The genus Equus originated in N America. About
a dozen American species are known from the late Pliocene and the Pleistocene
(some, e.g. the Laurentide horse, E. laurentius, survived until the
early Holocene). During the Pleistocene equids migrated into Eurasia and
Africa, giving rise to Old World horses, asses, onagers, zebras and quaggas. The
systematic position of various Eurasiatic equids remains debatable, but the
recent tendency is to separate the species typified by the domestic horse,
E. caballus (32 pairs of chromosomes), from the Asian wild horse,
E. ferus (33 pairs). Przewalski's horse, E. ferus przewalskii,
is a Mongolian variant of the latter; very probably the recently discovered
Riwoche Valley horse from Tibet is another (at any rate it has the same
chromosome count); other varieties of the species, including the Siberian tundra
pony, are extinct. Genetic evidence suggests that E. caballus and
E. ferus diverged about 200,000 years ago, and the fact that no known
breed of domestic horse has 66 or 65 chromosomes (the latter number is found in
hybrids between the two species) confirms that E. ferus did not
contribute significantly to the genetic makeup of domestic horses.
The West European Ice Age pony of Lascaux cave paintings
was morphologically of the ferus type, though it isn't clear whether it
should be regarded as conspecific with Przewalski's horse. There were several
(difficult to classify) varieties of wild horse in post-glacial North Africa and
Europe; some of them (e.g. the Scandinavian forest horse) were much larger and
heavier-built than the Asian wild horse and don't seem to have belonged to the
same species. The smaller frizzly-maned and mouse-coloured "forest tarpan" of
Northern Germany, Poland and Lithuania is quite mysterious, though it survived
in the wild until the 19th century. It was regarded as a game animal in the
Middle Ages. Since the so-called Polish "koniki" ('little horses'), which
are descended partly from the captive wild "tarpans" of Bialowieza Forest, show
no Przewalski-like genetic traits, the Central European forest horse was in all
likelihood a local variant of wild E. caballus, closely related to the
true tarpan.
The East European steppe tarpan, E. caballus
gmelini, used to range from western Ukraine and Romania to southern
Russia. Like the domestic horse, and like the extinct wild horses of the Middle
Eastern deserts, but unlike any other living equid, it had a floppy mane with a
forelock. Its range extended more or less to the Volga; the wild horses of
Kazakhstan were already of the Przewalski type (though a hybrid zone may have
existed north of the Caspian Sea). Obviously, the hydrological conditions of the
steppe region separated the two varieties through most of the Pleistocene and
part of the Holocene, creating an effective reproduction barrier and
letting them evolve into different species.
From the genetic point of view only the European steppe
tarpan and some of the extinct European and Middle Eastern wild horses can
be taken into consideration as possible ancestors of the extant breeds of
the domestic horse. It follows that the domestication of the horse can't have
taken place farther east than the Volga, and that all the domestic horses of
Central Asia came from the west. Presumably Caspian ponies and Arabians derive
mainly from the steppe and desert subspecies of E. caballus, while the
North European ponies and heavy horses have been influenced by the local forest
varieties. North African desert horses possibly contributed to the athletic Barb
and Iberian breeds.
I invite comments on the implications of horse biology
for IE origins.
Piotr