Dire wolves had a burst of newfound fame with their appearance in Game of Thrones, where they were portrayed as a far larger version of more mundane wolves. Here in the real world, only the largest populations of present-day wolves get as large as the dire wolf, which weighed nearly 70 kilograms. These animals once shared North America—and likely prey—with predators like the smilodon, a saber-toothed cat. Prior to the arrival of humans, dire wolves were far more common than regular wolves, as indicated by the remains found in the La Brea tar seeps, where they outnumber gray wolves by a factor of about 100.
Like the smilodon and many other large North American mammals, the dire wolf vanished during a period of climate change and the arrival of humans to the continent, even as gray wolves and coyotes survived. And with their departure, they left behind a bit of a mystery: what were they?
A new study uses ancient DNA from dire wolf skeletons to determine that they weren't actually wolves and had been genetically isolated from them for millions of years.
When it comes to canids, species boundaries and their relationship to anatomy are fuzzy. Domesticated dogs have incredibly diverse morphologies yet are all part of one species and can still interbreed with the gray wolves from which they were derived. Gray wolves and coyotes can also interbreed. So the question of whether gray wolves and dire wolves are closely related, as suggested by their similar appearances, would also hint at whether the dire wolf has made genetic contributions to any current species.
The new work, done by a large, international collaboration (full disclosure: I've been rock climbing with one of the authors) started with the traditional means of trying to answer this question: by looking at the skeletons of dire wolves. But this analysis, which involved over 700 individual skeletons, didn't produce much. Although they differ by enough that the two species could be consistently identified, there were no dramatic differences that suggest a significant evolutionary distance.
So the team turned to more modern methods. It's possible to isolate fragments of collagen, a protein that's a major component of bone and frequently has subtle differences between species. The sequence of dire wolf collagen had enough differences to suggest that dire wolves were distantly related to many other modern dog-like species. But the similarities among all of them were high enough to keep the researchers from resolving the relationships among these species.
Next, the team started isolating DNA from remains and managed to obtain some from five samples, originating from across the United States, from Idaho to Tennessee. The skeletons ranged in age from 13,000 to over 50,000 years old. An analysis of the shorter mitochondrial genome produced results similar to collagen. It indicated that dire wolves were a distinct lineage that was distant from that of wolves and coyotes, but the analysis was confused both by small differences between species and the fact that some lineages (like wolves and coyotes) had interbred.
That left the analysis dependent on the sequence of the regular nuclear genome. Because of the age of the samples, the DNA was very damaged and yielded only a small portion of the animals' full genomes (only from 1 to 20 percent of the genome was obtained, depending on the sample). To fully explore the canid lineage, the researchers also obtained the genome sequence from a North American wolf and two jackals—most other species on this branch of the evolutionary tree had been sequenced.
The tree based on these sequences indicate that the last fox species branched off from the rest of the canids about 7 million years ago. The next branch, occurring about 6 million years ago, produced a branch that includes both jackal species and the dire wolf. Everything else, including African wild dogs, various wolf species, and the coyote, were all on a separate branch of the lineage and are far more closely related to each other than to the dire wolf.
There's a fair amount of uncertainty here, as there typically is in evolutionary sequence analyses; the split between the dire wolf and the other major branch could have occurred anywhere from 4 to over 8 million years ago. But it's clear that, despite their physical similarities, dire wolves are part of a distinct lineage that's only distantly related to dogs.
Given that other canid species seem to have engaged in semi-regular interbreeding, the researchers checked for signs of that here. This analysis showed that there was no clear indication that dire wolves had interbred with either wolves or coyotes, despite sharing a continent for many thousands of years. There is a hint of a possibility that dire wolf ancestors had interbred with the ancestor of wolves, coyotes, and dholes somewhere around 3 million years ago, but the signal for this is somewhat weak.
Overall, this result is interesting as a fact itself. A couple of generations of Dungeons & Dragons players have probably been told that a dire wolf is a really big wolf, and we now know that's wrong. But it's also likely to cause us to go back and rethink some fossils that we've had for years. We have a number from North America that have been interpreted under the view that all the canids on the continent were part of a closely related cluster of species. Now that we know there were two very distinct lineages here, we can go back and try to determine if any of the older fossils are more closely related to one or the other lineage.
Interesting article. Thanks for sharing.