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Genome sequenced for 700,000 year old frozen Yukon horse

The skull of a 700,000-year-old horse found frozen in the Yukon permafrost. (Credit: D.G. Froese)

After years of painstaking puzzlework, a prehistoric horse fossil found in Canada's Yukon permafrost has yielded enough DNA for scientists to sequence the genome of the 700,000-year-old animal.

It is the oldest animal genome ever sequenced, the researchers say, and an effort that has shed light on the evolutionary history of the horse. The results are published in the journal Nature.

The fossil was discovered in the Canadian Arctic by a University of Alberta researcher, Duane Froese. He immediately recognized the fossil as an important find, according to a news release announcing the new research: it was much larger than the ice-age horse fossils that can be found across the polar north. It was also very well preserved.

Two European researchers, Dr. Ludovic Orlando and Prof. Eske Willerslev of the Natural History Museum of Denmark, worked with colleagues for the next three years to scrape together genetic material from the fossil and assemble the information.

Researchers had previously sequenced the genome of animals around 70,000 to 130,000 years old. But no one has ever successfully pieced back together a genome of this age, the scientists say.

Orlando and Willerslev first began examining the fossils and discovered intact collagen. Molecular signatures from collagen, a common protein found in the connective tissues of most animals, has been used by scientists recently to study evolutionary links between extinct and living species.

So the collagen was an exciting find in and of itself. But the scientists, as it turned out, discovered something even better: blood proteins. The temperature of the permafrost had helped preserve these materials.

The fossil, despite its age, still carried short pieces of DNA molecules. But fitting the pieces together like a puzzle was a huge challenge.

For one, much of the genetic material discovered on the fossil came from unrelated microorganisms that were living in the bone. So the scientists had to differentiate between that and the material they were actually interested in. And when they did isolate the horse genetic material, much of it was in rough shape. It was a huge challenge, they say, to reconstruct the whole genome.

What the final product reveals is that the last common ancestor of horses, donkeys, and zebras lived twice as long ago as previously thought.

And it also told researchers that Przewalski's horse, an animal that lives in the Mongolian Steppes, is the last wild horse. Whether the horse was truly wild had been a source of debate among researchers. But the new evolutionary insights provided by the ancient genome show that Przewalski's horse is in fact truly wild and split from the lineage leading to domestic horses around 50,000 years ago.

Most importantly, the work dramatically pushes back the time frame for sequencing ancient genomes, and could open new doors for examining genetic material in very ancient fossils.

Kate Allen is the Star's science and technology reporter. Find her on Twitter at @katecallen.




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