Dear Emma,

My favorite animated GIFs are the ones with cats riding unicorns. I’m delighted to tell you about a real unicorn that lived a long time ago: the Siberian unicorn.

The Siberian unicorn was bulky and furry. It had a big hump on its back. Its horn was three feet long. That’s as big as a human preschooler!

This real-life unicorn was a kind of rhino from Eurasia. But it was bigger than modern rhinos and probably galloped like a horse.

Scientists have known about Siberian unicorns since 1808. For a long time, they thought the unicorns went extinct 200,000 years ago. Recently, that changed. Now they think the unicorns went extinct closer to 39,000 years ago.

A grey-scale profile of the head and shoulders of a Siberian unicorn. It's furry and rhino-like with a large horn coming from the top of the head
An artist’s idea of what the Siberian unicorn looks like, 1878, Rashevsky, under supervision of A.F. Brant


To understand how scientists worked that out, I talked with my friend Kimberly Sheets. She’s an anthropology graduate student at Washington State University. She uses radioisotope dating in her research.

A radioactive isotope is an element with extra energy. That makes it unstable. Over time, it breaks down into something else.

Scientists analyzed the Siberian unicorn fossils with carbon dating. This uses a radioactive isotope of carbon called carbon-14.

“Over 5,730 years, carbon-14 decays into nitrogen-14,” Sheets said. “By measuring how much carbon-14 is in a fossil, you can tell when it stopped absorbing carbon from the environment.”

All living things are made of carbon. Plants pull carbon out of the air. Then they store it in their leaves, stems and roots. Animals get carbon by chowing down on those plants. While they’re alive, plants and animals take in carbon all the time.

When plants and animals die, they stop taking in carbon. Their bodies break down into the soil. Microbes help release the carbon back into the air.

Rarely, some plants or animals turn into fossils. They’re buried in the earth. Eventually their bodies absorb minerals from the earth. They turn to stone. Their carbon is trapped in the fossil.

Most carbon is carbon-12. It’s stable. The amount of carbon-12 in a fossil will never change. It just sits there. But carbon-14 begins to break down as soon as the plant or animal dies. It breaks down at a regular rate.

That rate is called a half-life. The half-life of carbon-14 is 5,730 years. After that amount of time, half the carbon-14 will be gone.

Scientists measured the amount of carbon-12 and carbon-14 in the Siberian unicorn fossils. Then they compared those numbers to calculate the fossil’s age.

Carbon dating only works for fossils less than 60,000 years old. For older fossils, scientists measure other elements that break down more slowly.

If Siberian unicorns went extinct around 39,000 years ago, they lived at the same time and place as humans. I’m pleased to tell you that cats lived then, too.

I can’t promise that those humans or cats rode Siberian unicorns. In fact, scientists think they were super-fast and rare, so it seems unlikely. But, based on math alone, unicorn-riding cats aren’t impossible.


Dr. Universe