Debate over the tailbone, and if it will stick around

Dr. Universe,

Why do we have a tailbone?

— Tyler, 15, East Liverpool, England

At the very bottom of the human spine is a bone that sticks out a bit called the coccyx (cox-ix). We sometimes call it the “tailbone,” but it is actually made up of several different spinal bones.

In some animals that actually have tails, those different bones at the bottom of the spine help them move their tail around. But in humans, those bones partially fused together.

You may already know a thing or two about the tailbone if you’ve ever hit a big bump while sledding or you’ve fallen on your behind. It can be pretty painful. You might have even thought that a tailbone seems kind of useless for a human that doesn’t even have a tail.

I decided to ask my friend Erika Deinert about these bones in the human body. She’s a tropical biologist and adjunct professor at Washington State University.

Even though your parents and grandparents didn’t have tails, if we went back in time and looked at ancestor species that we have in common with other primates, we would see some tails, Deinert explained.

These early primates did not stand upright, so they needed something to help them balance. That’s where a tail can come in quite useful. In fact, the tail was almost like having an extra arm or leg to help them move around. Tails are also useful for swinging in trees, which can help you get where you need to go, like to a nice piece of ripe fruit.

But as your primate ancestors started to stand upright, they didn’t really need a tail to help them balance anymore. For a moment, Deinert and I imagined what it would be like if humans had tails these days.

“What would a tail really do for us? It would be cool in some ways, but it doesn’t help with balance or moving around because we aren’t in trees,” she said.

Even though we don’t see fully grown humans walking around with tails, we do see tails as they develop in the womb. The tail-like structure forms during early development but is usually absorbed before birth.

Along with the tailbone, there are a few other traits in humans that no longer serve their original functions. For example, wisdom teeth. We just don’t really need them anymore, so we go to the dentist to have them removed. Scientists call these kinds of things vestigial traits. Vestigial comes from the Latin word meaning “footprint” or “trace.”

Deinert told me that some scientists are still debating why we still even have a coccyx if we don’t really use it anymore. While some people say it is an important part of our anatomy — there are some ligaments and muscles that attach to it — others say the tailbone is on its way out. While we may have different ideas about it, only time will tell if the tailbone will stick around.

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