Wednesday, July 31, 2013

700,000 year old horse DNA sequenced

Researchers have constructed the DNA from a 700,000 year old horse fossil from the far north west of Canada, which has challenged the accepted version of equine evolution.

Indeed, the widely accepted understanding of the horse's evolution is of a gradual increase in size from small forest-dwelling animals to the larger animals of today.

The fossil, found in the Klondike area of Yukon by  University of Alberta researcher Duane Froese, raised questions about this version of events. Not only was the  fossil considerably older than those commonly found in ice age deposits in the Yukon, Alaska and Siberia, it was also at least as large as many modern horses.

The layer of permafrost in which the fossil was found has been dated  by the volcanic ashes it contained as originating about 700,000 years ago. This puts it among the oldest known ice in the northern hemisphere.

An international research team, led by Ludovic Orlando, and colleagues at the University of Copenhagen, Denmark, managed to extract fragments of ancient DNA from the fossil. After several years work, they succeeded in constructing  a draft genome of the horse. This is by far the earliest genome sequence yet recorded. “Our data represent the oldest full genome sequence determined so far by almost an order of magnitude” they say in the report of their work, which is published in the journal Nature.

It is unlikely that the research would have been successful had the DNA not remained frozen over the past 700,000 years.

Having reconstructed the complete genome of the horse, the team compared it with DNA from a horse dating from the Late Pleistocene (about 43,000 years ago), and from five contemporary domestic horse breeds, a Przewalski's horse and a donkey.

By looking at differences between the DNA of the various horses, they could estimate the rate at which mutations occurred in the genetic code over time.

Analysis of differences between these genomes indicated the last common ancestor of all modern equids ( horses, zebras and donkeys ) would have lived about 4.0- 4.5 million years ago. This is about twice as far back than previously thought.

The results also indicated that Przewalski’s horse― an endangered subspecies native to the Mongolian steppes ― diverged from the lineage that gave rise to modern domesticated horses about 50,000 years ago.

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