eli5: Why are bones and shells made of calcium?

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eli5: Why are bones and shells made of calcium?

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It’s believed that bones and other hard, calcified structures evolved first as a way to store calcium, which is a vital part of a lot of organic processes. I couldn’t tell you all of them, but [Wikipedia can](https://en.wikipedia.org/wiki/Calcium_in_biology).

Calcium needs to be pretty tightly regulated in your body. It’s not something that you want floating around freely, because it can cause problems. However, you also don’t want to simply flush out any extra calcium, because it can be hard to find and hard to get. So, organisms began sequestering the calcium away in little calcified bits. These were *not* bones, as we know them, but they *were* hard knobby bits.

Well, it turns out that having hard knobby bits is *super* useful for a lot of things. For starters, the calcium was often kept close to nerves, since calcium is very important for keeping nerves working. This meant that for the very early precursors to vertebrates, it followed their central nerve, what in us is our spinal cord. That’s *very* useful for protecting such a vulnerable but important part of your body. So, the knobby bits evolved from “place to store calcium” (which they still very much are, even for us) into “hard thing that predators can’t easily bite through”. For vertebrates, that was a spinal cord. For inverts, it became a hard shell.

Speaking of biting, how are you gonna get through *their* protective calcium bits? Or the chitinous exoskeletons of arthropods? Use your own hard calcium bits, often *pointy* hard calcium bits. Teeth are pretty damn useful, and really changed how predation in the oceans worked when they evolved. Teeth also really need something to be stuck in, though – a squishy mouth can’t really exert enough force to *really* take advantage of hard teeth. So, hard jaws evolved for teeth to sink into.

Hard calcium jaws aren’t just important for holding the teeth. When your muscles pull, they need to pull *against* something. They need strong anchor points. A calcium bone jaw is perfect for that. You’ll notice that soft-bodied animals tend to wiggle with their whole bodies, because the only thing for their muscles to pull against is other muscles. They can also use hydrostatic pressure to keep *kind* of stiff – that is, filling up their cells like balloons so they are tight against each other and a little more stiff. But bones and shells changed the game. Muscles anchored to bones can exert a *lot* of force – way more than if they can only anchor against squishy water balloon cells. So, what began as “armor for squishy bits” and “tooth holders” became muscle anchors for improved strength and faster locomotion.

So, the evolution is:

1. Calcium is a very important element for biological processes, especially nerves.

2. Organisms need a way to store extra calcium to use as needed, and build little calcium clumps throughout their bodies.

3. Those calcium bits make it harder for predators to eat them – literally, their bodies become too hard to chew for organisms without teeth.

4. Hard calcium bits tend to form near important nerves, like the spinal cord and brain. That means you have hart calcium bits near your mouth (which is near your brain). You can use those to break and chew the calcium bits in prey. Suddenly, teeth.

5. Teeth work better when they’re anchored to something. Bone jaws evolve. Bones also help with movement, so as the spinal column evolves to protect the spine, those vertebrae also get used to help move, especially the whole-body side to side motion of swimming.

6. Simultaneously, the organisms that don’t have much of a spine and don’t move around much are building hard calcium shells that the newfangled teeth can’t get through.

7. Skeletons continue to evolve in vertebrates to help with movement. Sharks and skates evolve a cartilage skeleton, but other vertebrates evolve to use calcium bones. Note: bone is *still* used as a calcium reservoir. Your body pulls calcium from your bones to use for all those biological functions, and replaces it when you consume calcium. The amount you use this way is tiny, though, which is why you don’t see your bones fluctuating wildly in strength. If your diet is very low in calcium, though, you will see bones deteriorate as your body keeps pulling calcium out of your bones to use for more important things, like your nerves continuing to work.

8. Basically, all that but also for inverts as they build shells. And then, just to be confusing, some of them evolve to *not* have shells again (like octopuses).

There are, perhaps, elements that would make for better skeletons or shells. Beaver teeth are very high in iron and some deep sea snails [have evolved to have a lot of iron in their shells](https://en.wikipedia.org/wiki/Scaly-foot_gastropod). But, calcium is pretty biologically *available* since every other animal is using it, and a lot of plants need it for things as well. That makes it plentiful in the food web. Enzymes to manipulate it are plentiful in our genes, since organisms have needed to manipulate it for hundreds of millions of years. And, even if you had Wolverine’s metal skeleton you would still need calcium anyway for all of the things living things needed calcium for in the first place. So, why *not* calcium? If a better alternative arrives, natural selection will (probably) favor that and calcium skeletons and shells will fade away, but it hasn’t happened yet.