Debate over the tailbone, and if it will stick around

Dr. Wendy Sue Universe, Ask Dr. Universe

Dr. Universe,

How did the first horse change into the horses of today?

— Ava, 7, Kennewick

We can learn a lot about animals of the past from fossils, the imprints or remains we find in rocks. One fossil found in the Bighorn Basin of Wyoming helped us learn about the oldest known horses.

These horses are called Sifrhippus (siff-RIP-us). They had four toes on each foot and were very small.

Believe it or not, these tiny horses weighed only about 10 pounds. That’s just a bit heavier than your average house cat. According to the fossil records, Sifrhippus lived somewhere between 54 and 30 million years ago.

When I went to visit my friend, Lane Wallett, she told me all about the history of horses. As a veterinarian and a paleontologist at Washington State University, she is very curious about both horses and fossils.

Wallett’s favorite fossil is actually housed at the American Museum of Natural History. She has a picture on her desk of the pregnant Protohippus. Protohippus lived about 14 to 6 million years ago. It probably looked a bit like a modern-day donkey and had three toes.

If you want to learn about the history of horses, North America is a great place to look. In fact, the entire fossil history of early horses through the last Ice Age has been found here, Wallett said.

It wasn’t until about 3 or 4 million years ago that we started to see some fossils that looked more like the horses we know today, she adds. The big fossils were found in Hagerman, Idaho.

Those fossils are the earliest record of Equus, the group, or genus, that includes zebras, donkeys and horses. Horses like the one in Hagerman have also been found in Nebraska, Florida and Texas

“The animals that were best suited for the conditions in those times were those that could run fast on a single toe and eat grass,” Wallett said.

If we mapped out the family tree of horses, it wouldn’t just go in a straight line, but instead would have a lot of different branches. Wallett said that, at any given time, the animals that are most successful in having babies are the ones that are able to continue their family lines.

Because of this, the horse family tree has changed, or evolved, over millions of years. It’s pretty amazing that horses have gone from having four toes and weighing 10 pounds to weighing a thousand pounds and having just one toe on each foot. Many horses have also become domesticated.

For a long time, horses were living out in the wild. It wasn’t until more recently that humans started domesticating them — that is, taming them, working with them and caring for them. Wallett said there is still quite a bit of debate among scientists about when exactly humans domesticated horses.

While we have learned a lot about horses and their history, we still have a ways to go. Who knows, maybe one day you will study a fossil or look for clues in ancient DNA to help us learn more of the answer to your question.

Have a science question? Ask Dr. Wendy Sue Universe, WSU’s resident science cat and writer, by email at, on her website at, via Twitter at @AskDrUniverse or at

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