Around 500 million years ago, early vertebrates in the seas became fish, adopting an inner skeleton and a flexible spine based on a nanocomposite of fibers and mineral, known as bone material. This "invention" of evolution was so successful that the basic structure was also adopted for later vertebrates that lived on land.
source https://www.lifetechnology.com/blogs/life-technology-science-news/neutron-experiments-reveal-what-maintains-good-function-in-bones