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Where did Horses Come From?

Last revised April 23, 2017.

Przewalski’s Horses grazing.1

Where did the modern horse come from, and how did it get there?

Speciation

I remember reading about this when I was in the 5th grade. Back then, there was Eohippus, then Mr. Horse lost some toes, grew larger, and presto: something for Roy Rogers, you and me. I thought that complicated sequence was amazing. Then time passed, and research revealed more…

The modern horse has its ancient roots in North America.2

  • Eohippus/Hyracotherium. 55-52 million years ago, its ancestors were the size of foxes, with eyes on the front of their heads. Its legs were becoming longer, but leg bones were not yet fused, and this protohorse could rotate his legs. Five toes on all feet, though only four on the front touched the ground, and only three on the rear touched the ground. Toes touching the ground had small hooves. Their molars had developed horse-like ridges.
  • Orohippus. 52-45 million years ago. Orohippus remains have been found in Wyoming and Oregon.
  • Miohippus. 32-25 million years ago. There were now a dozen genera of early horses scattered from the Great Plains to Florida.
  • Parahippus. 24-17 million years ago. This little horse had moved out of the forests and onto the plains of North America. Its eyes had moved toward the sides of its head, and the two toes on the sides of it foot had begun to disappear, leaving it with three toes to run on.
  • Merychippus. 17-11 million years ago. This early horse had grown taller, and looked like a modern horse with a long face, long legs, and high crowned cheek teeth, making it the first known grazing horse. It still ran on three toes.
  • Pliohippus. 12-6 million years ago. Larger than its predecessors, with a deeper barrel, stronger and longer legs, and more noticeable withers. Ranged from Canada through South America, perhaps with concentrations in the Great Plains.
  • Equus. 5 million years ago to present. Dispersed from North America to every continent except Australia and Antarctica.

But the evolutionary sequence described above trivializes what really happened. During the first third of its evolution, the horse family remained small. Then different lines of descent changed irregularly, not in unison. Some groups grew, some grew than shrank. The number of transitional fossils that we now have our hands on is vast and diverse. The chart below only shows the main events.

Horse Family Tree.3

What happened to Eohippus? Like Pinto the Wonder Horse, Eohippus is dead.4 The name “Eohippus” is no longer used by paleontologists, because this dawn horse had already been discovered and named earlier, so Eohippus validus is now recognized as a “junior synonym” of Hyracotherium angustidens. And my fifth grade reading had this little horses’s size wrong, too. It wasn’t the size of a “small fox terrier”, but more than twice that size. The fifth grade was also when I learned that “geese fly in formation to improve their aerodynamics, and the strongest ones lead the flock.” And that plate tectonics was not possible. Maybe I should have skipped the fifth grade.

The truth about the origins of the horse is as fabulous and complex as the horse itself. And it gets more amazing.

Ice Ages

About 160,000 years ago an Ice Age lowered the sea level, creating a land bridge between North America and Asia. Horses and other species took advantage, and traveled to eastern Eurasia and then westward5. The horse was first domesticated in the western steppes of Eurasia — Ukraine and northwest Kazakhstan — in the same areas as pointed to by archaeological evidence.6

About 14,100 years ago, the planet experienced a prolonged cooling, which lasted for about 1,300 years. The cooling tied up huge quantities of water as ice, and the sea level dropped, forming land bridges from North America and Asia, and North America and Europe. Species dispersed over these land bridges.

There is paleontological evidence that Pleistocene horse species, including the wild ancestor (Equus ferus) of the domestic horse, spread west from North America, migrating into Eurasia over the Bering land bridge7. There was a wave of horse species that spread out of North America at that time, including Anchitherium and Sinohippus. Some headed west to Asia, and some may have gone east, to Europe, over another land bridge.

