Researchers who upended the history of horse domestication in findings published last week are not ruling out the possibility that modern domesticates originated outside Central Asia.
The earliest credible evidence of horse domestication dates back to the Botai culture, who are believed to have tamed horses in northern Kazakhstan around 3500 BC.
Until last week, models suggested that all modern domesticated horses descended from those first tamed by the Botai people.
But scientist Ludovic Orlando and his colleagues, writing in the journal Science, turned that belief on its head last week.
Their DNA testing revealed that Botai horses did not give rise to today’s domesticated horses, but were in fact direct ancestors of Przewalski’s horses.
Stunningly, the Przewalski’s horse, commonly thought to be the last true wild horses in existence, are actually the descendants of the first horses ever to be domesticated.
Przewalski’s horses (Equus ferus przewalskii) are a different species to the modern domestic horse (Equus caballus), with a different number of chromosomes.
Therefore, the site of the first domestication of E. Caballus, with which we are all familiar, remains shrouded in mystery.
“With the genome data that we currently have, it is impossible to locate the source that gave rise to the modern horse,” says Orlando, who is Professor of Molecular Archaeology at the University of Copenhagen, Denmark, and research director at the French National Center for Scientific Research.
What is clear, though, is that none of the ancient Eurasian horses that the team has analyzed so far within the last 4100 years or so relates to Botai.
Patterns of mitochondrial DNA variation also indicate that the horse population expanded drastically between around 4100 and 5,000 years ago.
It thus looks like humans somehow encountered and developed a new type of domesticated horse at that time, and that this horse became extremely popular and perhaps even facilitated the expansion of human populations.
“We are looking at a series of candidate places that are known sources of major human expansions in the third millennium BCE,” Orlando says.
Interestingly, the earliest fossils presently sequenced of this new horse type come from Hungary, Romania and the Pontic Caspian steppe. The team thus does not dismiss a possible origin outside Central Asia.
Antoine Fages, a doctoral student in the Orlando team who carried out the experimental work for the study, said: “This work clearly illustrates the power of harnessing ancient genome sequencing. It really allows us to discover a series of key evolutionary processes that have left little signature in the genomes of living populations.”
Orlando continues: “Ancient genomics just rejected Botai as the ancestors of modern domesticates. We are confident that ancient genomics will soon help identify the tempo and locus of horse domestication.”
It is an important field of research, given that the domestication of the horse is one of the key transformative changes that revolutionized human history.
Horseback riding allowed us to travel well above our own speed, spreading our genes, diseases and culture over vast geographical regions.
The development of cavalry has also been paramount to the battlefield until the mechanization of weaponry. Many conquerors are remembered as exceptional horse masters, Orlando notes.
Alexander the Great and Genghis Kahn are perhaps the most famous. Some of their horses, such as Bucephalus, are no less legendary.
“Horse breeders have considerably transformed the horse during history, especially during the last few centuries with the development of intensive selective breeding,” Orlando says.
“It is thus almost impossible to reconstruct early stages of horse domestication by looking at the genome of modern horses.”
In recent work, his team reconstructed the genomes of Scythian horses and clearly established that the genome of the animal has been dramatically modified during the last 2000 years.
By targeting the Botai horses, the team aimed for a deeper time travel, some 5500 years ago.
“Sequencing the genomes of the earliest domestic horses would catch evolution red-handed at the time we first tamed wild horses. It would thus reveal the biological changes underlying the process of animal domestication as it started,” Orlando says.
However, the findings amounted to a remarkable curve ball. Rather than being the source of modern domesticates, the Botai horses appeared to be the direct ancestors of another group of horses that lived in Kazakhstan around 5000 years ago and the Przewalski’s horses, long considered the last truly wild horses in existence.
“Our findings literally turn current population models of horse origins upside-down: what we used to understand as the last wild horse on earth is in fact the descendant of the earliest domestic horses, which simply escaped human pressure and became feral during the last few millennia,” Orlando says.
The study takes advantage of the extensive genome dataset produced to identify the genomic changes underlying this feralization process.
One such change affects a variant of the TRPM1 gene involved in leopard spotting, which used to be present amongst Botai horses but was eliminated from the Przwewalski’s horse gene pool. As such a variant is also associated with color night blindness, it is likely that it could only be artificially maintained by human herders but was rapidly lost by natural selection after the horse turned feral.
Doctoral student Charleen Gaunitz, who also worked on the study within the Orlando team, said: “Ironically, we used to think that the endangered population of Przewalski’s horses should be preserved as the last wild horses in the planet. We now find that they must be preserved as the closest descent of the earliest domestic horses.”