We’re so used to seeing horses, we often forget just what a bunch of weirdos they are. Let’s start with their feet: can you name another animal with only one hoof? If you said cow, sheep or goat, then you need to take a closer look next time you’re on the farm, because they all have two hooves per foot. Deer – likewise. The next curious question: how did such a powerful animal come to have an upstart ape lead it around by the mouth, and sit on it?
The evolutionary history of horses is relatively well understood, but there are still some gaps in our knowledge, and new discoveries continue to be made. So where do these strange animals come from, and how did we end up with the domesticated breeds so familiar to us today?
Talking about toe-numbers is not arbitrary. Horses belong to a group called perissodactyls, or odd-toed ungulates. This group includes horses, tapirs, rhinos and a wealth of extinct animals such as the clawed, long-forelimbed chalicotheres and the rhino-like brontotheres. They contrast with even-toed ungulates, or cetartiodactyls. This group include hippos, deer, giraffes, alpacas and most common farm animals such as pigs, sheep and cows. Also included are the cetaceans: the whales and dolphins. Although they may have shed their toes on their journey to a streamlined aquatic life, we know from genetics and the shape of their skeletons that they share an ancestor with the rest of the even-toed ungulates. Cetartiodactyls are by far the most populous of the two ungulate groups today, but it wasn’t always so.
As their names suggest, the main way odd- and even-toed ungulates are distinguished from each other is by how many little piggies they have on each foot*. Both groups share a common ancestry that reaches to just after the extinction of the non-avian dinosaurs around 66 million years ago, but the exact relationships between the mammals that took advantage of the post-saurian world remain unclear to palaeontologists. In the 10 million years after the mass extinction, the first perissodactyls emerged from the confused wastebasket of animals called condylarths. Their odd-feet had been reduced from the ancestral five toes to only three – sometimes with a vestigial fourth – losing their pinkie and big toes. Tapirs and rhinos have stuck with this ever since.
Horses took things an odd-toed step further.
The first horses appeared around 56 million years ago, but you’d have been hard-pressed to spot one in the wild, let alone identify them in a line-up. These little proto-horses included the “dawn horse”, Eohippus, which stood at barely 50cm at the shoulder, and the only slightly larger Mesohippus. These early horses and their relatives lived in dense forests where they browsed on foliage. Over the next 50 million years, as Earth’s climate dried and cooled and forests were replaced by grasslands, some horses adapted to these new, open landscapes.
While the traditional tale of the horse is told as a linear progression towards horsedom, the truth is that for much of their evolution, multiple branches of horse relatives – of different sizes and toe counts – coexisted across the varied habitats of our planet. By elongating their limbs and reducing their toe-number, some of these horses became swift grassland runners that could cover greater distances. At the same time, their teeth changed shape to shear tough grass instead of chewing soft leaves, and grew longer to counteract a lifetime of abrasion. By around eight million years ago in one lineage of horses – the equine equids – the single middle toe had become a sole weight-bearing hoof. They were the ancestors of today’s horses.
Recent research, however, suggests the picture is not so simple. Evidence of the second and fourth digits of the horse’s foot are still visible, greatly reduced and incorporated into the dominant digit as though giving us the permanent middle-finger. But a team of researchers in America have been taking a closer look at horse limbs. They believe they can still find the remnants of the inner and outermost digits incorporated into the scant second and fourth digits. This gives palaeontologists and evolutionary biologists a new perspective on the mechanisms that allow animals to reduce their digits and limbs. It also reminds us that even when we think we understand something completely, it pays to look twice.
The horse is worth ongoing research not just because it is an evolutionary and biological marvel, but also because it’s an anthropological one. “Domestication of horses revolutionised human history,” says Professor Ludovic Orlando from the Natural History Museum of Denmark. “With horses, we travelled way faster, and could transport goods, people, germs and culture at unprecedented speed.
“In short, horse domestication is a turning point in history.”
Orlando and his coauthors have just published a new study examining the DNA of 88 different horses, more than half of them from archaeological sites spanning over 5,000 years of horse and human history. These include horses from the Botai: a Central Asian steppe culture with the earliest archaeological evidence for horse husbandry, beginning around 5,500 years ago. Their research is the latest in a line of genetic studies trying to unravel the way in which humans have changed horse genes through selective breeding, and to trace the origins of the first domestication of wild equine ancestors.
