Jan. 13 (UPI) -- To find an ancestor shared by the dire wolf and gray wolf, one must trace the branches of the Canidae family tree backs some six million years.
That was the last time a relative of the dire wolf and a relative of the gray wolf got together.
The revelation was made possible by a tireless effort to extract and analyze DNA from dire wolves. Those efforts, detailed Wednesday in the journal Nature, recently -- finally -- yielded results.
The gray wolf is the only wolf species still living on the North American continent.
The dire wolf, however, long extinct, is one of North America's famous prehistoric carnivores -- and was made more famous in the "Game of Thrones" books and blockbuster TV show.
But until now, scientists weren't sure of the relationship between the two species.
Since paleontologists first began sifting through the remains of Ice Age megafauna preserved in Los Angeles' La Brea Tar Pits, some 4,000 dire wolves have been unearthed.
All that fossil evidence had failed to provide much clarity to the question of the dire wolf's evolutionary origins.
Bones and skeletons can offer insights into what wolves looked like, as well as where and how they lived. But to understand the species' evolutionary origins and its relationship to the gray wolf, scientists needed DNA.
"Because the dire wolf tended to live in temperate and tropical environments it's much harder to get well preserved DNA from their remains compared to animals that live in high-latitude or high-altitude parts of the world," study co-lead author Kieren Mitchell told UPI in an email.
"Lower temperatures are much better for long term DNA preservation, which is why most of the very old animal DNA you read about comes from species like horses, mammoths and rhinos that lived in cold environments," said Mitchell, an evolutionary biologist at Adelaide University in Australia.
La Brea's sticky pits of tar were ideal for trapping ancient Ice Age specimens, but less ideal for preserving DNA.
Over the last decade, however, scientists have developed technologies to piece together bits of degraded DNA.
These technologies have not only gotten more efficient, but less expensive. Improved computer models have also made it easier to analyze large genomic datasets.
These developments helped scientists finally gain insights into the dire wolf's evolutionary history.
After building a genomic portrait of the dire wolf using bits of DNA extracted from bones dated between 13,000 and 50,000 years old, scientists compared the species' genetic profile with the genomes of North American gray wolves, coyotes and three ancient dogs.
The analysis revealed no evidence of gene transfer -- that is, no evidence of interbreeding.
Until now, most scientists assumed the dire wolf was a very close relative of the gray wolf. The dire wolf looked a lot like a gray wolf, only bigger. But despite living alongside one another for at least 20,000 years, the two species never interbred.
Scientists suspect that -- based on the latest genomic analysis -- the two species evolved in isolation for millions of years before briefly sharing territory at the end of the last ice age.
"Our best guess is that the ancestor of dire wolves arrived in the Americas at least hundreds of thousands if not millions of years ago and stayed," co-lead author Angela Perri told UPI in an email.
"From there, dire wolves arose in the Americas and also stayed," said Perri, an archaeologist at Durham University in Britain.
Gray wolves spread across North America, too, but they seem to have preferred colder environs -- dire wolves, not so much.
"We think that a major factor limiting the distribution of the dire wolf to the Americas was that it was adapted to warm environments," Mitchell said. "This would have prevented it from migrating out of North America into Eurasia, because the Bering Land Bridge -- which connects modern day Alaska and eastern Russia -- is only exposed during Ice Ages."
"The image we get from Game of Thrones of dire wolves roaming around freezing, snow-covered, northern lands couldn't be more wrong," Mitchell said.
Meanwhile, the movements of gray wolves were likely limited by the presence of large ice sheets and glaciers.
Researchers estimate all that time apart yielded too many genetic differences for the two to produce healthy hybrid offspring.
To identify the last time the ancestors of gray wolves and the ancestors of dire wolves had relations, scientists had to go back nearly 6 million years on the evolutionary timeline.
After this surprisingly early split, dire wolves formed their own distinct evolutionary lineage.
The other lineage eventually yielded gray wolves, wild dogs and Ethiopian wolves, all of which are more closely related to one another than they are to dire wolves.
"Dire wolves are unique in that they are the only known species in the genus we propose to move them to, Aenocyon," Perri said. "I suspect after this work other fossils that appear to be ancestral to dire wolves will also move into that genus."
The newly revealed evolutionary separation between dire wolves and other Canidae lineages may explain why the dire wolf suddenly disappeared some 13,000 years ago.
Having evolved in isolation, and with no gene transfer with other canids, the dire wolf was rather set in its ways.
"Dire wolves likely had millions of years to evolve their own specialized behavior and biology that was very different to grey wolves and coyotes," Mitchell said.
At the end of the last ice age, the large mammals that dire wolves preyed upon began dying off. Declines in bison, horse and camel populations left dire wolves with less and less to eat.
"Presumably dire wolves weren't able to adapt successfully to preying on smaller animals -- like deer, rabbits, or even mice -- and couldn't migrate to other areas with more abundant large prey," Mitchell said.
Unlike the dire wolf, other wolf lineages have regularly hybridized, trading genes with one another, over the course of their evolution.
This process can aid the spread of beneficial genes, lending each lineage newfound resiliency against the hardships brought on by climate change and ecological disruption.
But the dire wolf isn't the only wolf to have disappeared from the fossil record. Other wolf lineages, species that were present just a few centuries ago, have also fizzled out. Scientists hope their latest efforts will serve as a model for investigating the fates of other missing wolves.
"Many of these populations have been recently persecuted by humans and some were driven to extinctions over the last 100 years, including multiple subspecies of gray wolves, red wolves and coyotes," study co-author Laurent Frantz told UPI in an email.
"We'd like to know how diverse they were just before people started exterminating them to better understand how much we have lost and how can we better protect these species in the future," said Frantz, a professor at Ludwig Maximillian University in Munich, Germany.