Horses have been invaluable partners as our species spread around the globe. Originating on the grassy plains of Central Asia, they are thought to have been domesticated around 5,000 years ago, and are now found on every continent – except Antarctica – as they have provided us not only with transport, but meat and milk too.

Investigating this deep history in order to uncover clues as to how we domesticated the beasts of burden, researchers have analysed the genomes of the sacrificed remains of horses unearthed in Eastern Kazakhstan. They are thought to have been killed some 2,300 years ago as part of a ritual by the Scythian people, a group of accomplished equestrians who are thought to have been some of the first people to perfect mounted warfare.

Coupled with the genetic analysis of another horse dating back to 4,100 years ago, the researchers were able to get a glimpse of what domestic horses looked like 2,000 years after they were first domesticated. It turns out they were being bred for sturdy legs and different color variations. The study, published in Science, also suggests that one main aspect we thought we knew about horse domestication might actually be wrong.

^{The remains of horses sacrificed by the Scythians. Michael Hochmuth/German Archaeological Institute, Berlin}

It is thought that horses were domesticated in multiple places across the steppes of Central Asia. This was thought to be supported by the fact that in modern horses, there is great diversity within the females, but highly limited diversity within the males – which all contain the same, or very similar, Y-chromosome.

This suggests that the milk-producing mares were taken from the wild multiple times in many different places, while the stallions were only domesticated once. But this latest study now questions that hypothesis.

^{Kazakh horses. Ludovic Orlando/Natural History Museum of Denmark, CNRS}

The team found that the genetic diversity in the ancient horses was incredibly high and that none of them were inbred, implying the breeders were not relying on single horse lineages to keep specific traits. What’s more, they found that the Iron Age horses had much more varied Y-chromosomes. This suggests that the lack of diversity in the male lineage seen today is not a result of only a few males being domesticated at the start, but is a result of breeding within the last few thousand years.

This also matches up with the fact that the ancient horses show fewer traces of genetic diseases. They found that over the past 2,000 years, horse management has negatively impacted the health of the animals, as not only is there now incredibly limited male diversity, but the demographic collapse has also caused an accumulation of harmful mutations, such as ones thought to contribute to dementia and seizures.