Nature Podcast：恐狼的神秘灭绝学术资讯

来源：Nature自然科研

又到了每周一次的 Nature Podcast 时间了！欢迎收听本周由Benjamin Thompson 带来的一周科学故事，本期播客片段里讨论了体型庞大，已经灭绝的恐狼。欢迎前往iTunes或你喜欢的其他播客平台下载完整版，随时随地收听一周科研新鲜事。

音频文本：

Host: Nick Howe

America was once home to a massive species of wolf known as the dire wolf. They could reach nearly 70 kilograms, and they shared the land mass with several other now extinct mammals up until the Late Pleistocene when they all but vanished. Luckily there were lots of them and so they left an extensive mark in the fossil record. For instance, there have been thousands of them excavated from the famous La Brea tar pits in California alone. But why they went extinct and how they’re related to their modern wolf family is still up for debate. Along with his team, Laurent Frantz from Queen Mary University of London and the University of Munich turned to ancient DNA, recovering dozens of partially fossilised remains – subfossils as they’re known in the business – providing a clearer story of this ancient American predator. Reporter Geoff Marsh found out more.

Interviewee: Laurent Frantz

We’re talking about a time at which the climate was on average colder than we are now, with quite a few glacial periods where a large proportion of North America would have been covered by a very thick ice sheet, and everything that was living south of the Canadian-American border would have been living in a sort of dry and relatively warm environment. We have multiple predators around – the American lion and you have these sort of giant short-faced bears as well – but really the most common and the one that is the most ubiquitous is the dire wolf. And then later on, the grey wolf and the cayote, we don’t know exactly when but they were there before the dire wolf went extinct for sure, and probably tens of thousands of years before dire wolf and other mega-carnivores like the lion and the bear went extinct. So, I’m Laurent Frantz. I’m a professor of palaeogenomics at the University of Munich and a senior lecturer at Queen Mary University of London.

Interviewer: Geoff Marsh

So, I guess the big mystery is that we know for a time that dire wolves were sharing America with those other canids like the grey wolf and the cayote, but somehow the grey wolf kind of ended up as this top predator and the dire wolves ended up in evolution’s waste basket.

Interviewee: Laurent Frantz

I mean, in the end, things have reversed a little bit. Now, the grey wolf is almost extinct from North America and the one that’s found everywhere now in North America is the cayote, the smaller version that was maybe the outsider for a long time and seems to have come out as the winner in the end.

Interviewer: Geoff Marsh

Okay, so, really, what you’re interested in is how these dire wolves are related to these other wolf-like animals.

Interviewee: Laurent Frantz

So, for a long time, people have been looking at the morphology because this is all we had, and when you look at the morphology of a dire wolf, it’s just very strikingly similar to a wolf, and you can use fancy statistics and sort of 3D modelling of their skulls and put their skulls through scanners and compare the skulls to all these other canids, and when you’re comparing the skull’s teeth or other elements to other species, there’s a striking relationship between grey wolves and dire wolves. They look almost identical. It’s not really a shape difference but just a size difference, and that’s what led people to think they just are a species or subspecies. That’s what you see from a morphological perspective.

Interviewer: Geoff Marsh

Sure, but to properly resolve the story, you and your crew turned to ancient DNA, didn’t you?

Interviewee: Laurent Frantz

So, we turned to ancient DNA and also ancient proteomics. There’s not just only DNA that survived in these subfossils, also the proteins survived. It stays a lot longer than DNA and in fact, we managed to get some of these from the La Brea tar pits which are highly decorated material in which there is no DNA whatsoever. So, that gives you also an answer, and the answer we got from that specific sequence was that yes, they were different from grey wolves but it was really difficult to say more than that. You need genomes for this. So, we had 46 subfossil specimens from places like Tennessee and Wyoming. Of those 46, we only identified 5 that actually possessed enough DNA that we could sequence and reconstruct parts of their genome. We also sequenced genomes of African relatives to make sure that we had the genomes of all the closely related species, and then you use an algorithm to construct a tree, and then you could say, well, the dire wolf was actually extremely far away from the grey wolf. You can also use various methods to try to estimate how long ago were their ancestors living, so how long ago did they separate into two different species, and it was millions of years ago, and that was extremely surprising.

Interviewer: Geoff Marsh

What was it, 5 million years ago?

Interviewee: Laurent Frantz

5-7 million years ago, about. A large confidence interval, but still, a long time ago.

Interviewer: Geoff Marsh

And does that locate where the dire wolf originated?