The generally accepted story of horse domestication begins with humans hunting them for food, as depicted in Palaeolithic cave art. It was thought that they were first domesticated in the Asian steppe for food and milk, perhaps in a similar manner to the reindeer herded in Scandinavia today. They were then increasingly used for labour and transport, and finally in hunting and warfare. The Botai were credited with first domesticating horses, and it was thought these domesticates subsequently spread throughout Eurasia along with migrating human populations, leading to our modern domestic horses.
Orlando’s study tells a different story: “We find that Botai horses are not the ancestors of the modern domestic horses, nor of any domestic horse that we have sequenced within the last four thousand years. They are instead the direct ancestors of Przewalski’s horses, which we used to call the last truly wild horses in the planet.”
Found in zoos alongside their cousins the zebra and onager, Przewalski’s horses have long been considered to be the only remaining wild horses. All others have either gone extinct, or been replaced by domesticated populations in the last few thousand years. Orlando and his colleagues’ findings suggest Przewalski’s horses are actually more like the American mustang or Australian brumby: domesticated horses that have since gone feral. “Wild is generally taken as devoid of any human influence,” Orlando says. “Hence, mustangs, even though living in the wild, are not wild, as their direct ancestors used to be domesticated.”
This means that there are no true wild horses left on earth.
Alan Outram, who was also part of the research, puts these findings into context: “Archaeologists and palaeoecologists are increasing finding that things that were once considered ‘pristine’ and never impacted by humans, actually have been. Another example is the Amazonian rain forest, which was once thought of as pristine, but we now know there are many ‘dark earths’ caused by past human horticulture, [and] the forest later recovered.” However, this doesn’t make these animals and habitats any less valuable: “It’s important to stress that this slightly more complex history doesn’t undermine the need for modern day conservation to maintain biodiversity and protect important lineages. The Przewalski just got more interesting, not less.”
So despite first appearances, it turns out horses still have all their fingers and toes – they are just hidden in their bones. Although the mystery of the earliest ungulate origins over 60 million years ago is still unresolved, we can trace their emergencefrom forest dwarves to elegant grassland specialists, in step with changes in our planet’s climate and vegetation. This odd-toed animal has arguably had the greatest influence on humankind of any domesticated species. While the unexpectedly unique origin of one horse subspecies is now revealed thanks to ancient DNA, work continues to try to trace the origin and domestication of the rest of the world’s horse breeds.
“Understanding the mode and tempo of horse domestication,” Orlando says, “is truly understanding the mode and tempo of human history.”
* There are exceptions to the toe-number rule in both groups, especially in their early evolutionary history as toes were being reduced and lost in different lineages at different times. Other traits in their skeletons help palaeontologists make sense of these complicated relationships.
References
Gaunitz C, fa*ges A, Hanghøj K, Albrechtsen A, Khan N, Schubert M, Seguin-Orlando A, Owens IJ, Felkel S, Bignon-Lau O, de Barros Damgaard P, Mittnik A, Mohaseb AF, Davoudi H, Alquraishi S, Alfarhan AH, Al-Rasheid KAS, Crubézy E, Benecke N, Olsen S, Brown D, Anthony D, Massy K, Pitulko V, Kasparov A, Brem G, Hofreiter M, Mukhtarova G, Baimukhanov N, Lõugas L, Onar V, Stockhammer PW, Krause J, Boldgiv B, Undrakhbold S, Erdenebaatar D, Lepetz S, Mashkour M, Ludwig A, Wallner B, Merz V, Merz I, Zaibert V, Willerslev E, Librado P, Outram AK, Orlando L. 2018 Ancient genomes revisit the ancestry of domestic and Przewalski’s horses. Science 10.1126/science.aao3297
Solounias N, Danowitz M, Stachtiaris E, Khurana A, Araim M, Sayegh M, Natale J. 2018 The evolution and anatomy of the horse manus with an emphasis on digit reduction. Royal Society Open Science 5: 171782.
