Our skeletons are made of bone. This is a fact so simple as to seem mundane. It’s easy to forget the evolutionary alternatives.

We don’t have to look very far afield on the tree of life to see that skeletons needn’t be made of bone. Sharks and rays get along just fine with skeletons made of flexible cartilage. And with the prehistory’s perspective, an even odder alternative emerges. The very first bones were dramatically different from our own.

Our bones, held inside our bodies, have cells inside. Osteoblasts become new bone cells – called osteocytes – while osteoclasts go about resorbing old bone. Our bones are not static, but, thanks to these cells, are dynamic tissues. Not so for the original bones.