The Holocene

The Holocene is the current geological epoch. It began about 11,500 years ago, when the glaciers of the last ice age began to retreat. It is sometimes called the Anthropocene — the age of man — because this epoch has been increasingly dominated by a single species. In the scale of geologic time, the Holocene is like the thickness of the glue on the back of a postage stamp sitting on the top of the Empire State Building — not very long.

During our evolution, humans spent a very long time living in Africa. During that time, the animals of Africa and the humans had a chance to co-evolve. Humans adapted to hunting, and the animals of Africa became more adept at avoiding hunters. It was an uneasy stand-off. But when humans began to migrate out of Africa, they encountered species that had no special training in avoiding us. Humans could walk right up to a mammoth and put a spear in its side. So humans spread around the world, snacking as they went. Given the ease of catching something for dinner, humans preferred big mammals, birds, and tortoises — the megafauna — over little ones.

The percent of megafauna on different land masses over time, with the arrival of humans indicated.8

In the history of human dispersion, the pattern of extinctions has always been the same: add humans, subtract species. We learned to hunt, but never learned the difference between kill and overkill.

Many wish to believe that climate change produced a change of habitat which had fatal effects on the horse population of North and South America, and that this happened to coincide with the arrival of humans. But another view is possible:  climate change produced land bridges, humans arrived, the horses disappeared, and the habitat they once maintained through grazing now changed.9

Lucky for us horse lovers that horses had already radiated out from North America over earlier land bridges, because humans traveled the land bridges in the opposite direction in a later ice age, reaching North America from Asia. As the cooling created the land bridges, it also changed habitats, likely causing some extinctions and putting many animals at risk.

As humans poured into Asia, Europe, North America, and other places, they discovered animals that had not yet learned to fear them, and were fairly easy to approach and kill. It took a while for us to kill all the megafauna — giant beavers, mammoths, dire wolves, camels, and other wonders. But Rome wasn’t built in a day, either.

Prior to domestication, it is very likely that horses were hunted as any other large mammal, for meat. Cave paintings in France and Spain from around 15,000 years ago show humans hunting horses for meat and hides. The remains of horses have been found at archaeological sites up to about 9,000 years ago, when they become rare. Hunting was the likely cause of the extinction of wild horses in Eurasia10, North America, and South America.

Domestication of the horse involved those that had dispersed to Asia and Europe, and that survived our arrival. Research has narrowed the hunt for the most recent domesticated ancestors of the horse to Eurasia, where the horse was also believed to have been first domesticated, and to the Iberian Peninsula (Spain, Portugal, and Andorra). Domestication requires both humans and horses, so we can presume that it only occurred after humans arrived and in areas offering suitable habitat for modern wild horse populations. Such habitat is open plains, not forests; much of central Europe was forested back in those days, and would not have been good habitat for modern horses.

As a population migrates, its genetic diversity is reduced. Only some of the travelers will make the trip, and they will breed along the way. When long distances are involved, it takes a long time to blur and hide the subtle genetic differences that remain along the route.11 To determine the direction that horses spread, German researchers examined the current genetic diversity of horses from several parts of Europe and mapped it. The maps below show the areas with the greatest genetic diversity and the areas of grassland or savannah at the time of presumed domestication. These researchers had no samples of horses of the steppes of Asia, so we don’t know from their research if the maps below tell the whole story or just part of it, but it now seems as if the modern horse radiated from the lower right corner of the map below.

High diversity in European horses mirrors the distribution of open landscape in the mid-Holocene.12 Maps A and B show interpolations of two measures of genetic richness, and show the greatest richness in the Iberian peninsula and the Caspian region. The greater the diversity, the more likely we have found the origin. The third map shows the distribution of biomes in the area 6,000 years ago.