Interviewee: Laurent Frantz

It does to some extent. It allows us to sort of think about what we call a biogeographic model. We think that the dire wolf was part of this lineage that was in America for millions of years, and most of its evolution took place in the Americas as opposed to all of the other living canids that live nowadays, even those that are in the Americas nowadays.

Interviewer: Geoff Marsh

So, like the cayote, the grey wolf, they came later, quite recently.

Interviewee: Laurent Frantz

They came later, much more recently, yes.

Interviewer: Geoff Marsh

You mentioned earlier, though, didn’t you, that the canids themselves, these wolf-like animals, they are kind of well-known for their interbreeding and their hybridisation, aren’t they, so how do we know that wasn’t going on in the dire wolves?

Interviewee: Laurent Frantz

So, with genomes, you can actually test this, and so we could do the same thing with the dire wolf, and when you’re looking at the genus Canis, which is most of the canids that we think of today except not foxes, but cayotes, grey wolves, the dhole in southeast Asia but also the African wild dog and the jackals, there is gene flow all over the place. It seems like canids interbreed, they will separate into species, stay away for a few million years and then they’ll meet again somehow and then interbreed again, and then we see a lot of lineages that are almost equal hybrids between two species. So, what we were expecting was basically, okay, if you overlap with other canids for a long time, there will be some gene flow, and it turns out that the dire wolf actually didn’t really interbreed with either grey wolves or cayotes, which was also extremely surprising, even though they’re morphologically so similar.

Interviewer: Geoff Marsh

Well, one thing we can be quite certain about is that the dire wolf, along with lots of other megafauna, massive animals from the end of the Pleistocene, all went extinct. What does your genetic analysis do for our understanding of perhaps why the dire wolf didn’t make it through and yet the grey wolf and the cayotes did?

Interviewee: Laurent Frantz

It seems that there is one rule for this Canis species, one very important rule in their evolution is mix up with whatever is living there. So, if you arrive in a new environment then you can mix up with the species that live there, borrow a few genes and a few behaviours and sort of adapt yourself faster, or you can also do that with a new species coming in when there are changes in the environment. What we think maybe is happening is that the environments were changing quite rapidly at the end of the Pleistocene and so the dire wolf wasn’t able to actually adapt fast enough and it wasn’t able to adapt fast enough potentially because it wasn’t able to borrow these genes from these incoming species. And actually, it seems like the grey wolf has been able maybe to survive in some parts, like in Yellowstone Park where we see black grey wolves. These black grey wolves probably acquired this black coat colour through interbreeding, in this case, with dogs that came even later with humans, and that black colour seems to have been highly beneficial for them and maybe allowed them survive longer. So, the dire wolf didn’t have this potential mechanism. It seems to have broken maybe rule number one of surviving as a Canis species.

Interviewer: Geoff Marsh

Yeah, so they were these kind of really specialist top predators, maybe too specialist on these big prey, and because they stayed so isolated and didn’t mix up their genetic toolkit, they were stuck when things changed around them maybe.

Interviewee: Laurent Frantz

They were definitely too specialised. We have a lot of evidence for this. They were clearly big, they were clearly morphologically made for attacking large prey, so if large prey disappeared then that’s it. But you could think that maybe someone of your genes could have been surviving in these grey wolf populations that were living afterwards and, in a way, they would not have gone completely extinct the way they are now.

Interviewer: Geoff Marsh

One thing I thought was does the fact that they’ve been genetically isolated and yet they have this striking morphological resemblance of current wolves, does that say anything about the body plan of a wolf, because it strikes me that evolution has gone, ‘This is a brilliant, lethal blueprint. If it ain’t broke, don’t fix it.’ You hear that about the crocodilians, don’t you? They’re body plan hasn’t changed much for millions of years.

Interviewee: Laurent Frantz

So, yeah, this is something that we’ve been asking ourselves. Are we looking at a convergence between a grey wolf and a dire wolf, or are we actually looking at the ancestral form, right? This is this sort of form that has been around for a very long time because it’s so efficient. So, yeah, I think it speaks for that body plan as something really adaptive. But I think the grey wolf had something else. What’s interesting about the grey wolf is that it’s extremely plastic. They have what we call ecotypes. Some of them hunt bisons and some of them hunt rabbits, and they walk in teams or they are more solitary, they are widely different sizes and have widely different behaviours. It’s an incredibly plastic and sort of flexible species, so this plus that sort of killing body plan I think make it like an almost indestructible species.

Host: Nick Howe

That was Laurent Frantz. For more on those ancient apex predators, we’ll put a link to the paper in the show notes. And thanks to Yellowstone National Park’s sound library for some of those canid recordings you heard throughout the story.

《自然》论文：

Dire wolves were the last of an ancient New World canid lineage

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