The horse and the donkey are the fruits of a long evolution that has been going on throughout the Tertiary and Quaternary eras, for 60 million years. The image of man on horseback seems so commonplace that one cannot imagine that it is so recent, only four or five millennia, in the already long history of the relationship between man and animal. When were horses born? In which countries where they born and how did they spread gradually through history? How did the horse’s ancestors turn into the animal we know today? In this article, we will go back over the history of the horse, the evolution of equids, to offer you a journey through time with the animal that has always inspired men.
The horse as a mythical companion of man
The “fusional relationship” between the horse and man, only a few hundred years after it was domesticated, is pushed to the extreme in the ambivalent myths of the Centaurs, the Amazons and Pegasus, the winged horse, and Bellerophon, its inspired rider. The myth of the Centaurs would have its origin in the incursions of barbarian horsemen who would have settled in the Salty Thes and would have been fought there by the Lapithes, whose women they wanted to take. As for the Amazons, they would have been inspired by the Sarmatians. Among the Pontic “horse people” of the Bronze and Iron Ages (Cimmerians, Scythians, Sarmatians…), the close communion of man and horse is apparent in death, when the heroic dead were buried with their mounts or their chariots, and this in the very areas where their predecessors or ancestors had domesticated horses.
The psychopomp horse, companion of the souls of the deceased, spread from Iberia (Vallfogona de Balaguer) to Altaic Siberia (Pazryk) and to Shang China (Anyang). There are only two animal species that have had such a close relationship and common destiny with man: the horse and the dog.
The ancestors of equids
The European Hyracotherium and its alter ego the American Eohippus lived, at the beginning of the Tertiary era, in shrubby or forest environments. They are at the origin of an abundant progeny, in which appear the ancestors of the horse.
These had numerous collaterals, whose lines, after having been maintained more or less long, eventually became extinct. Ignoring this fact, the first authors placed the known “ancestors” of the horse on the same ascending line. This vision is at the origin of the notion of orthogenesis, which is now outdated, and which has also been applied to the human lineage. In fact, in both cases, the “evolutionary radiation” was intense, and many branches could have been perpetuated until the present time, as it is the case in other zoological groups (bovids, rodents…) which comprise a flowering of similar and yet varied species. It is therefore necessary to oppose the concept of evolutionary radiation to that of orthogenesis: man does not descend from the ape, but from one ape among many others.
The promoters of the equidae family, Hyracothe- rium from the Eocene (55 to 50 million years ago), Miohippus from the Oligocene (27 million years ago) were from the size of a cat to that of a fox for the first, and from that of a deer for the second. They were unspecialized animals, leaf grazers, with tubercular teeth (bunodonts) and a low crown (hrachyodonts) and their almost digitigrade limbs, resting on their pads like the present-day tapirs, had 4 fingers on the hand and 3 on the foot. Following one of the laws of evolution, the size of the animals tended to increase. In an environment of grasses, bushes, and shrubs, to escape predators, one had to either stay low to the ground and hide, or break free, see them coming from a distance and run away more easily. The combined effects of biological variation and natural selection validated the second solution: growing above the vegetation cover, the equids started to live in hierarchical groups; while some grazed, nose in the grass, others watched the surroundings, to detect any approach by a predator.
The evolution of the horse’s anatomy
Their dental apparatus specialized in an exclusively herbaceous diet, acquired at the beginning of the Miocene (20 to 17 million years B.C.), in Merychippus and Anchitherium, so-called grazing animals. The teeth compensated for the abrasion caused by these woody and siliceous plants by the prolonged growth of their crowns, which became extremely high (hypsodonty). In response to the diet, which required real rasps and the formation of enamel ridges (lophodont teeth) and in relation to the increase in the volume of the animal, the wear table of the teeth became more complicated (plexodontia). Functionally and morphologically, the premolars came closer to the molars (homodon- tie): “molarization of the premolars” increased the triturating surfaces for chewing plants. These cumbersome and heavy dental rows, on the one hand, and the function of absorbing and obstinate chewing, on the other hand, made the facial extremity of the equids progressively lengthen. At the same time, the limbs became looser, freeing the body from the entanglement of grass and brush. In the second half of the Miocene (12 million years ago), the tribe of Hipparionini, animals the size of ponies, which still have three fingers, is recognized; the tribe of Equini, whose number of fingers on each limb has been reduced to one, appears in the Pliocene (around 4.5 million years ago). The organization of the locomotor apparatus and of the neuromuscular systems were adapted to support a large body mass, to be on the alert and to run on open ground, the horse being fundamentally a walking and running animal rather than a jumper. Many surprising mechanisms have developed, some related to the status of prey of the species, always ready to flee, such as the static blocking of the limbs (the horse sleeps standing up, and with one eye) and the solidarization of their joint movements (economy of means in the context of the reduction of muscles around the single finger and the increase in the efficiency of the muscular effort), others related to the “onguligradie”, such as the mechanism of the “spring foot”. Beyond the locomotor apparatus, all the anatomy and physiology of the horse make it a system perfectly integrated to its way of life and its environment.
