A dog bone found at an Irish Stone Age tomb has helped to shed new light on the possible dual origins of pet dogs.
The dog bone, believed to date back almost 5,000 years, was unearthed at Newgrange in County Meath - an ancient monument built by Stone Age farmers.
Scientists at Trinity College Dublin used it to sequence the dog's genome.
The research suggests that modern dogs may have emerged from two separate domestications of wolves, on opposite ends of the Eurasian continent.
It challenges previous theories that man's best friend originated from a single domestication of wolves in Asia.
The DNA analysis conducted in Dublin formed part of a major international study of dog domestication, led by Oxford University in the UK.
The Newgrange dog's genetic blueprint was compared to that of 59 ancient dogs, some dating from as far back as 14,000 years.
It was also contrasted to the DNA of 2,500 modern dogs.
In a paper published in the Science journal, the authors said: "Our analyses revealed a deep split separating modern East Asian and Western Eurasian dogs."
They added that their findings "suggest that dogs may have been domesticated independently in Eastern and Western Eurasia from distinct wolf populations.
"East Eurasian dogs were then possibly transported to Europe with people, where they partially replaced European Paleolithic dogs."
'Like a time machine'
Newgrange, which has been designated as a UNESCO World Heritage Site, pre-dates Stonehenge in the UK and the Great Pyramids of Giza.
Scientists involved in the project have hailed the quality of the Newgrange fossil in helping them to understand how domestic dogs evolved.
Dan Bradley, Professor of Population Genetics at Trinity College and a senior author of the study, said: "The Newgrange dog bone had the best preserved ancient DNA we have ever encountered, giving us prehistoric genome of rare high quality.
"It is not just a postcard from the past, rather a full package special delivery."
Lead author Dr Laurent Frantz from Oxford University agreed saying: "Reconstructing the past from modern DNA is a bit like looking into the history books, you never know whether crucial parts have been erased.
"Ancient DNA, on the other hand, is like a time machine, and allows us to observe the past directly."
His Oxford colleague Professor Greger Larson said: "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.
"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."