The horse DNA was extracted from a hind toe bone found in the Thistle Creek region of the Yukon’s Klondike gold mines. It owes its remarkable longevity to the bone having been buried in permafrost, which kept the DNA both very cold and very dry.

The researchers who discovered the bone, Duane Froese of the University of Alberta and Eske Willerslev, an expert on ancient DNA at the University of Copenhagen, first estimated its date from the layers of volcanic ash where it was found. They also conducted tests that showed that the horse bone, despite its age, was likely to contain DNA, even though the chemical starts to degrade as soon as an animal dies.

To help establish that the DNA from the horse bone was really 700,000 years old, Dr. Willerslev and Ludovic Orlando, a colleague at the University of Copenhagen, started an ambitious project to analyze the genomes of many other members of the horse evolutionary tree. These include the horse that lived 43,000 years ago, before horses were domesticated; a Przewalski’s horse, a species thought to represent the last living wild horse population; five domestic horse breeds (Arabian, Icelandic, Norwegian fjord, Standardbred and Thoroughbred); and Willy the donkey.

Dr. Orlando said the range of genetic variation in the Thistle Creek bone lay outside that of all the other horses, showing it could not have been contaminated by modern horse DNA. Also, the DNA in the bone was much more fragmented than that of late ice age horse bones from the same region, indicating it was older. The geological evidence for the bone’s age is “very secure,” Dr. Orlando said. With this and other data he is confident that the Thistle Creek horse and its genome are indeed 700,000 years old.