Science

DNA from 700,000 year-old horse is oldest ever sequenced, reveals story of equine evolution

By James Maynard , Jun 26, 2013 08:27 PM EDT
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Horse DNA from 700,000 years ago has been used to create the oldest complete genome known - by far. In addition, by comparing that sequence to those of modern horses, donkeys and mules, researchers were able to trace their common ancestor back to an animal which lived over four million years ago.

Previous to this, the oldest complete DNA sequences were those of mastadons and polar bears who lived over 50,000 years ago. The oldest hominid DNA sequence dates from 70,000 years in the past. This makes this new sequence the oldest one known by over half-a-million years.

"We knew that sequencing ancient genomes as old as 70,000 to 80,000 years old was possible. So we said, why not try even further back in time?" Ludovic Orlando, an evolutionary geneticist with the Natural History Museum of Denmark, asked.

The sample was taken from a six-inch-long leg bone discovered in the Yukon, in western Canada, during a 2003 expedition. It is believed to date from between 780,000 to 560,000 years ago.

By targeting collagen, a tissue containing large concentrations of DNA, and using multiple techniques to fill in missing gaps, the team was able to read DNA much older than what was previously possible. This technique could be used to sequence the DNA of long-extinct species.

Tracing the common ancestor of the Equus genus of animals to a time between four and four-and-half million years ago is over two million years further in the past than previously believed.

They also were able to determine that the Przewalski breed of wild horses in the world, the last wild ones in the world, split off from modern horses about 50,000 years ago.

The animal lived during the Pleistocene, a period marked by ice ages, the disappearance of saber-tooth tigers and wooly mammoths, along with the rise of hominids.

Although DNA degrades over time, it is believed that the cold in the region kept water from ruining the genetic structure. This may be possible to recover old DNA from other species as well.

"We can always keep our fingers crossed that (DNA from) an ancient hominid will be found in one of those environments that have been cold," Edward Rubin, who heads the U.S. Department of Energy's Joint Genome Institute, said. Rubin was not involved in the study.

Another challenge the researchers had to overcome was the microbes that had inhabited the bone after the animal's death left their own DNA behind - 99.7 percent of the 12 billion strands examined.

Study into the sequencing of the 700,000 year-old horse DNA was detailed in the journal Nature.

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