Researchers analyzed the whole genome sequence of the fossils of dogs found in Germany that date to 7,000 and 5,000 years ago, as well as that of a dog that lived about 5,000 years ago in Ireland.
The Irish fossil was has been used in previous studies. The skull of the 5,000-year-old dog was found in 2010 in the excavation of a cave in Bavaria, while part of the skull of the 7,000-year-old dog was found by accident when Trinity College Dublin professor Dan Bradley was screening bones dating to the Neolithic period, or the New Stone Age.
These aren't the oldest fossils of domesticated dogs; one jawbone dating to 14,700 years ago was found in Germany, and older fossils seem dog-like, but lack confirmation.
But they shed light on the fact that domesticated dogs living 7,000 years ago, alongside some of the first European farmers, are the ancestors of the dogs kept as pets around the world today, said Krishna Veeramah, study author and genetics professor at Stony Brook University.
Dogs and humans may have had a similar relationship as far back as 14,000 years ago in hunter-gatherer communities.
About 7,000 years ago, these farming communities had just arrived in Europe, replacing hunter-gatherer societies and creating denser groups of people. This would have probably altered the dogs' behavior as well, as they adapted to be around more people.
Rather than being kept as house pets, these dogs probably roamed close to or within villages. And food-wise, they may have needed to fend for themselves and scavenge, according to Adam Boyko, assistant professor at Cornell University's department of biomedical sciences, who was not associated with the study.
Unlike modern dogs, they wouldn't have yet adapted to eat foods that were rich in starch. But they also wouldn't have looked like wolves. Some of the same hallmarks we associate with modern dogs were present in these Neolithic dogs, like floppy ears and skulls that were smaller than those of wolves.
The 5,000-year-old dog contained an "additional component" more similar to dogs originating from Central and South Asia, and it was found close to skeletons of people from the migrating Indo-European Corded Ware culture during the Neolithic period. The researchers believe this component is due to the herders bringing their own dogs, which then mixed with the local farming dogs.
The researchers were also able to determine that the genome of a modern dog was very similar to the genome of these ancient dogs, further suggesting this single origin.
The fact that these, as well as the oldest known remains, have all been found in Europe suggests that the location is important for future studies as the researchers try to nail down where domestication took place.
Europe also plays a part in the history of domesticated dogs because most of the breeds we know and love today came about due to selective breeding during England's Victorian period.
The new study is in contrast to one released last year in the journal Science
, in which the genome of the 5,000-year-old Irish dog was sequenced. Combined with other data, the University of Oxford group suggested that dogs were independently domesticated twice from gray wolves during the Old Stone Age, once in Asia and once in Europe. The researchers said that a sub-population of the dogs from Asia moved to Europe during the New Stone Age and replaced the Old Stone Age European dogs but that the ancestry was preserved in the fossil.
"By sequencing two ancient dogs from Germany, one 7,000 years old and one 5,000 years old, and reanalyzing the Irish dog, we found that this claim was not supported," Veeramah said. "Instead, we found that the dogs present in Europe 7,000 years ago, and perhaps even 14,000 years ago, were essentially same dogs present in Europe today. We found no evidence of a replacement, and instead we found that the current data still support a scenario that dogs were domesticated just once, sometime between 20,000 to 40,000 years ago."
Veeramah and his research team will turn their focus next to what happened to dogs after the New Stone Age and how their relationships with humans evolved.
They also want to know where domestication actually took place. "This is likely to be the big question answered by [University of Oxford professor] Greger Larson's marvelous project
(involved in the 2016 Oxford study), as they are looking to sequence the genomes of much older specimens from other regions such as East Asia and the Americas," Veeramah said.