Transcript:

[0:05]

Today we’re going to go on a journey millions of years back. I know it’s tough to wrap our heads around that, but stick with me. We need to go back about 65 million years ago. The planet Earth was very different. You had massive dinosaurs — everybody’s favorite or least favorite — the Tyrannosaurus rex. Very popular in culture, in movies, but we also had flying reptiles, swimming reptiles, and other fantastical creatures walking the planet.

[0:48]

Then, bam — something tragic and massive happened that changed the trajectory of Earth. In the last episode, we talked about how important forces were. This fifth mass extinction was caused by an asteroid estimated to be 6 miles (10 kilometers) wide. It hit the planet near modern-day Mexico, in the Yucatán Peninsula, and changed the history of the planet. Many species died out that day, and over the weeks and next thousand years, dinosaurs went extinct. Overall, 76% of all species went extinct in this mass extinction.

[1:35]

Yet life survived. Some species, like our early mammal ancestors, endured. Early mammals survived in caves or underground areas protected from the superheated atmosphere that followed the asteroid impact. Around the planet, there was a lack of food — plant life died off — so how these animals survived still baffles scientists today. It took thousands of years for the Earth to recover. Gone were the large dinosaurs and reptiles that had suppressed mammals. This marked the shift from the Age of Dinosaurs to the Age of Mammals. It took about 10 million years after that asteroid for mammals to diversify.

[2:35]

This is where a small creature — not much bigger than a small border collie, about 12 inches (60 cm) at the shoulder — emerged. We call it Eohippus, the first horse or “dawn horse.” We’re going to talk about how this small woodland creature, who browsed on leaves and branches in dense forests, became the massive grazing animals we know today, roaming the plains eating grass. We’ll explore what science tells us about how they did it, why we know they did it, and why it matters.

[3:15]

A lot of people ask: why do I need to know this? The evolution of the horse explains why they are the way they are — why domestic horses eat grass, why they are hindgut fermenters, why they behave as they do, why they have a single hoof. This is also true of donkeys, wild asses, Przewalski’s horses, and zebras. Evolution has shaped them into who they are today, and we’re going to tell that story.

[4:51]

Anytime we study evolution, it’s difficult to wrap our heads around. A year, a decade — we can grasp those. But hundreds, thousands, millions of years? It’s not tangible. Today I’ll try to put that in perspective and make it tangible, so you can follow step-by-step how this process happened. Before we dive into the horse specifically, we’ll cover some key concepts: what evolution is, how fossils form, and why we find so many fossils of ancient horses — more than almost any other species. For the last 60–80 years, the horse has been one of the species scientists point to as proof of evolution.

[6:30]

We, like horses, are complex organisms made of DNA — our blueprint. DNA mutates naturally over millions of years, leading to changes in species. Each of us has about 42 genetic mutations from our parents, out of 3.2 billion pieces of code. Over thousands and millions of years, these mutations add up, changing species. Natural selection drives many of these changes — as the environment changes, species adapt or go extinct.

[8:57]

We can see evolution-like changes in real time with things like antibiotic resistance in medicine, or dewormer resistance in horses. A small percentage of worms survive treatment due to mutations, and over time, those resistant worms become the majority. The same happens with pathogens and antibiotics. Scale that up to millions of years, and small advantages — running faster, digesting better — determine which species survive.

[11:16]

Now, fossils. Fossils form when an animal’s remains are buried quickly in sediment or other material before they decompose. Over time, minerals replace the bone, creating a rock-like replica. Horses have an extensive fossil record because they evolved in North America, where conditions for fossil preservation are favorable. Dry, arid regions — like parts of the U.S. and Mongolia — preserve fossils better than tropical forests.

[14:55]

The horse has been evolving for over 55 million years, from Eohippus to modern Equus. One of the earliest major papers on horse evolution was published in 1926 by W.D. Matthew of the American Museum of Natural History. He noted the horse’s fossil record is the most complete among large animals, and its skeletal and dental changes are easy to understand.

[17:03]

Back in the Eocene (56–34 million years ago), the world was hot and humid, with dense forests in North America. Eohippus, the dawn horse, lived here — about 12 inches tall, weighing 12–60 lbs. It had four toes on the front feet and three on the back. It was a frugivore, eating fruits, nuts, seeds, and browsing leaves. Over time, predators and environmental changes pressured it to evolve — from multi-toed feet to a single hoof, and from low-crowned teeth for soft food to high-crowned teeth for grinding grass.

[24:12]

By the Oligocene (33.9–23 million years ago), forests receded and grasslands spread. Horses like Mesohippus emerged, about 24 inches tall, with three toes and longer legs for running. Diet shifted toward leafy browsing and some grass. Later, in the Miocene (23–5 million years ago), Merychippus appeared — about 36 inches tall, still with three toes, but the side toes were reduced. It had high-crowned teeth and a diet dominated by grass.

[30:57]

Dinohippus, about 3.6 million years ago, was the first fully single-toed horse and had a stay apparatus allowing it to sleep standing up — an advantage for escaping predators. This species gave rise to modern Equus.

[32:20]

Modern Equus appeared about 3.2–4 million years ago, with fossils like the Hagerman horse found in Idaho. From here, horses radiated to other parts of the world, giving rise to zebras, wild asses, and domesticated horses. We’ll tell that story in the next episode.

[33:39]

This was a quick 30-minute overview of a field people dedicate their lives to. If I got anything wrong, please email me at podcast@madbarn.com. Understanding horse evolution gives me a deep appreciation for why they are the way they are today, and how they’ve thrived for millions of years. Stay tuned for the continuation of their story.