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We have a lot to learn about our bodies, but there are some things you’d think you’d know for sure. Like, how many bones you have.

You might even have heard a number for this: 206. But... that number is wrong, and without a scan of some kind, no one can tell you exactly how many bones you have. And that’s because, no matter what age you are, you’re continuing to lose them!

Now, when I say you’re losing bones,. I’m talking about your total bone count. Don’t get me wrong, if you’re an adult, you’re also probably losing bone tissue.

We tend to have the highest total bone mass at some point in our twenties, and bone density drops from there. But in this case, I’m talking about how many bones, a number which decreases when separate bones grow together to form one. We tend to associate this bone fusion with babies and childhood.

But research suggests that it doesn’t consistently stop at any particular age! Everyone starts growing bones during fetal development. First, the basic skeletal template is laid out.

Then, the cells that secrete bone material group into clusters called ossification centers, which is where bones begin to form. At the beginning of the third trimester, fetuses have around 800 of these clusters in the central part of each developing bone. So, you could say you once had 800 teeny tiny baby bones.

But by the time you were born, many of those had already grown together, leaving you with around 300 bones. And over the first couple decades of life, lots more bone fusion happens. Take this long bone right here, called the humerus, for instance.

It started out as multiple pieces of cartilage, which turned into bones over my first few years. Then, by the time I finished puberty, those separate bones had fused into the one I have now. And you’ve probably heard about how babies have soft spots on their heads.

That’s because the 6 major bones that make up the top of their skull are only connected by tough membranes at birth. They typically fuse together by the time the kid’s in preschool. Meanwhile, my collarbone didn’t become a singular collarbone until my 20s.

In fact, it’s often talked about as the last bone to fuse. But it’s not. The appendicular skeleton — basically the limbs — is pretty stable by a person’s 20s, unless they have a bone disorder that causes too much fusion.

But the axial skeleton, which includes the bones in the head, sternum, ribs, spine and pelvis, continues to fuse throughout adulthood. For instance, bones in the sternum and the hyoid or “tongue” bone may not fuse until a person’s well over the age of 30. And the older you get, the more likely it is that your coccyx, or tailbone, has fused to the sacrum — the large bone right above it — even into your 80s and beyond.

A lot of the research on these later-fusing bones has come out more recently. That’s in part because, well, people just kind of assumed there was a simple, round number for skeletal bones. In fact, that number has been a subject of debate for centuries.

People have proposed everything from 197 to 307. Though, most of that variability has been due to inconsistency in counting certain bones, like the tiny bones in the ear, or whether you include teeth. Then, in the mid 19th century, the famous anatomist Henry Gray settled on 206 — so that's the number he put in his classic textbook Gray’s Anatomy.

And it got repeated in textbook after textbook — until it became gospel. The problem is, 206 came from very limited data which came from dissections he’d performed. Gray did the best he could given what he had to work with.

While it was legal at the time to study cadavers that nobody claimed, there weren’t a lot of them to go around. So he didn’t exactly have lots of replicates or a wide variety of ages to examine. But that ultimately meant he ended up counting things kind of weirdly.

As one pair of researchers put it, it’s as if he counted the bones in the head of a young adult but the body of someone in their golden years. More detailed analyses in the century since have revealed a lot about how our bones fuse, including that the timing can depend on the person and their life. In addition to inheritable differences in fusion speed and timing, research suggests certain activities and conditions can impact the total number of bones you have at a given age — things like singing, clenching your jaw, or being pregnant.

But when you average everything out, young adults tend to have around 215 separate bones — a number which can drop to less than 190 late in life. That’s 25 bones! So the skeleton of the adult human body simply isn’t set in stone.

Or in this case, in bone. Thanks for watching this episode of SciShow! If you liked learning about the quirky history of bone counts,.

I bet you’ll love our podcast, SciShow Tangents. In it, some of the fun people involved in SciShow get together for a lightly competitive knowledge showcase. Over the course of a season, they rack up points for teaching the others— and everyone listening at home— the most mind-blowing science facts related to the week’s theme.

There was a whole episode on bones, for instance! And it included some really wild stuff about glowing bones... Anyhow, if you love science, laughing, and lighthearted, nerdy competitions, you should check it out!

You can find SciShow Tangents anywhere you get your podcasts. [ outro ].