“Here we see them as breeders,” said Ludovic Orlando, a professor of molecular archaeology at the University of Copenhagen in Denmark, who led the research. “We reveal part of their management strategy and part of their knowledge 2,300 years ago.”
The findings also fit an emerging theory of how domestication in general changes animals as they become intertwined with humans.
“It’s great stuff,” commented Greger Larson, director of the paleogenomics and bioarchaeology research network at the University of Oxford in England, who was not involved in the research. “It demonstrates the power of ancient whole genomes to understand the pattern and the process of domestication.”
Among the farm animals whose lives have become entwined with people, horses were a late addition.
Dogs were the first animal friends of humans — wolves that scavenged for food among garbage piles and turned docile about 15,000 years ago, or possibly much earlier. Cattle, chickens and pigs were domesticated by people in different parts of the world between 8,000 and 11,000 years ago.
It was only about 5,500 years ago that people in Central Asia started catching and keeping wild horses for meat and milk. Riding horses came later.
In the new research, the scientists used a bit of bone from the horse skeletons — less than half a gram in most cases — to extract DNA. They were able to decipher the genomes for 11 of the 13 horses from the Scythian mound. They also analyzed the DNA of two stallions from a royal Scythian tomb 400 years earlier, and one mare, dating to 4,100 years ago, that belonged to a nearby, earlier people, the Sintashta, who had already figured out how to use horses to pull two-wheeled chariots.
Credit Michael Hochmuth/German Archaeological Institute, Berlin
From the DNA, the scientists found that the Scythians bred for certain characteristics: stockier forelimbs that were thicker. The horses also had genes for retaining water, perhaps indicating that the mares were milked for human consumption. Many, although not all, of the horses possessed genes associated with racing speed that are found in today’s thoroughbreds.
The genes also showed a variety of colorings — cream, black, spotted, bay and chestnut.
Many of the genetic changes were related to the “neural crest” — a line of cells along what becomes the spinal cord during embryonic development, but which migrate to various parts of the body. That fits in with an idea proposed in 2014 of how domestication and the initial goal of breeding tamer animals able to live and work with people also led to a series of other traits commonly observed among domesticated animals: smaller brains, floppy ears, curly tails, varied colorings.
“Most of them have a neural crest derivation,” said Adam S. Wilkins, a visiting scientist at Humboldt University in Berlin and one of the authors of the hypothesis.
The genetic changes may slightly reduce the number of neural crest cells, and that may lead to smaller adrenal glands, which produce “fight-or-flight” hormones. The result may be animals that are less likely to startle, and are more amenable to being handled by people.
“This begins to support a sort of grand unified theory of domestication,” said Daniel Bradley, a professor of genetics at Trinity College Dublin in Ireland.
What the researchers did not find is the gene that enables certain horse breeds today to “amble” — a gait that is faster than a walk but slower than a gallop.
Unlike modern horses, the Scythian horses’ DNA showed no signs of inbreeding. “This is extremely surprising in horses,” Dr. Orlando said.
The Y chromosome tells the genetic story of males of a species. The mitochondria — energy factories within cells — contains DNA passed down only from mothers. In modern horses, the Y chromosomes in stallions are almost identical, reflecting the breeding technique of using a single stallion with desired characteristics to father many offspring.
That indicates that the Scythians maintained the natural herd structure of horses, Dr. Orlando said. He said additional studies had revealed when and where the genetic diversity of stallions crashed later, but he would not say publicly until he finished the scientific paper that laid out the answer.
For Melinda Zeder, a Smithsonian Institution scientist who studies domestication, that fits in with other research that indicates the narrow genetic variation among many domestic animals — which sometimes leads to prevalent diseases — is a recent development, not an inevitable consequence of domestication.
“I think that’s a very important lesson for the future,” she said. “A red-flag warning we would do well to pay attention to.”
The findings also point to the profound impact that humans have had on the environment and the evolution of other species for millenniums. “It is something humans have been doing for a long time,” Dr. Zeder said. “It’s not always detrimental.”