WASHINGTON， June 2 (Xinhua) -- Dogs may have been domesticated not once， as widely believed， but twice， once in Europe and once in Central Asia or China， a major study said Thursday.
Scientists have debated where domestic dogs come from for a very long time. Some argued that humans first domesticated wolves in Europe， while others claimed this happened in Central Asia or China.
All these claims， according to the new study published in the U.S. journal Science， may be right.
The study， supported by funding from the European Research Council and Britain's Natural Environment Research Council， said that man's best friend may have emerged independently from two separate， possibly now extinct， wolf populations that lived on opposite sides of the Eurasian continent.
The team， led by the University of Oxford and including French researchers based in Lyon and at the National Museum of Natural History in Paris， made the conclusion after reconstructing the evolutionary history of dogs.
They first sequenced the genome of a 4，800-year-old medium-sized dog from bone excavated at the Neolithic Passage Tomb of Newgrange， Ireland.
Then， they obtained mitochondrial DNA from 59 ancient dogs living between 14，000 to 3，000 years ago and compared them with the genetic signatures of more than 2，500 previously studied modern dogs.
Their analyses revealed a genetic separation between modern dog populations currently living in East Asia and Europe， one they said occurred several thousand years after the first known appearance of dogs in Europe.
The new genetic evidence also showed a population decline in Europe that appeared to have mostly replaced the earliest domestic dog population there， which supported the evidence that there was a later arrival of dogs from elsewhere.
Lastly， a review of the archaeological record showed that early dogs appeared in both the East and West more than 12，000 years ago， but in Central Asia no earlier than 8，000 years ago.
Combined， these new findings led the researchers to believe that dogs were first domesticated from geographically separated wolf populations on opposite sides of the Eurasian continent.
At some point after their domestication， the eastern dogs dispersed with migrating humans into Europe where they mixed with and mostly replaced the earliest European dogs.
Most dogs today are a mixture of both Eastern and Western dogs -- one reason why previous genetic studies have been difficult to interpret， they said.
The international project is currently analyzing thousands of ancient dogs and wolves to test this new perspective， and to establish the timing and location of the origins of our oldest pet.
""Animal domestication is a rare thing and a lot of evidence is required to overturn the assumption that it happened just once in any species，"" senior author Greger Larson， professor of the Oxford University， said in a statement.
""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，"" he said.
""Maybe the reason there hasn't yet been a consensus about where dogs were domesticated is because everyone has been a little bit right，"" he added. Enditem
" This photo provided by Nasdaq shows Nasdaq Tower in New York's Times Square Adam Shaheen Youth Jersey , as the display shows that the Dow Jones industrial average crossed the 20,000 mark for the first time, Wednesday, Jan. 25, 2017. (Rohini ShahriarNasdaq via AP)
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