Biologist Ludovic Orlando of the University of Copenhagen, with colleague Eske Willerslev, reporting on their research in the journal Nature, say the findings suggest the ancient ancestor of the modern Equus genus, which includes horses, donkeys and zebras, branched off from other animal lines about 4 million years ago.
The sequence extracted from a foot bone of a horse that lived between 780,000 and 560,000 years ago, compared with that of five modern domestic horse breeds, a wild horse known as Przewalski's' horse, and a donkey, allowed the detailed tracing of the evolutionary history of the horse family, the researchers said.
"We have beaten the time barrier," Orlando said.
The researchers said they were able to sequence the DNA, much older than any animal DNA recovered before, partly because freezing ground temperatures in permafrost in the area where the bone was found slowed the rate of DNA decay.
"All of a sudden, you have access to many more extinct species than you could have ever dreamed of sequencing before," Orlando said.
The scientists said their findings also support the contention that Przewalski's horse, brought back from near-extinction in Mongolia by captive-breeding programs, is in fact the last remaining true wild horse when compared genetically with domesticated horses.