Genetic research on horses continues, with exciting results. For example, we have recently learned that ancient Chinese domestic horses are more related to modern Mongolian horses than the Przewalski horse.13

Domestication

For decades, scientists have studied horse domestication — in part because this may have been the most fateful of all domestications. Societies built around the horse were able to travel long distances, be more effective in warfare, and carry on more efficient trade. Genghis Khan demonstrated this, much to the dismay of those his armies encountered. (Genghis went exploring between 1162 and 1227. By then, horses had made it to Europe, and the cavalry existed. It just wasn’t ready for him.) The horse must get most of the credit for the spread of Indo-European languages and culture, and the collapse of ancient societies.14

As their numbers diminished, humans may have decided that they (or perhaps just mares) could be raised for milk (and hides and meat when needed), reducing the need for hunting them. Eventually horses went extinct in the wild — first the ancestors of what is now the domestic horse — Equus ferus caballus — and more recently the ancestors of Przewalski’s horse (Equus ferus przewalskii. A few of this subspecies have been captive bred and returned to the wild, so they are no longer “extinct in the wild”).

The evidence for domestication origins in the Eurasian steppes is strong. Horse remains found at archaeological sites in the Eurasian grassland steppes date back about 6,000 years.15 Other archaeological evidence includes finding horse milk residues in pots and tooth wear on horse skulls that resembles that of frequently bitted horses dating back to around 3,500 BCE16  — about 5,500 years ago. Other research has found that different coat colors — a likely consequence of domestication — first appeared in Siberia and Eastern Europe.17 The evidence of bits is evidence of riders, and what they did to control their steeds.

Bone growth on the bars (red arrows) caused by the wear of the bit. Some horses have a groove across the bar near the lower first molar (blue arrow), where bone has withered away from bit pressure.18

There is evidence that the ridden horse had become important in human culture by 1,000 BCE,19 about 500 years after Moses. If so, it would not be likely that Egyptians chasing Moses in “The Ten Commandments”20 would have been riding horses or using them to pull carts and chariots if horses had not yet been domesticated.) Early humans did not spend much time at their typewriters, and didn’t leave us notes on their lives, so details of this transition from dinner to transportation are sketchy. Ever since the first riders managed to hang on, horses have been with society, as war horse, draft horse, and now as a sporting and companion animal.

Horses may have made it to Europe prior to domestication. However, we know more about the horses that were domesticated and made the trip from western Eurasia. Researchers have found evidence that showed that the spread of horse domestication out of the western Eurasian steppe was accompanied by high levels of introgression (repeated backcrossing) from local wild populations.21 It is easy to imagine that wild horses often escaped from domestic control, and that wild horses often found opportunities to breed with captive horses. The early days of the domestic horse may have been as filled with adventure as the characters in Days of Our Lives.

Further, because these researchers found greater genetic diversity in mares than stallions, they believe that introgression from the wild was mainly by adding captured wild mares to existing herds of mares. This may be because the wild horse (as demonstrated by Prezewalski’s horse) was so difficult to breed in captivity. Some genetic diversity may have been lost in wild males: I think it possible that early man captured the mares — and focused on mares with young foals — as sources of milk and later meat, because they were easier to manage than stallions (and also easier to milk).

Although the horses in Asia and Europe likely originated in North America, they went extinct, and modern North American horses originated in Asia and Europe. Horses were re-introduced to the American west by Spanish explorers in the 16th century, established herds, and were adopted by Native Americans, who probably followed the same pattern as the Natives of western Mongolia years before. These re-introduced horses were likely of Barb and Andalusian blood lines, native to the Iberian Peninsula. The wheel turns.

In the early days of domestication, humans learned from each other, and in some localities, attempts must have been repeatedly made to domesticate the wild horse. All such efforts put the ancient wranglers — and their horses — at great risk.

That risk is not much diminished today. Horses are surely less flighty than in those early days. Our thoroughbreds must be faster, and our draft horses must be stronger. But 3,500 years or so is not much time for evolution to do anything for an animal with a low reproductive rate. Fruit flies sure, horses nope. So while they are domesticated enough that we can ride them, it is also possible to ride a rhino22 or zebra23 without first domesticating them. And slightly domesticated caribou24 seem to be willing mounts in our lives, too.

