Transcript:

[0:04]

We ended last week with the emergence of the Hagerman’s horse, the first species termed Equus. If you can travel back in time with me again, we’ll continue their story — the story of how we got to today’s versions of horses, donkeys, and all the wild equids. Hagerman’s horse emerged in North America, and two and a half million years ago we were in the height of an Ice Age. Large ice sheets extended from the Arctic through Canada into parts of the northern United States. Similarly, in Europe and Asia, massive ice sheets pushed south.

[0:54]

Hagerman’s horse lived in what is now Alaska. They crossed ice sheets and eked out a living in the cold. To find food, they scraped away ice and snow to get bits of grass, lichen, or other plants that modern northern herbivores survive on. Picture an animal with the face of a donkey, the ears and body of a horse, probably a short mane like Przewalski’s horses and zebras, and possibly striping across its body. You might see a small band — a resident stallion, a few mares, foals, and yearlings — surviving in the frigid environment.

[1:46]

For some reason, perhaps driven by environmental pressures, they turned west toward Asia. At the time, the ocean — what is now the Bering Sea — was frozen. Crossing this massive ice sheet, they found no food and had to rely on their internal reserves to travel 50–100 miles (hundreds of kilometers) to Asia. They walked until they set hoof on the mainland in modern-day Siberia, scraped for small amounts of grass or lichen, then likely migrated farther south until they reached the grasslands of Asia. This happened two and a half million years ago, perhaps with multiple small bands making the journey. They now had the rest of the world ahead of them.

[4:15]

Telling the story of Equus and today’s modern horses is always exciting. Last week, we covered 55 million years of history, from Eohippus — 12 inches at the shoulder, four toes on the front feet, three on the back, forest-dwelling and eating fruits and nuts — through the adaptation to grasslands. We saw the shift to three toes with Mesohippus, increasing size, receding side toes, and teeth adapting from soft diets to coarse grasses. We reached Dinohippus, the proto-horse, about the size of a pony (14 hands, under 5 feet or 1.5 m at the shoulder), just before the emergence of Equus.

[5:57]

Why does this matter? Understanding their history gives us insight into modern equids — horses, donkeys, zebras, wild asses — and explains their behaviors, diets, and care needs. It shows how they survived millions of years and informs our approach to training, feeding, and working with them. Next week, we’ll jump into domestication and how we came to have horses so integrated into human life that children can lead them today.

[7:45]

Returning to Hagerman’s horse and the term “equid”: a few hundred years ago, Swedish taxonomist Carl Linnaeus classified these animals and coined the genus name Equus, from the Latin for “horse,” derived from ancient Greek. From there came the scientific classification still used today. Equus simplicidens refers to Hagerman’s horse; Equus caballus is the domestic horse; Equus ferus refers to the extinct true wild horse; donkeys are Equus asinus (or Equus africanus asinus), descended from the African wild ass.

[9:47]

Fossils of Hagerman’s horse date back almost 4 million years, but it was around 2.5 million years ago that they successfully crossed the ice sheet into Asia. Horses also migrated into South America about 3 million years ago, after the Isthmus of Panama connected the continents. They went extinct in both North and South America around 10,000 years ago, likely due to a combination of climate change at the end of the Ice Age and hunting by early humans.

[12:59]

Once in Asia, Equus radiated outward, eventually reaching Africa. Today’s equids fall into two main lines: caballine (true horses, including domestic horses and Przewalski’s horse) and non-caballine (zebras and wild asses). Domestic horses number over 60 million worldwide and have 64 chromosomes; Przewalski’s horses have 66 chromosomes and are a distinct species. Nearly extinct 50 years ago, their population has been restored to over 2,500, with reintroductions in China and Mongolia.

[16:03]

Zebras include the endangered Grevy’s zebra (46 chromosomes, ~2,500 left), the near-threatened plains zebra (44 chromosomes, ~1 million), and the vulnerable mountain zebra (32 chromosomes, ~35,000 left). Wild asses include the critically endangered Somali wild ass (~700 left, 62 chromosomes) and Asiatic wild asses such as the onager (~55,000) and the kulan (~1,300). Domestic donkeys, descended from African wild asses, number about 53 million worldwide.

[18:47]

Hagerman’s horse is the ancestor of all modern equids — horses, zebras, and asses. After reaching Asia, equids diversified. Cold climates, grasslands, and high-fiber diets shaped their evolution, much like wild horses survive today in places like Nevada. While we can trace broad patterns, precise evolutionary paths are harder to determine due to the similarity of equid bones and the lack of preserved soft tissue. Genetics is now helping to clarify relationships.

[22:04]

Hagerman’s horse is the only known equid species to migrate from North America to Asia. Studies, like a 2021 paper in Nature by Cirilli et al., examined over 30 species from Hagerman’s horse onward, using skull, jaw, and limb measurements to trace evolutionary shifts toward zebras, asses, and other species. Minor anatomical changes — such as a slightly different palate — could indicate adaptations to diet and lead to new species. Zebras emerged about 700,000 years ago; wild asses diverged from Hagerman’s horse about 2 million years ago; African wild asses appeared around 1.5 million years ago.

[26:12]

In South America, horses evolved into a different genus, Hippidion, before going extinct 10,000 years ago. For the last million years, wild types remained relatively stable until about 6,000 years ago, when humans began domesticating horses. That’s where we’ll pick up the story next week.

[27:46]

As always, I acknowledge that this is not my field of expertise — I’m an equine physiologist — and I respect the work of equid paleontologists. I believe telling their history is important, which is why we start here before moving into topics like domestication and modern horse distribution. There are about 60 million horses in the world today, plus millions of donkeys and other equids, and they’re found in nearly every country. If you have comments or topic suggestions, email me at podcast@madbarn.com. You can also visit madbarn.com for articles on equine health, reproduction, and nutrition, and follow us on social media. Please subscribe to the podcast for more episodes.