(Picture from here.)
I’ve been getting over an injury I got doing judo about a month ago. So, of course, I started thinking about bones.
The skeleton is one of our most obvious anatomical features. I know we can see and feel skin and eyes and hair. But the skeleton is one of our clearest examples of an anatomical system. The bones articulate. They move together. Muscles attach to them.
Where did they come from?
Well, we’re vertebrates. That meas we have a dorsal notochord. There’s an erroneous concept that vertebrates are animals with backbones. But that’s a little tough since chondrichthyes (cartilaginous fish) don’t have a “bone” in their skeleton, excepting teeth. The structural members are all cartilage, not bone. A more precise definition is that vertebrates are animals that have a vertebral column, which may or may not be composed of bone. The column is composed of different elements, called vertebrae, and house the spinal cord. And that sharks have, too.
The two great branches of fish in the vertebrates are the chondrichthyes and the osteichthyes. And that’s where mammalian heritage of bone begins. We are descended from the osteichthyes and the hard, calcium rich substance has been with us since.
Fish evolved cartilage before they evolved bone. The cartilaginous fish evolved into two groups: Agnathostomata (fish without jaws) and the Gnathostomata (fish with jaws.) Agnathostomata include such pleasant fish as lampreys and hagfish.
The Gnathostomata are no stranger to bone. Placoderms showed up over four hundred million years. Some species have bone; some do not. The bones serve as armor, teeth or other purposes. They do not server as structural members. That came later.
The critical feature of bone is the mineralization of the softer tissue. At some point, we developed the ability to impregnate that nice soft tissue with rock. More importantly, we impregnated particular tissue with rock. Structural tissue. Cartilage, though, is crucial. The processing of cartilage is a necessary precursor to bone growth– mice grown without crucial genes involving cartilage development lack bone. (See here.)
So, how did mineralization get going.
Well, about 1.5 billion years ago a tremendous amount of Calcium Carbonate (CaCO3) were washed into the oceans from volcanic and other sorts of tectonic activity. So many organisms took advantage of this new found chemical trove. A lot of weird animals in the Cambrian showed up a bit more than .5 billion years ago. Animals that wore their skeletons on the outside.
A skeleton, inside or outside, soft or hard, gives muscles something to pull against. The structural skeleton can be made out of enclosed water, cartilage, other muscle or bone. The important thing is to give a rigidity to the structure so that organized movement can occur– at least more than squishy oozing along.
We had, back then, no shortage of skeletons on the outside. This should not come as any sort of surprise. Lots of animals had hard outer bits. Clams, for example, Growing hard bits on the outside was relatively common. So it doesn’t seem all that hard to put hinges between the hard bits and get crustaceans and trilobites.
Vertebrates belong to the phylum Chordata, animals with notochords. We have notochords, too. Inside our vertebral column. We like to call it a spinal cord. What’s different about chordates is they put their notochord on the dorsal (back) side instead of the belly side. Very early on, we wrapped that notochord with vertebrae and we were off. The skeleton evolved from the vertebrae. Consequently, our skeleton evolved from the inside out while the other animals with hard parts evolved their skeleton from the skin inward.
All of the animals at that time with moving hard parts refined them. We end up with shrimp, crabs and the afore mentioned trilobites. Back in vertebrate company, we evolved fishes, fishes with jaws and jawed fish with bony armor. But no mineralization of the structural members.
One big shift in the vertebrate world was to shift from our friend calcium carbonate, so beloved by our exoskeletal brothers, to calcium phosphate. (CaPO4). This was in the form of calcium hydroxyapatite. (Ca5(PO4)3(OH))
Why change strategies when calcium carbonate had been around for a long, long time?
One idea was that the original use of calcium phosphate wasn’t as bone at all. Instead, it was a storage mechanism of phosphorus– often a biochemical limiting factor. Anyone who has used phosphate based fertilizer knows its utility. ATP/ADP (adenosine tri-phosphate and adenosine di-phosphate) are the way the cell stores and release energy. That P in the abbreviation is for phosphorous. But as nice as this is, why did vertebrates evolve it and not invertebrates? After all, it would have been good for both. It could be that the change to phosphorous was for a wholly different reason and the storage advantage was a happy accident.
Another idea derives from vertebrate activity. From the fossil record and from observation of vertebrate animals in the wild, it looks like we’re active creatures. We don’t sit around. We run. We hunt. We don’t sit and wait for our food to come for us. We go and get it. One of the side effects of this activity is a change in pH– the acid or base values of the tissue. Calcium carbonate is much more soluble material than calcium phosphate. Consequently, fish that tried to swim hard could find their hard bits not so hard.
The first structures resembling bone we find in the fossil record are teeth or teeth like structures. They are with us to this day. Sharks have teeth. Lampreys have teeth. Teeth are a bit different from normal bone. Both bone and teeth are calcium phosphorous structures but teeth are much harder than bone. But bones can heal. Teeth can’t. However, the biochemical structures are close enough that one wonders if the biochemical pathway that brought forth teeth was torqued a bit to bring forth bone.
There are also teeth like structures in the skin to form shields. Remember placoderms? One of the arguments in paleontology is which came first? Teeth or shields? Genetically, they appear to derive from the same source.
Early skeletons were cartilaginous but were not based on collagen, the primary structural protein in connective tissue. Later, when collagen evolved, it was used in the skeleton such as those involving sharks and such. It fell to the ancestors of the bony fishes to invent ossification.
There are two mechanism of ossification: intramembranous ossification, where the bone is laid down directly into connective tissue, and endochondral ossification, where cartilage serves as a template for the bone. Intramembranous ossification happens in bone repair and in the early construction of certain head structures. Endochondral ossification is how the skeleton gets formed.
It didn’t happen all at once. Apparently, endochondral ossification started with surrounding connective tissue– biochemically similar mechanisms for embedding bone in the skin. Eventually, the process of cartilage replacement occurred. How this occurred. There is fossil evidence that early sharks had the ability to deposit bone in tissue though it was not used structurally.
There is also current evidence (see here.) that some sharks can mineralize cartilage in a similar way to how bony fish do it. That said, is this development that occurred since sharks diverged from the rest of the fish? After all, the biochemical means by which mineralization occurs is very old in the vertebrate family tree– recall our constant friend, the placoderm. And the mechanism for laying down a supportive skeleton is represented in both the boney and cartilaginous fish. We often look at what we call primitive animals and forget that they’ve been around just as long as we have with just as nasty and powerful selective pressures on them. Sharks diverged from our line 400 million years ago. But they haven’t been sitting around since then. They’ve been evolving, too.
Once structural mineralization occurred, though, the advantage it gave was tremendous. It enables fish to swim fast. In fact, the cartilaginous fish have to derive structures analogous to bone in order to get that upper speed.
Marlins, for example, have a bony skeleton stiffened not only by having just a few vertebrae but also by strapping those vertebrae together with bony strips and ropes of connective tissue. Mako sharks are similarly fast but don’t have any such things. How do mako sharks swim fast? They increase their internal pressure against a skin which does not stretch. In this way they mimic what the marlin does. Essentially, they create a fluid skeleton to make up for any deficiencies in their skeleton.
Not to mention that a mineralized skeleton set the stage for invading the land. There are a lot of reasons even an average sized vertebrate dwarfs the largest land invertebrates that ever lived. Skeletal scaling is one of them.
Which brings me back to my ankle. I wish its intramembranous ossification process would get its act in gear.