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A social gathering in the Cambrian era, beginning some 540 million years ago, might have resembled an underwater war game—all life resided in the ocean then and almost every creature present would have been wearing some sort of external armor, complete with spiked helmets. The ancestors of insects and crustaceans wore full exoskeletons, probably made from a mixture of protein and chitin like the shells of modern lobsters. Starfishlike organisms and mollusks manufactured their body armor from calcium carbonate extracted from seawater. Even one fishlike evolutionary dead-end, the ostracoderm, managed somehow to swim while encased in scales and heavy plates made of true bone—that is, mineralized cartilage rich in calcium and phosphates.
It was the mild-mannered softies of the period, however, that would first develop internal bones. Wormlike organisms, such as the conodonts [see “Teeth,” on page 75], started to mineralize the cartilage surrounding their primitive spinal cords, becoming the first vertebrates. Bony cranial coverings came next, and other creatures with more extensive cartilaginous internal skeletons soon followed suit. Because these swimmers used muscle contractions to propel themselves, having muscles anchored to solid bone would have provided greater strength. The hardened skeleton also offered a more solid scaffold for bodies to grow larger and to diversify, adding limbs to their repertoires.
Serving as a massive and highly responsive storage depot for critical minerals, particularly calcium, is a role that likely evolved later but is now one of the most important functions of human bone. Without calcium, the heart cannot beat and brain cells cannot fire, so far from being inert, bone is in constant flux between growth and self-demolition to meet the body’s needs and to maintain its own structure. Cells called osteoclasts (“bone breakers”) destroy old or dead bone tissue, and osteoblasts (“bone growers”) give rise to new bone cells. Working together, these cells replace about 10 percent of the skeleton every year. In the shorter term, if blood calcium levels are too low, osteoclasts destroy bone to release the mineral. Conversely, if exercise produces larger muscles, osteoblasts get to work building new bone to withstand their pull.