When we think about the relationship between humans and horses, images of elegant races or majestic horses in exhibitions likely come to mind. However, the reality is that our bond with equines has been far more pragmatic and eventful than typical history books suggest. For millennia, these animals have been the driving force behind human progress, facilitating everything from the transport of goods to the conquest of entire territories, although our understanding of their genetic roots has been completely transformed thanks to paleogenomics.
Modern science, relying on the extraction of genetic material from fossil remains, has begun to disprove the idea that there was a single "eureka moment" of domestication. Instead, it seems we are facing a much more complex scenario , where the horse was domesticated in different parts of the world and at different times, creating a mosaic of lineages that interbred and disappeared over the centuries, leaving invisible traces in the breeds we ride today.
The debate on domestication hotspots
For a long time, it has been debated whether all domestic horses descended from a single event that occurred in the steppes of Eurasia. However, the study of mitochondrial DNA (mtDNA) , which is inherited exclusively from the mother, has revealed such a vast diversity that it points to the existence of multiple centers of domestication. A clear example is the D1 group; for years it was thought that this haplogroup, very common in Iberian and North African horses, could be the key to a local domestication event in the western Mediterranean.
To resolve the doubts, researchers analyzed horse samples from the Neolithic period to the Middle Ages at sites such as Cova Fosca and El Portalón in Atapuerca. Interestingly, group D1 only appeared in medieval samples, suggesting that this lineage was likely an immigrant that arrived on the Iberian Peninsula much later than previously thought, ruling out its role as the original driver of prehistoric domestication in the region.
The genetic treasure of the Iberian Peninsula
While group D1 wasn't the main focus, the C haplogroup of Lusitano horses certainly has a fascinating history. This lineage was present in wild horses 6.200 years ago and persisted throughout the Bronze Age. Most incredibly, this prehistoric Iberian genetic line has survived to the present day, appearing in breeds such as the Lusitano, the Argentine Criolla, and the Paso Fino of Puerto Rico.
This opens the door to two possibilities: that a completely independent domestication process occurred in Iberia, or that local wild mares mixed with horses brought from elsewhere. Either way, it is clear that Iberian mares played a decisive role in the formation of the domestic populations that would later travel to the New World, leaving an indelible mark on American equine genetics.
The loss of diversity and the myth of the pure race
It hasn't all been evolution and expansion; the last few centuries have been quite harsh on the genetic health of horses. The rise of the "purebred" concept has led to an alarming decline in genetic diversity , with a reduction of between 14% and 16%. By using a few selected stallions to reproduce aesthetic or speed traits, we have fallen into the trap of inbreeding.
This phenomenon has led to an increase in deleterious mutations in modern horses —mutations that don't kill the animal but diminish its ability to survive or reproduce. It's ironic to think that the horse we know today is, genetically speaking, much more limited than the horse of two thousand years ago, since genome modification in the last 200 years has been more aggressive than in the previous 4.000 years of domestication.
Lost lineages and historical conquests
Ancient DNA has also rescued from oblivion species whose existence we didn't know. Lineages have been identified in Siberia (such as Equus lenensis ) and another in the Iberian Peninsula that lived around 4.000 years ago. This latter Iberian lineage is like the "Neanderthal" of horses: it existed, left some minimal traces, but was not the direct ancestor of today's domestic horses.
- Islamic expansions: Between the 7th and 9th centuries, the European genetic landscape changed radically. European horses became very similar to those of the Sanids of Persia, resulting in a new, more successful anatomy.
- The Botai case: Evidence of domestication was found in Kazakhstan around 3500 BC, and surprisingly, these horses turned out to be the ancestors of Przewalski's horse, which we thought was completely wild.
- Early selection: 125 genes have been identified that were the first targets of human selection, related to cognition, behavior, and locomotion.
From the analysis of Middle Pleistocene genomes, dating back some 700.000 years, to the sequencing of the extinct Quagga, science has demonstrated that the genus Equus has undergone collapses and expansions linked to climate. The horse, being the last of the large herbivores to be domesticated, has been a key element in warfare, agriculture, and communication, transforming the social structure of humanity as we, changing the course of history, altered its genetic code to suit our whims and needs.