A new study, co-written by a Texas A&M University professor, is shedding light on the remaining vestige that connects the modern dog to its ancestors from thousands of years ago.
The study, titled "The evolutionary history of dogs in the Americas," dispels the myth that the first American dogs descended from North American wolves; rather, American dogs' complex lineage originates in Siberia. According to the study, man's best friend first arrived to North America about 9,000 years ago. There were three other arrivals of canines on the continent during the past 1,000 years: Arctic dogs brought by the Thule population, European canines and Siberian huskies.
The research appeared in the latest issue of Science, an academic journal from American Association for the Advancement of Science. Anna Linderholm, co-author of the study and A&M anthropology assistant professor, said researchers were not originally looking to discover how dogs were brought to North America.
"Our team started this project to investigate when and where dogs were domesticated," Linderholm said in an email. "It is assumed to have taken place in the Old World anywhere from 36,000 to 16,000 years ago. When looking for samples in museums across the world, [we saw] a lot of American dogs and decided to start sampling them to investigate who brought the dogs to America and when."
According to the study, the arrival of European settlers in the 15th century led to the erasure of native dog populations in North America. The study cites various factors as to why these settlers contributed to a decrease in native dog populations: "Shifts in cultural preferences, the persecution of indigenous dogs" and infectious diseases brought by European canines.
Linderholm said although native canine populations can be traced to Siberia, many of the dogs people see today in North America have greater resemblance to canines brought over by Europeans.
"We also suspect that the waves of dogs that came with the European settlers and the Thule population [were] stronger and more equipped for the new land and work that needed to be done," Linderholm said.
The only viable trace connecting modern dogs to native dog populations is a cancer known as canine transmissible venereal cancer, according to the study. Linderholm said it's likely the cancer also contributed to the decrease in native dog populations.
Heather Wilson-Robles, an associate professor from A&M's department of veterinary medicine and biomedical sciences, said the cancer is not particularly common within the U.S.
"[Transmissible venereal tumor] is a tumor that's been around for thousands of years," Wilson-Robles said. "It's certainly not a new thing. As oncologists, we've been treating it for decades. This particular tumor is not particularly common in this area. It tends to be common in very rural areas or areas with a large feral dog population like Caribbean Islands [or] kind of closer to the equator."
The cancer is mainly spread through mating practices, Wilson-Robles said, and dogs that are spayed or neutered are not susceptible to getting the disease. Wilson-Robles, who has treated dogs with this specific tumor, said the disease is usually treated through doses of the chemotherapy drug Vincristine.
"One of the really nice things about this particular tumor is that it's very easy to treat. We usually will cure it," Wilson-Robles said. "Five, six [or] seven doses of Vincristine is enough to cure most dogs with this tumor even if it spread beyond the primary site or metastasized."
Wilson-Robles said it's intriguing that the tumor has withstood the test of time.
"It's interesting because there's two, maybe three, species on the planet that have a transmissible tumor, and dogs are one of them," Wilson-Robles said. "There's all kinds of immune system interactions that have to be bypassed for this tumor to grow. ... This is sort of an immortalized tumor cell that's basically the same as the original dog that lived 9,000 years ago."
Linderholm said what she mainly wants people to draw from the study is how correlated the evolution of dogs are to human migration.
"Whenever people [moved], they took dogs with them," Linderholm said. "They have been a constant companion to us humans for at least 16,000 years [and] truly a man's best friend."