Domestication of The Horse - Genetic Evidence

Genetic Evidence

See also: Genomics of domestication and History of horse domestication theories

The domestication of stallions and mares can be analyzed separately by looking at those portions of the DNA that are passed on exclusively along the maternal (mitochondrial DNA or mtDNA) or paternal line (Y-chromosome or Y-DNA). DNA studies indicate that there may have been multiple domestication events for mares, as the number of female lines required to account for the genetic diversity of the modern horse suggests a minimum of 77 different ancestral mares, divided into 17 distinct lineages. On the other hand, genetic evidence with regard to the domestication of stallions points at a single domestication event for a limited number of stallions combined with repeated restocking of wild females into the domesticated herds.

A study published in 2012 that performed genomic sampling on 300 work horses from local areas as well as a review of previous studies of archaeology, mitochondrial DNA, and Y-DNA suggested that horses were originally domesticated in the western part of the Eurasian steppe. Both domesticated stallions and mares spread out from this area, and then additional wild mares were added from local herds; wild mares were easier to handle than wild stallions. Most other parts of the world were ruled out as sites for horse domestication, either due to climate unsuitable for an indigenous wild horse population or no evidence of domestication.

Genes located on the Y-chromosome are inherited only from sire to its male offspring and these lines show a very reduced degree of genetic variation (aka genetic homogeneity) in modern domestic horses, far less than expected based on the overall genetic variation in the remaining genetic material. This indicates that a relatively few stallions were domesticated, and that it is unlikely that many male offspring originating from unions between wild stallions and domestic mares were included in early domesticated breeding stock.

Genes located in the mitochondrial DNA are passed on along the maternal line from the mother to her offspring. Multiple analyses of the mitochondrial DNA obtained from modern horses as well as from horse bones and tooth from archaeological and palaeological finds consistency shows an increased genetic diversity in the mitochondrial DNA compared to the remaining DNA, showing that a large number of mares has been included into the breeding stock of the originally domesticated horse. Variation in the mitochondrial DNA is used to determine so-called haplogroups. A haplogroup is a group of closely related haplotypes that share the same common ancestor. In horses, seven main haplogroups are recognized (A-G), each with several subgroups. Several haplogroups are unequally distributed around the world, indicating the addition of local wild mares to the domesticated stock. One of these haplotypes (Lusitano group C) is exclusively found on in Iberian Peninsula, leading to a hypothesis that the Iberian peninsula or North Africa was an independent origin for domestication of the horse. However, until there is additional analysis of nuclear DNA and a better understanding of the genetic structure of the earliest domestic herds, this question cannot be answered.It remains possible that a second, independent, domestication site might exist but, As of 2012, research has neither confirmed nor disproven that hypothesis.

Even though horse domestication became widespread in a short period of time, it is still possible that domestication began with a single culture, which passed on techniques and breeding stock. It is possible that the two "wild" subspecies remained when all other groups of once-"wild" horses died out because all others had been, perhaps, more suitable for taming by humans and the selective breeding that gave rise to the modern domestic horse.