The ancient, extinct dire wolf may have been among the lonest of the wolves - so genetically distinct from its closest wolf relative that it could no longer interbreed, forcing it into an evolutionary dead end when it died out 13,000 years ago.
That's the finding based on a new study, the in-depth analysis of DNA retrieved from ancient dire wolf bones from across North America. Once dire wolves (Canis dirus) diverged from grey wolves millions of years ago, they seem to have never mingled since.
In fact, so different is their genetic lineage from other canids that the research team proposes that dire wolves be placed in another genus completely - that they be reclassified as Aenocyon dirus, as was first proposed all the way back in 1918.
"Dire wolves are sometimes portrayed as mythical creatures - giant wolves prowling bleak frozen landscapes - but reality turns out to be even more interesting," said palaeobiologist Kieren Mitchell of the University of Adelaide in Australia.
"Despite anatomical similarities between grey wolves and dire wolves - suggesting that they could perhaps be related in the same way as modern humans and Neanderthals - our genetic results show these two species of wolf are much more like distant cousins, like humans and chimpanzees."
Dire wolf remains can be found in the fossil record from 250,000 to around 13,000 years ago, and seem to have dominated the carnivore scene during the last Ice Age in what is now North America.
In the famous La Brea tar pits alone, excavated dire wolf individuals outnumber the slightly smaller grey wolf (Canis lupus) more than a hundredfold.
But how they diverged, evolved and ultimately went extinct towards the end of the Last Glacial Period, about 11,700 years ago, has been challenging to piece together. So an international team of scientists set to work on one of the only clues we have: bones.
"Dire wolves have always been an iconic representation of the last ice age in the Americas, but what we know about their evolutionary history has been limited to what we can see from the size and shape of their bones," said archaeologist Angela Perri of Durham University.
But sometimes palaeontological remains can contain other information inside: DNA preserved well enough to be sequenced. And that's what the team investigated.
They obtained five samples of dire wolf DNA from over 50,000 years ago to 12,900 years ago, from Idaho, Ohio, Wyoming and Tennessee, and sequenced them.
Then, they compared them to genomic data from eight canids that are living today, obtained from a genomic database: grey wolf, coyote (Canis latrans), African wolf (Canis lupaster), dhole (Cuon alpinus), Ethiopian wolf (Canis simensis), African wild dog (Lycaon pictus), Andean fox (Lycalopex culpaeus) and grey fox (Urocyon cinereoargenteus).
They also generated new genome sequences for the grey wolf, the black-backed jackal (Canis mesomelas) and the side-striped jackal (Canis adustus).
They found that, unlike other wolves that migrated between regions, the dire wolf stayed put, never straying out of North America.
And, fascinatingly, even though they shared space with coyotes and grey wolves for at least 10,000 years, they never seem to have interbred with them to produce hybrids.
"When we first started this study, we thought that dire wolves were just beefed-up grey wolves, so we were surprised to learn how extremely genetically different they were, so much so that they likely could not have interbred," said molecular geneticist Laurent Frantz of Ludwig Maximilian University in Germany and Queen Mary University in the UK.
"This must mean that dire wolves were isolated in North America for a very long time to become so genetically distinct."
In fact, according to the team's analysis, the dire wolves and grey wolves must have diverged from a common ancestor more than 5 million years ago. When you consider that dogs and wolves diverged between 15,000 and 40,000 years ago, that's a very long time indeed.
Interbreeding between canid species whose territories overlap is quite common. The hybrid of a coyote and a wolf is so common that it has a name - coywolf - and wolf-dog hybrids aren't unknown either (although breeding them as pets is extremely controversial in the US). So for dire wolves to have spent so long in proximity with canids without interbreeding is highly unusual.
And, although the team did not explore this possibility, the genetic isolation could have contributed to the ancient beast's eventual demise, as it was unable to adapt to a changing world with new traits.
"While ancient humans and Neanderthals appear to have interbred, as do modern grey wolves and coyotes, our genetic data provided no evidence that dire wolves interbred with any living canine species," Mitchell said. "All our data point to the dire wolf being the last surviving member of an ancient lineage distinct from all living canines."
The research has been published in Nature.