Dogs have been man's best friend for up to 40,000 years, a major genetic study has found. 

A DNA analysis of the world's oldest known dog remains has revealed that dogs were domesticated in a single event by humans living in Eurasia.

The findings disprove previous research finding that dogs were domesticated twice by separate groups living in east and western Eurasia.

Instead, all modern dogs are thought to have descended from animals that were domesticated by people living in Eurasia from 20,000 to 40,000 years ago.

Scientists agree that dogs stem from wolves.

But over the past few years, exactly where and how many times dogs were domesticated has been a matter for debate.

Dr Krishna Veeramah, an assistant professor in evolution at Stony Brook University, said: 'We've found clear evidence that dogs were domesticated 20,000 to 40,000 years ago.

'New research last year provocatively suggested that dogs could have been domesticated twice but our conclusion was there is no evidence for dual domestication.

'We would argue that finding evidence for only one domestication event is a big deal because it is very important to helping us understand how domestication works.'

His research found that dogs evolved to be a separate species from wild wolves sometime between 20,000 and 40,000 years ago. 

Dr Veeramah said that because dog domestication has only occurred once over the course of human history, it suggests that the process is complex and dependent on a very specific set of conditions. 

'The process of dog domestication would have been a very complex process, involving a number of generations where signature dog traits evolved gradually,' he said.

'The current hypothesis is that the domestication of dogs likely arose passively, with a population of wolves somewhere in the world living on the outskirts of hunter-gatherer camps feeding off refuse created by the humans.

'Those wolves that were tamer and less aggressive would have been more successful at this, and while the humans did not initially gain any kind of benefit from this process, over time they would have developed some kind of symbiotic [mutually beneficial] relationship with these animals, eventually evolving into the dogs we see today.'

For the study, researchers extracted DNA from the skeletal remains of Neolithic dogs from archaeological sites in Germany and Ireland.

One of the animals, which lived around 7,000 years ago, is the oldest recorded known dog specimen in the world, while the other two lived around 4,500 years ago.

However it is likely that older specimens exist, but distinguishing between 'large dogs and small wolves' can be troublesome, according to Dr Veeramah.

Scientists compared DNA taken from these animals to DNA taken from modern breeds of dog.

He found that there was a very high degree of genetic continuity between dogs from the Neolithic period and modern breeds.

This means that the DNA of modern dogs is very similar to that of domesticated canines that lived thousands of years ago.

Dr Veeramah said: 'We found that ancient dogs look a lot like modern European dogs.

'Dogs that farmers had in Europe 7,000 years ago are very similar, genetically speaking, to modern dog breeds.'

This finding strengthens the idea that the first domesticated dogs lived somewhere in Eurasia,  Dr Veermah said.

But it is still not possible to pinpoint the exact location where domestication first occurred, he added.

'We know when dogs were domesticated but the question remains where they can come from,' he said.

In order to answer this question, researchers must find and analyse specimens that lived around the time that dog domestication occurred, he added.