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The Convergence of Ecology & Selection for the Domestication of the Horse

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Two new recent studies have been published which attempt to pinpoint the how and why of prehistoric horse domestication in the Eurasian Steppe. We have a rough ideas that the domestication of the horse began about 5,500 years ago, where Central Asian peoples caught and kept horses for meat and milk. Riding horses came a couple thousand years later.

A Mongolian horse breeder catching horses. LUDOVIC ORLANDO / NATURAL HISTORY MUSEUM OF DENMARK / CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE

Uushgiin Ovor Deer stone site near Mörönь, Khovsgol aimag, Mongolia

The first study was published in the March 2017 edition of the Journal of Archaeological Science. It uses the archaeological record from Deer Stone-Khirigsuur Complexes found in Central Asia. The standing stones at these sites represent deer and khirigsuurs represent burial mounds. The remains of hundreds, sometimes thousands, of horses have been found at these burial mounds.

Lead author William Taylor, a postdoctoral research fellow at the Max Planck Institute for the Science of Human History, identified that burial ritual of horses spread rapidly across the Mongol Steppe at around 1200 BC. Which is several hundred years before mounted horsemen are clearly documented historical records. Taylor and colleagues’ radiocarbon dating and correlation with ecological records indicate that early horsemanship took place earlier than thought, during a wetter, more productive climate period. This may have given herders more room to experiment with horse breeding and transport, which leads us into the next study.

The second study was published last week in Science. An international team of researchers looked at similar horse burial rituals of the Steppe people. The study analyzed the ancient DNA of 13 stallions and one mare that were buried in a well-preserved permafrost mound in what is now Kazakhstan. The mound is attributed to the Scythians who roamed from Siberia to the Black Sea. Known for their equestrian battle skills, including the ability to shoot arrows while riding, and for the brutality as documented by Herodotus, who wrote that the Scythians blinded their slaves, and the warriors drank the blood of the first enemy they killed in battle.

The site of a royal Scythian tomb where scientists assessed the DNA of two stallions. MICHAEL HOCHMUTH / GERMAN ARCHAEOLOGICAL INSTITUTE, BERLIN

The authors extracted 11 genomes of the 13 horses excavated from the Scythian mound. Ancient DNA of two stallions from a royal Scythian tomb dated 400 years earlier, as well as one mare, dating to 4,100 years ago, that belonged to a nearby, earlier chariot wielding people, the Sintashta, were also included. The found out the Scythians bred for stockier, thicker forelimbs, retained water more effectively with a variety of colors from cream to black, spotted to bay and chestnut. The genes that were ultimately identified and selected for belonged to the neural crest lineage, an idea that was initially proposed in 2014, as a site of domestic selection. This theory is that the selected genetic changes ultimately slightly reduced the number of neural crest cells. In turn, that lead to smaller adrenal glands. Leading to a lower amount of catecholamines, which in results selected for less “flight or fight,” less startle, ultimately animals that are more amenable to being handled by people.

Both these studies corroborate each other’s points. Around ~4.1 and 2.3 thousand years before present, we now know that prehistoric Eurasian people were domesticating horses in a much more wet climate, a climate where they could experiment more. Their selection patterns support the neural crest hypothesis, which created the perfect convergence of environment and ecology meeting the profound impact of late Bronze Age human domestic selection on the evolution of the horse.

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