Following the violent maneuvers of tectonic plates ~1.5 billion years ago, huge quantities of minerals, including calcium carbonate (CaCO3), were washed into the oceans. This abundance provided the possibility for inhabitants to get hard.
At first, unicellular organisms employed CaCO3 to regulate mineral intake. By a half billion years later, calcium carbonate was being put to more creative purposes: strength, stability, protection, and motility. The evolutionary explosion of the Cambrian period owed largely to mineralized body parts. Shells, spines, and skeletons evolved.
For building bones, the employment of calcium carbonate ceded to calcium phosphate, which afforded greater chemical stability, especially in the acidic environments that are created after intense physical activity.
Bone is an intriguing composite of essentially 2 materials, the flexible protein collagen and the hard mineral called apatite. ~ physicist and physical chemist Roland Kröger
The evolved properties of bone owe to an intricate, fractal, hierarchical organization. The principal building blocks of bone at the nanometer scale are curved needle-shaped nanocrystals that form larger twisted platelets which resemble propeller blades.
“At the smallest scale, needle-shaped mineral units form platelets that organize into stacks bridging multiple collagen units.” ~ structural biologist Natalie Reznikov et al
The platelet blades continuously merge and split during bone development. Interweaving mineral and protein form continuous networks that provide superior strength.
“The combination of the 2 materials in a hierarchical manner provides bone with mechanical properties that are superior to those of its individual components alone. There are 12 levels of hierarchy in bone.” ~ Roland Kröger
Besides numerous nested structures in bone, a common feature of all of them is a slight curvature, effecting twisted geometry. The mineral crystals are curved, the protein strands (collagen) are braided, the mineralized collagen fibrils coil, and entire bones themselves have a twist, such as those seen in the curving shape of a rib.
Clearly, bones evolved not only with purpose, but with a highly coherent structure that optimizes certain properties inherent within the chemistry of the ingredients and the combination of materials. The employment of fractals in the construction of bone illustrates Nature’s fondness for novelty and labyrinthine order via modular self-similarity.