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1 Image source: photo byJairo S. Feris Delgado. https://commons.wikimedia.org/wiki/File:Equus_przewalskii_Le_Villaret_02_2006-07-20.jpg

2 “Horse Evolution Over 55 Million Years” http://chem.tufts.edu/science/evolution/horseevolution.htm

3 Image source: “Horse Evolution Over 55 Million Years” http://chem.tufts.edu/science/evolution/horseevolution.htm

4 For lyrics to my favorite song about horses, see Tom T. Hall “Pinto The Wonder Horse is Dead” https://genius.com/Tom-t-hall-pinto-the-wonder-horse-is-dead-lyrics

5 Warmuth, V., Eriksson, A., Bower, M.A., Barker, G., Barrett, E., Hanks, B.K., Li, S., Lomitashvili, D., Ochir-Goryaeva, M., Sizonov, G.V. and Soyonov, V., 2012. Reconstructing the origin and spread of horse domestication in the Eurasian steppe. Proceedings of the National Academy of Sciences, 109(21), pp.8202-8206.

6 Outram AK, et al. (2009) The earliest horse harnessing and milking. Science 323: 1332–1335.; Olsen SL (2006) Documenting Domestication: New Genetic and Archaeological Paradigms,

eds Zeder MA, Bradley DG, Emshwiller E, Smith BD (Univ of California Press, Los Angeles), pp 245–269.

7 Prothero, Donald R., and Robert M. Schoch. Horns, tusks, and flippers: the evolution of hoofed mammals. JHU press, 2002.

8 Image source: The timing of extinctions follows the “march of man” (after Martin, 1989) Martin P. S. (1989). Prehistoric overkill: A global model. In Quaternary extinctions: A prehistoric revolution (ed. P.S. Martin and R.G. Klein). Tucson, AZ: Univ. Arizona Press. pp. 354–404. ISBN 0-8165-1100-4. https://upload.wikimedia.org/wikipedia/commons/a/ad/Extinctions_Africa_Austrailia_NAmerica_Madagascar.gif

9 Zimov, S. A.; Chuprynin, V. I.; Oreshko, A. P.; Chapin, F. S.; Reynolds, J. F.; Chapin, M. C. (Nov 1995). “Steppe-tundra transition: a herbivore-driven biome shift at the end of the Pleistocene”. The American Naturalist. 146 (5): 765–794. doi:10.1086/285824. JSTOR 2462990.

10 Clutton-Brock, Juliet. Horse power: a history of the horse and the donkey in human societies. Natural History Museum Publications, 1992.

11 See Linz B, et al. (2007) An African origin for the intimate association between humans and Helicobacter pylori. Nature 445:915–918.; Tanabe K, et al. (2010) Plasmodium falciparum accompanied the human expansion out of Africa. Curr Biol 20:1283–1289.; François O, Blum MGB, Jakobsson M, Rosenberg NA (2008) Demographic history of European populations of Arabidopsis thaliana. PLoS Genet 4:e1000075.; Manica A, Amos W, Balloux F, Hanihara T (2007) The effect of ancient population bottlenecks on human phenotypic variation. Nature 448:346–348.; Prugnolle F, Manica A, Balloux F (2005) Geography is a better determinant of human genetic differentiation than ethnicity. Curr Biol 15:R159–R160.; Ackland GJ, Signitzer M, Stratford K, Cohen MH (2007) Cultural hitchhiking on the wave of advance of beneficial technologies. Proc Natl Acad Sci USA 104:8714–8719.

12 Image source: Warmuth, V., Eriksson, A., Bower, M.A., Cañon, J., Cothran, G., Distl, O., Glowatzki-Mullis, M.L., Hunt, H., Luís, C., do Mar Oom, M. and Yupanqui, I.T., 2011. European domestic horses originated in two Holocene refugia. PloS one, 6(3), p.e18194.

