Final answer:
The initial formation of bone in humans begins during early embryonic development, with ossification starting at a primary ossification center in the middle of the bone. This process continues for several decades after birth before the adult skeleton is fully formed.
Step-by-step explanation:
The initial formation of bone in humans begins during early embryonic development. Initially, the skeleton is made almost entirely of cartilage, which gradually turns into hard bone through a process called ossification. Ossification begins at a primary ossification center in the middle of the bone and later occurs at secondary ossification centers in the ends of the bone. This process continues for several decades after birth before the adult skeleton is fully formed.