NEW YORK (GenomeWeb) – A new study appearing in Science suggests dogs may have been domesticated from wolves twice, with dogs from the more recent event mixing with and replacing those domesticated earlier.
Researchers from the University of Oxford, Trinity College Dublin, and elsewhere sequenced the complete genome of a 4,800-year-old Neolithic dog bone sample from Newgrange, Ireland. They also assessed new and existing genomic data for hundreds of ancient dogs, modern dogs, and wolves.
In the process, the team uncovered a split between East Asian and European dogs that took place thousands of years after the available archeological evidence places dogs in Europe — results that point to the possibility of parallel dog domestication events taking place thousands of years apart.
"Our ancient DNA evidence, combined with the archaeological record of early dogs, suggests that we need to reconsider the number of times dogs were domesticated independently. Maybe the reason there hasn't yet been a consensus about where dogs were domesticated is because everyone has been a little bit right," co-senior author Greger Larson, director of the Paleogenomics and Bio-Archaeology Research Network at the University of Oxford, said in a statement.
Most prior studies have indicated that grey wolves were coaxed along the road to becoming dogs during a single domestication event, though there is ongoing debate amongst researchers over where domestication took place. For example, a study published in the Proceedings of the National Academy of Sciences last October argued that dog domestication occurred in Central Asia, while a more recent Cell Research paper placed it in southern East Asia.
A 2013 study in Nature Communications, meanwhile, uncovered ties between indigenous dogs in China and the ancestors of domestic dogs and estimated that domestication took place some 32,000 years ago. But Swedish and American researchers found evidence for earlier domestication and suggested that multiple domestication events may have occurred.
For the latest analysis, Larson and his colleagues sequenced the genome of the 4,800 year old Newgrange dog to an average depth of 28-fold coverage using a sample obtained from a Neolithic grave at that site. They also did mitochondrial DNA sequencing on 59 European dog samples from 3,000 to 14,000 years ago. The team then compared the new genome to the mitochondrial sequences and to genetic data for hundreds of modern-day dogs, including 80 genome sequences and array-based genotypes for more than 600 dogs.
Phylogenetically speaking, a dog breed from the Netherlands called the Sarloos clustered apart from other domestic dogs, the researchers noted, likely due to its heavy captive wolf and German shepherd ancestry. But genetic variants in dog genomes also revealed a split between dogs from East Asia and Western Eurasia that went back between 6,400 and 14,000 years.
The split took place long after domestic dogs' first archeological appearance in East Asia or Europe, which appears to go back some 15,000 years or more. And the team saw signs of both older and newer domestic dog ancestry in the Newgrange individual, suggesting dogs from a second domestication event may have mixed with, and to some extent replaced, the earliest domestic dogs.
Still, the study's authors cautioned that "our scenario remains hypothetical" based on the data available so far. "Genome sequences derived from ancient Eurasian dogs and wolves, combined with detailed morphological and contextual studies of the archeological remains, will provide the necessary means to assess whether dog domestication occurred more than once," they wrote.