13 Cai, Dawei, Zhuowei Tang, Lu Han, Camilla F. Speller, Dongya Y. Yang, Xiaolin Ma, Hong Zhu, and Hui Zhou. “Ancient DNA provides new insights into the origin of the Chinese domestic horse.” Journal of Archaeological Science 36, no. 3 (2009): 835-842.

14 Anthony, David W., Peter Bogucki, Eugen Comşa, Marija Gimbutas, Borislav Jovanović, J. P. Mallory, and Sarunas Milisaukas. “The” Kurgan Culture,” Indo-European origins, and the domestication of the horse: a reconsideration [and comments and replies].” Current Anthropology 27, no. 4 (1986): 291-313.; Diamond, Jared M. “The earliest horsemen.” Nature 350 (1991): 275-276.

15 Anthony, D. W. in Horses Through Time, S. L. Olsen, Ed. (Roberts Rinehart for Carnegie Museum of Natural History, Boulder, CO, 1996), pp. 57–82. ; Clutton-Brock, J. A Natural History of Domesticated Mammals (Cambridge Univ. Press, Cambridge, ed. 2, 1999).; Bennett, Deb, and Robert S. Hoffmann. “Equus caballus Linnaeus, 1758 Horse.” Mammalian Species 628 (1999): 1-14.

16 Anthony DW, Bogucki P, Comşa E, Gimbutas M, Jovanović B, et al. (1986) The “Kurgan Culture,” Indo-European Origins, and the Domestication of the Horse: A Reconsideration [and Comments and Replies]. Curr Anthropol 27: 291–313.; Outram AK, Stear NA, Bendrey R, Olsen S, Kasparov A, et al. (2009) The earliest horse harnessing and milking. Science 323: 1332–1335.

17 Ludwig A, Pruvost M, Reissmann M, Benecke N, Brockmann GA, et al. (2009) Coat Color Variation at the Beginning of Horse Domestication. Science 324: 485–485.

18 Image source: Sandin, Theresa. “The Bridle and the Bit”. Aug 29, 2005. http://www.sustainabledressage.net/tack/bridle.php

19 Clutton-Brock, Juliet. Horse power: a history of the horse and the donkey in human societies. Natural History Museum Publications, 1992.

20 “The Ten Commandments (1956 film)” Wikipedia. https://en.wikipedia.org/wiki/The_Ten_Commandments_(1956_film)

21 Cieslak M, Pruvost M, Benecke N, Hofreiter M, Morales A, et al. (2010) Origin and history of mitochondrial DNA lineages in domestic horses. Plos One 5: e15311.; Lira J, Linderholm A, Olaria C, Brandström MD, Gilbert MTP, et al. (2010) Ancient DNA reveals traces of Iberian Neolithic and Bronze Age lineages in modern Iberian horses. Mol Ecol 19: 64–78.

22 https://www.google.com/search?q=ride+a+rhino&num=100&rlz=1C1CHFX_enUS628US628&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwiKqNKPgo7SAhXExYMKHWNiCqgQsAQIMA&biw=1536&bih=686

23 https://www.google.com/search?rlz=1C1CHFX_enUS628US628&biw=1536&bih=686&tbm=isch&sa=1&q=ride+a+zebra&oq=ride+a+zebra&gs_l=img.3..35i39k1j0i30k1j0i5i30k1j0i24k1.39883.40906.0.41147.5.5.0.0.0.0.61.272.5.5.0.voicev1e%2Cevd%3D1%2Ccfro%3D1…0…1.1.64.img..0.5.272…0i10i24k1.qYxGX715WiI

24 See Froelich, Amanda. “16 Photos Of A Magical Mongolian Tribe That Rides Reindeer And Hunts With Eagles” http://www.trueactivist.com/16-photos-of-a-magical-mongolian-tribe-that-rides-reindeer-and-hunts-with-eagles/”

 

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