:26Speaker 03

  So say this dog encounters a human for the very first time. They're going to register this human smell and hear their voice and see their arms and legs and face, all in one go.

• 17:37Speaker 04

  When they're taking in all that information, they can take it in through all of their senses. And so they get a very full picture of the world.

• 17:45Speaker 03

  Dogs learn to love us pretty quickly. But after wolves go through this period, they end up very different. Catherine has a guess about what makes this happen, but it's subtle. She thinks wolves might go through this critical period of socialization just two weeks earlier than dogs do. And when wolves start going through this period, they can only smell. Sight and hearing turn on a little bit later, one at a time.

• 18:11Speaker 04

  And so what would happen then is they'd have a really good olfactory picture of the world. But as they go through this critical period, they're sort of layering on information about what it sounds like and what it looks like, which is not as solid a picture of things.

• 18:27Speaker 03

  So say a wolf puppy is just starting this period and they meet a human. They register the human smell. And once they're used to it, they might just start liking us.

• 18:37Speaker 04

  But then they start hearing and we sound really scary, but they compare that to what we smell like. And then they start seeing, we look really scary, but they compare that to what we sound like and what we smell like and keep going. So basically, the whole thing would get interrupted by the development of the senses.

• 18:54Speaker 03

  Dogs also develop their senses one by one. But their critical period of socialization starts later, when they have all their senses already turned on. But Catherine thinks wolves start this period earlier, when they can only smell. So they have a more jarring experience.

• 19:11Speaker 04

  And this would really explain a lot of those behavioral differences. It would explain why dogs are so easy to socialize because they have a full picture of a human during that time period. And so they just need to get an idea of what it sounds, smells, and looks like. And then moving forward, they're good to go on any of those things.

• 19:33Speaker 03

  Katherine thinks this difference of two weeks, this tiny developmental shift, could be the root of what separates dogs from wolves. But it's still just a hunch. So she's trying to figure out exactly when this period starts and ends.

• 19:48Speaker 04

  What we're interested in finding out is, when do dogs and wolves start walking? And when do they avoid novelty?

• 19:57Speaker 03

  To find when the critical period starts, Katherine's putting accelerometers on puppies that track their movement.

• 20:03Speaker 04

  Hi. We put a little harness on the wolves, and it looked like they were going to their first day of kindergarten. They look like they have these little backpacks on. It's kind of ridiculous.

• 20:16Speaker 03

  And to measure when the critical period ends, she's trying to figure out when puppies start being afraid of new things. So she puts wolf puppies in a little arena with an object that they've never seen before.

• 20:30Speaker 04

  And so then we can compare the dogs and the wolves and how much fear they show, how much time they spend with the object, and when they go, yeah, no, I don't want to check out that novel object anymore.

• 20:39Speaker 03

  After all these experiments, if this two-week difference proves to be true, the story of how wolves became dogs could be something like this.

• 20:50Speaker 03

  Something like 20,000 years ago, ancient wolves may have come across our roving camps and garbage. Most of them would have been hyper cautious of us, and they would have stayed away. But there might have been some wolves that risked getting closer and got more scraps. There could have been something biologically different about these wolves. Specifically, they might have started their critical period of socialization a little bit later.

• 21:14Speaker 04

  So if that difference was a slight change in the critical period, those animals would do better, be more likely to reproduce, be more likely to have offspring. So those offspring would then inherit that slightly different critical period and experience their life maybe a bit closer to people.

• 21:32Speaker 03

  Generation after generation, there'd be incremental shifts in this critical period. Until, one day, you'd have these puppies that start this period when they could see, smell, and hear the world.

• 21:44Speaker 04

  And boom, you'd get this giant change.

• 21:48Speaker 04

  Where those animals were now animals that could easily socialize with people, easily generalize, easily be OK with novelty.

• 21:55Speaker 03

  And this shift might have set the stage for them to eventually evolve into dogs.

• 22:00Speaker 04

  It's something I find endlessly exciting, that you could have just a two-week shift in something very early on and get this huge behavioral difference, just entirely different animal at the other side.

• 22:17Speaker 03

  This is still only a guess, just one possible story for what could have happened tens of thousands of years ago. So Catherine not only has to figure out if that two-week shift between wolves and dogs exists, she also has to figure out what's going on in their DNA. She's currently working with geneticists to find what genes are involved in this critical period.

• 22:39Speaker 04

  Eventually, if we can figure out genes or genetic mechanisms that are tied to this shift, and we could look at ancient remains and say, OK, do they have these differences that we are associating with domestication? That might help us get down to the question of where and when and how. That will be in the distant future, but eventually it might help us answer that question.

• 23:07Speaker 03

  Catherine's work is foundational, and it'll help us get clearer on these bigger questions. But for her, studying this evolutionary story goes beyond dogs and wolves. If dogs evolved because of our literal piles of garbage, there's something to learn about how any animal adapts to the changes that we've made in the natural world.

• 23:27Speaker 04

  If we look at all life on the planet right now, it's having to deal with our disturbances, with our changing ecosystems. And some animals are doing really well with that, while other animals are doing really poorly with that and going extinct and endangered and not able to keep up with this pace. So a better understanding how this might have happened long ago, historically, evolutionarily, might give us a better understanding also of how animals and plants and such today might be able to or not able to adapt to us.

• 24:03Speaker 03

  Humans are deeply involved in the evolution of other animals, whether we want to be or not. And in a way, we did create dogs. We welcomed these ancient dogs into our homes, and now we can't imagine our lives without them. But it probably had less to do with our intentions than it did with the craftiness of evolution.

• 24:23Speaker 04

  If we think about dogs as something we created, we tend to think of them as something degenerate, something that we made, something that we were under control of. I find it a bit egotistical that we think we could have somehow been like, aha. Heck, before we knew what domestication was, we went, you know what would be great? If we had this thing that could herd sheep and bark at people and sleep at my feet in front of the fireplace. What's a fireplace? I don't know. Instead of this really fascinating evolutionary story that a byproduct of our existence is garbage and refuse. It just is. We create a lot of stuff that animals and plants and microbes then take advantage of.

• 25:06Speaker 04

  And a lot of animals survive off of it today, including dogs. We are not separate from evolution. We like to think of ourselves as something special, above. We are just something else in nature that's happening, and evolution acts on that, too. Life just keeps adapting.

• 25:37Speaker 03

  This episode was reported and produced by me, Manning Nguyen. There was editing from Brian Resnick and Meredith Hoddinott. Mixing and sound design from David Herman, music from Noam Hassenfeld, and fact checking from Serena Solan. Special thanks to Eleanor Carlson and the folks at Wolf Park in Battleground, Indiana. If you have thoughts about this episode or ideas for the show, please email us. We're at [email protected]. Unexplainable is part of the Vox Media Podcast Network, and we'll be back next week.

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