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. When bone first evolved, there was not a cell in it.
Bone didn’t start off as an internal scaffold. In its primordial form, bone covered the bodies of jawless fish. Astraspis was one of these critters. This 455 million year old fish’s body consisted of a large head with a simple slit for a mouth, a tail for propulsion, and an external wrapping of tissues that represent the forerunners of our own bones and teeth.
Paleontologist Philip Donoghue and colleagues included a cross section of the outer Astraspis layers in a 2006 paper considering the origin of bone. The bumpy surface scales had enameloid caps over dentine cores – effectively, skin teeth – and lay on top of a “spongy” layer of what researchers call aspidin. And aspidin is bone in its earliest mineralized form.
Aspidin didn’t have cells inside. Cells laid down the mineralized material, but then died. In early fish like Astraspis, this tough stuff helped anchor the bumpy “teeth” of the skin and form a base layer underneath. The very first bones were armor.
[My thanks to paleontologist Adam Huttenlocker for leading a bone biology seminar that inspired this post.]
Donoghue, P., Sansom, I., Downs, J. 2006. Early evolution of vertebrate skeletal tissues and cellular interactions, and the canalization of skeletal development. Journal of Experimental Zoology. 306B: 278-294. doi: 10.1002/jez.b.21090