The geographical evolution of the horse
About 2,5 M.A. ago, the “hipparions” became extinct in Eurasia and the “equines” were left alone in the field. The genus Equus, which made its entrance in America (around 3,7 M.A. BC) with Equus simplicidens, appears in Eurasia, at the same time as the mammoth, at the beginning of the upper Pliocene (2,5 M.A. BC), with Equus lividens. C.), with Equus livenzovensis, then E. vireti (2.2 to 2.1 M.A. B.C.) and E. stenonis (in the Lower Pleistocene, 1.6 M.A. B.C., Val d’Amo, Tuscany). The genus explodes in a quantity of daughter species, so that up to 25 to 30 species have been recognized, more than in any other genus of the equidae family. The lineages of “Stenonian horses” (E. robustus, E. stehlini, E. altidens, E. bressanus, E. süssenbornis…) end with Equus hydruntinus, known to the Magdalenians. The current Grevy’s zebra would be an ultimate descendant of the Stenonian lineage. The “caballine horses”, stemming from E. simplicidens, appeared less than 1 million years ago in America (E. scotti), they passed into Eurasia about 500,000 years ago.
Equids today: recent and current horses
In the Pleistocene, the distribution area of the equines was generalized to any grassy area, hot or cold (savanna, pampa, steppe, tundra). The horse was present in the whole of the New World, up to Patagonia (Fell cave, 9000 years B.C., Palli-Aike cave, 7000 years B.C.); it then disappeared completely, for unknown reasons.
At the beginning of the Middle Pleistocene (700,000 BC), in the group of “caballine horses”, Equus mosbûchensis reached almost 160 cm at the withers. In response to the cooling of the climate during the Mindel and Riss glaciations (650,000 to 120,000 years BC), the size of the successive forms decreased from E. steinheimensis to E. chosaricus (the Tungus horse) and E. taubachensis (the Weimar horse) of the Riss-Würm (Eemian) interglacial.
At the beginning of the Upper Pleistocene (Eemian), E. achen- heimensis still evokes E. mosbachensis. Then, at the time of the last ice age (Würmian), the Remagen horse, Equus germanicus, measures only 141 to 150 cm at the withers; it is thought that the Neanderthals knew this horse, in the Middle Paleolithic. It gave way (around 30000 years BC) to the Solutré horse (Equus gallicus), which measured only 130 to 145 cm at the withers, as well as its more oriental contemporaries (Equus ferus), at the origin of the current horses. Around 15000 BC, E. arcelini, the “small Magdalenian horse” appeared. The Cro-Magnon men depicted the Solutré horse and the Magdalenian horse on the walls of the decorated caves, in forms diversified by the geography, the natural environment, the diachrony or the artistic style (Chauvet, Lascaux, Niaux, Le Portel, Ekaïn…). It is interesting to observe that, while the size of horses decreases, that of the teeth or enamel ridges of their crowns remains stable or increases: the one reflects the cooling of the climate, the other a tougher, xerophytic plant food.
Towards the end of the Pleistocene (60,000 to 10,000 years B.C.), horses populated the steppes of Eurasia, from the Atlantic Ocean (Portugal) to the Arctic Ocean (Kolteny Island) and the Pacific Ocean (Vladivostok), between the 35.38th to 40th and 50 to 52.75th parallel north, with an incursion into North Africa and the two Americas.
In the Holocene, it became scarcer in the west of the European continent. Paleontological and archaeozoological data show a general decrease in the size of horses between Eastern Europe and the Iberian Peninsula. The eastern wild horse, ancestor of the Tarpan and Przewalski’s horse, has recovered a height of around 140 cm at the withers, while the offspring of the Solutré horse did not exceed 134 cm, or even less in the Basque Country.
To learn more: the evolution of equids
