Scientists have established that the ancestor of the modern horse was grazing the planet between four and 4.5 million years agotwice as long as was previously thought.

They made the discovery by unravelling the DNA of a horse that lived 700,000 years ago.

The findings — reported in the journal, Nature — were due to the discovery of a piece of fossilised horse bone in the permafrost in the Yukon territory in Canada.

Bone this ancient does not usually harbour the biological materials — like connective tissue — that make DNA extraction possible.

But the discovery of collagen in the bone fragment gave researchers hope.

“It was a unique chance to push our technology to the limit,” said Dr Ludovic Orlando of the University of Copenhagen.

“To be frank, I didn’t think it would be possible when we first tackled the idea.”

But new technology meant they could extract tiny scraps of DNA sequence, which they reassembled into a full genetic code.

“It’s like mending a vase which has broken into a thousand pieces — only this one has billions of pieces!” said Dr Orlando.

The result is the oldest genome that has been fully sequenced.

Scientists then analysed the horse DNA sequence and compared it against the genome of a horse that lived some 43,000 years ago, as well as those of five modern horse breeds, the wild Przewalski’s horse and a donkey.

“Our analyses suggest that the Equus lineage giving rise to all contemporary horses, zebras and donkeys originated four to 4.5 million years ago — twice the conventionally accepted time,” said the study.