When University of Alberta researcher Duane Froese found an unusually large horse fossil in the Yukon permafrost, he knew it was important. Now, in a new study published online today in Nature, this fossil is rewriting the story of equine evolution as the ancient horse has its genome sequenced.

Unlike the small ice age horse fossils that are common across the unglaciated areas of the Yukon, Alaska and Siberia that date to the last 100,000 years, this fossil was at least the size of a modern domestic horse.

Froese, an associate professor in the U of A Department of Earth and Atmospheric Sciences, and Canada Research Chair in Northern Environmental Change, had seen these large horses only a few times at geologically much older sites in the region-but none were so remarkably well preserved in permafrost.

Froese and his colleagues from the University of Copenhagen, who led the study, had dated the permafrost at the site from volcanic ashes in the deposits and knew that it was about 700,000 years old-representing some of the oldest known ice in the northern hemisphere.

They also knew the fossil was similarly old. The team, which also included collaborators from the Yukon and the University of California, Santa Cruz, extracted collagen from the fossil and found it had preserved blood proteins and that short fragments of ancient DNA were present within the bone.

The DNA showed that the horse fell outside the diversity of all modern and ancient horse DNA ever sequenced consistent with its geologic age. After several years of work, a draft genome of the horse was assembled and is providing new insight into the evolution of horses.

The study showed that the horse fell within a line that includes all modern horses and the last remaining truly wild horses, the Przewalski's Horse from the Mongolian steppes.

The 700,000-year-old horse genome-along with the genome of a 43,000-year-old horse, six present-day horses and a donkey-has allowed the research team to estimate how fast mutations accumulate through time.

In addition, the new genomes revealed episodes of severe demographic fluctuations in horse populations in phase with major climatic changes.