A single trait―wild―defines and distinguishes the Przewalski horse from any of its congeners. In Mongolia, where it originates from, its name Takh or Takhi (монголын тахь) means spirit horse. Yet, the residents of the Haus zur Wildnis are tame, much tamer than some semi-feral and feral horses I encountered in Canada, and most people would probably not call them wild. Appearances are often misleading: their affable disposition results from decades of captivity, hence familiarity to men, and does not prejudice their wildness at all. To determine the wild trait, one must investigate the history of the species through its biology, i.e. its DNA.
Two DNA research cast light on the complex evolutionary history and phylogeny of the Przewalski horse within the Equus genus, which until now remained unclear. The intent of the first study was to determine the genetic relationship between Przewalski and modern domestic horses through assessing levels of genetic variation on sexual chromosomes and autosomes. The second research was performed on the foot bone of an equid that lived 560,000 to 780,000 years ago (Middle Pleistocene). Recently fetched from the permafrost soils of the Canadian Arctic near Thistle Creek (Yukon), the foot had been well preserved by the cold temperatures and the complete genome of the prehistoric horse could be successfully sequenced using state-of-the-art technology, making it the oldest ever fully sequenced genome. The genetic code was then compared with eight genomes of the Equus genus: those of a 43,000 year old horse from the Late Pleistocene, a Przewalski horse, a donkey, and specimens of five modern domestic horse breeds.
The Przewalski horse was found to be closely related to our modern domestic horse―especially the Mongol horse due to probable unsubstantial interbreeding on overlapping range in the past―as both appear to belong to the same phylogenetic clade, which means they do not descend from one another but share a common ancestor. Przewalski and domestic horse populations diverged sometimes between 72,000 and 38,000 years ago and there is no sign of recent admixture between them, their evolutionary histories are therefore distinct. More recently, horse domestication, which began circa 6,000 years ago, may have played a role in shaping horses’ genetics given the different levels of genetic variation observed in current Przewalski and domestic horses.
With 33 chromosome pairs (2n=66) against 32 for all domestic horse breeds (2n=64), Takhis possess the highest diploid chromosome number of all equine species. This extra chromosome pair either results from fission of chromosome 5 in Przewalski horses or fusion of chromosomes 23 and 24 in domestic horses through evolution. The specific genes code for some unique morphological characteristics such as near-absent forlop, erect mane, short guard hair on the upper part of the tail, shedding tail and mane as well as distinctive skeletal features. However, the fact that Przewalski and modern domestic horses produce fertile offspring when interbreeding proves that genetic differences are minor and confirms that both species belong to the same lineage―the caballines. By contrast, crossing between caballine and non-caballine species such as domestic horse with donkey or zebra, or donkey with zebra generally result in viable but unfertile hybrids due to more significant chromosomal disparity.
The wild trait of the Przewalski horse is therefore defined by its unique evolution and biological imprint. The only other horse species assumed to have shared such a privilege in historic times is the Tarpan (Equus ferus ferus), also known as the Eurasian Wild Horse, which was extirpated in the wild in c. 1897 before completely dying out in the early 1900’s. Horses colloquially referred to as wild such as the Mustang (USA), Cayuse (Canada), Exmoor and Dartmoor (UK), Brumbie (Australia), or Camargue (France) are in fact all feral or semi-feral because their direct and common ancestor―considered by many to be the Tarpan―was once domesticated. They are wild to the sole extent that they are untamed and free-range in a natural environment more or less secluded from men. It follows that the Przewalski horse is the very last extant wild horse and its ancient origins plunge us back to prehistoric times.
REFERENCES –  Horse Domestication and Conservation Genetics of Przewalski’s Horse Inferred from Sex Chromosomal and Autosomal Sequences, Lau et al., 2008.  Recalibrating Equus evolution using the genome sequence of an early Middle Pleistocene horse, Orlando L. et al., Nature, 4 July 2013.  A clade is a group consisting of an ancestor and all its descendants, a single “branch” on the “tree of life”. The ancestor may be an individual, a population or even a species (extinct or extant).  According to various authors cited in: Horse Domestication and Conservation Genetics of Przewalski’s Horse Inferred from Sex Chromosomal and Autosomal Sequences, Lau et al., 2008.  Horse Domestication and Conservation Genetics of Przewalski’s Horse Inferred from Sex Chromosomal and Autosomal Sequences, Lau et al., 2008.  Przewalski’s Horse: The History and Biology of an Endangered Species, Lee Boyd, Katherine A. Houpt, p 7.  Ungulate Taxonomy, Colin Groves and Peter Grubb, Johns Hopkins University Press, 1st Ed., 2011, p 13.