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DNA from 2,000-year-old stallions is helping rewrite the story of horse domestication.
Ancient domesticated horses had much more genetic diversity than their present-day descendants do, researchers report in the April 28 Science. In particular, these ancient horses had many more varieties of Y chromosomes and fewer harmful mutations than horses do now. Previous studies based on the genetics of modern horses concluded that domestication must have squeezed out much of the diversity seen in wild horses before the Ice Age. But the new findings suggest that the lack of diversity is a more recent development.
“Today, Y chromosomes of all horses are pretty much the same,” says evolutionary geneticist Ludovic Orlando of the Natural History Museum of Denmark in Copenhagen. As a result, scientists thought that ancient people started domesticating horses by breeding only a few stallions to many different mares.
“But when we look in the past — wow! — this is a whole new planet,” Orlando says.
Horses are thought to have been domesticated by about 5,500 years ago. Orlando’s group examined DNA from the bones of 15 Iron Age stallions from the ancient Scythian civilization: Two stallions were from a 2,700-year-old grave site in Russia and 13 were sacrificed in a burial ritual about 2,300 years ago in Kazakhstan. The team also looked at a 4,100-year-old Bronze Age mare from the Sintashta culture in Russia. Nearly all of the stallions had a different type of Y chromosome, Orlando says.
That finding challenges the idea that only a few stallions participated in the early stages of domestication. Loss of Y chromosome diversity among horses must have happened within the last 2,300 years, Orlando says, and maybe as recently as 200 to 300 years ago, when people started creating specific horse breeds.
Using genetic data from modern animals to figure out what went on in the past is like flipping to the end of a novel and reading only the ending; it shows how things ended up but doesn’t indicate how the story started or unfolded. Examining ancient DNA can fill in those gaps to give a better indication of how domestication took place and how ancient people interacted with animals, says Laurent Frantz, an evolutionary biologist at Queen Mary University of London.
Modern horses also carry mutations that can be harmful (SN: 1/10/15, p. 16), including ones involved in dementia and seizures. But the ancient horses didn’t have those mutations, indicating that those DNA changes happened sometime within the last 2,300 years.
“It really shows an awful lot has changed very recently, and it’s incredibly dangerous to model the deep past from modern genetics,” says zooarchaeologist Alan Outram of the University of Exeter in England. “You really need to carry out the ancient DNA studies.”
Orlando and colleagues also determined some genetic traits that were cultivated by the Scythians. Genes involved in mammary gland development and function had variants associated with greater milk production, perhaps indicating that the Scythians milked their horses. Outram and others have evidence that horse milking started at least 5,000 years ago (SN: 3/28/09, p. 15).
Also changed were genes involved in the function of neural crest cells. Those embryonic cells migrate to different parts of the body during early development and help form parts of the brain, some facial features and other tissues. One recent hypothesis is that changes in how neural crest cells work could lead to common characteristics shared by domestic animals, such as floppy ears, juvenile faces and spotted coats (SN: 8/23/14, p. 7).
Genetic results from the ancient horses provide evidence that the hypothesis might be true, says Frantz. Geneticists will have to work with experimental biologists to confirm that neural crest cells are involved in changing the appearance of domesticated animals. But, Frantz says, “this is the first step toward testing that hypothesis correctly.”
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