We take in calcium as +2 ions, and use it in the same state when making calcium phosphate for bones. That helps because calcium, like most metals, takes a lot o

We take in calcium as +2 ions, and use it in the same state when making calcium phosphate for bones. That helps because calcium, like most metals, takes a lot of energy to reduce to its metallic form. Titanium and iron are also naturally found as ions. Getting bulk quantities of the metallic form would be a waste from an evolutionary standpoint, when it could just make a strong mineral-based composite material like bone.