Researchers have reconstructed an ancient genome that is 10 times as old as any retrieved so far, and now say that DNA should be recoverable from animals that lived one million years ago. This would greatly extend biologists’ ability to understand the evolutionary past.

The genome was that of a horse that lived some 700,000 years ago in what is now the Yukon Territory in Canada, and its reconstruction has already led to new insights. The researchers who sequenced it then analyzed DNA from a less ancient horse, one that lived 43,000 years ago, as well as five contemporary horse breeds and a donkey named Willy that resides in the Copenhagen zoo. They concluded that the genus that gave rise to modern horses, zebras and donkeys — Equus — arose about four million years ago, twice as far back as had been thought.

Before this work, the oldest genome that had been recovered was that of a Denisovan human who lived 70,000 years ago. The new finding, if accepted, would extend by tenfold the reach of paleogenomics, the study of ancient genomes reconstructed from fossil bones. Within the last few decades this young science has become a powerful complement to paleontology, the study of fossils, as a way of reconstructing evolutionary history....