The iconic prehistoric red wolf, 'the dire wolf' that roamed parts of the Americas more than 11 millennia ago, was a distinct species from the somewhat smaller gray wolf, an international team of scientists has reported in the journal Nature With information from University of California.
The study, which poses a mystery that biologists have pondered for more than 100 years, was conducted by UCLA researchers, along with colleagues from Durham University in the United Kingdom, the Australian University of Adelaide and Germany's Ludwig Maximilian University.
"The dire wolf, a legendary symbol of Los Angeles and La Brea Tar Pits, earned its place among the many large and unique species that went extinct at the end of the Pleistocene epoch," said Robert Wayne of UCLA, a distinguished professor of ecology and evolutionary biology and co-author of the study. The Pleistocene, commonly called the Ice Age, ended about 11,700 years ago.
More than 4,000 dire wolves have been excavated from the La Brea tar pits, but scientists know little about their evolution or the reasons for their ultimate demise. Gray wolves, also found in the fossil-rich pits, have survived to this day.
"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 co-author Angela Perri of Durham University.
These bones are now revealing much more. Using cutting-edge molecular approaches to analyze five direwolf genomes from fossil bones dating from 13,000 to 50,000 years ago, researchers have been able to reconstruct the evolutionary history of the long-extinct carnivore for the first time.
Significantly, they found no evidence for gene flow between direwolves and gray wolves or North American coyotes. The absence of any gene transfer indicates that direwolves evolved in isolation from the Ice Age ancestors of these other species.
"We found that the dire wolf is not closely related to the gray wolf. Moreover, we showed that the dire wolf never interbred with the gray wolf," said co-leader Alice Mouton, who conducted the research as a UCLA postdoctoral researcher in ecology and evolutionary biology in Wayne's lab.
The ancestors of the gray wolf and the much smaller coyote evolved in Eurasia and are thought to have moved to North America less than 1.37 million years ago, relatively recently in evolutionary time. On the other hand, the dire wolf, based on its genetic difference from those species, is now believed to have originated in the Americas.
"When we started this study, we thought direwolves were just gray wolves, so we were surprised to learn how extremely different they were genetically, such that they probably couldn't have reproduced," said the study's last author, Laurent Frantz, a professor at Ludwig Maximillian University and Queen Mary University in the U.K. "This shouldmean that direwolves have been isolated in North America for too long to become genetically so distinctive."
"Giant wolves are sometimes portrayed as mythical creatures - giant wolves roaming bleak, frozen landscapes - but the reality turns out to be even more interesting," said Kieren Mitchell of the University of Adelaide, a lead co-author.
The direwolf was a "lone wolf" when it came to breeding
Interbreeding is quite common among wolf lineages when their geographic areas overlap. Modern gray wolves and coyotes, for example, are often interbred in North America. However, researchers, using a dataset that included a Pleistocene wolf, 22 modern North American gray wolves and coyotes, and three ancient dogs, found that the wolfterrible had not reproduced with any of the others - probably because it was genetically incapable of reproducing with those species.
"Our finding that there is no evidence of gene flow between red wolves and gray wolves or coyotes, despite substantial range overlap during the Late Pleistocene, suggests that the common ancestor of gray wolves and coyotes likely evolved in geographic isolation from members of the gray wolf lineage," Wayne said. "This result is consistent with the hypothesis thatthat direwolves originated in the Americas."
Another hypothesis about the dire wolf - one not tested in the current study - concerns its extinction. It is thought that due to its body size - larger than gray wolves and coyotes - the dire wolf was more specialized in hunting large prey and unable to survive the extinction of its regular food sources. The lack of interbreeding may have accelerated its disappearance,suggested Mouton, now a postdoctoral researcher at the University of Liège in Belgium.
"Perhaps the wolf's terrible inability to interbreed did not provide necessary new traits that could have allowed it to survive," she said.
Uncovering the mystery of the direwolf DNA
While the direwolves sequenced in this study had no ancestors of gray wolves, coyotes, or their recent North American ancestors, a comparison of direwolf DNA with that of gray wolves, coyotes, and a wide variety of other wolf-like species revealed a common but distant evolutionary relationship.
"The ancestors of dire wolves probably diverged from gray wolves more than 5 million years ago - it was a big surprise to find that this divergence occurred so early," Mouton said. "This discovery highlights how special and unique the dire wolf was."
Based on their genomic analyses, the researchers also concluded that there are three primary lineages that descend from shared ancestry: dire wolves, African jackals, and a group comprising all other extant wolf species, including the gray wolf.
Gray wolves, which now live mainly in wild and remote regions of North America, are more closely related to African wild dogs and Ethiopian wolves than to dire wolves, Wayne noted.
Study is first to report genome-wide data on direwolves
The genomic analyses - conducted in a joint effort at UCLA, Durham University, Oxford University, University of Adelaide, Ludwig Maximilian University and Queen Mary University - focused on both the nuclear genome and the mitochondrial genome, which is abundant in ancient remains.
"The reduced cost of sequencing analyses, plus state-of-the-art molecular biology methods for highly degraded materials, allow us to recover DNA from fossils," Mouton said. "Ancient genomic DNA analyses represent an incredible tool to better understand the evolutionary history of ancient and extinct species."
The study's 49 co-authors also include Blaire Van Valkenburgh, a UCLA distinguished professor of ecology and evolutionary biology who holds the Donald R. Dickey Chair in Vertebrate Biology; Julie Meachen, who earned her Ph.D. in ecology and evolutionary biology at UCLA and is now an associate professor of anatomy at Des Moines University in Iowa; and Colin Shew, a UCLA lab technicianin ecology and evolutionary biology; as well as dozens of other researchers from the UK, Australia, Germany, Russia, Spain, France, Denmark and other countries.
Funding sources for the research included the National Science Foundation, the Office of Naval Research, the Marie Curie COFUND, the European Research Council, the Natural Environmental Research Council, the Wellcome Trust and the Australian Research Council. The Nature article lists many other acknowledgements.