I understand there is a hypothesis that this is to do with the piezoelectric property of bone minerals. Under compressive stress a small electric potential is produced which may preferentially attract the osteoblasts (the cells that cause bone growth).
We, as humans, are conditioned so well to our planet. The processes that made this conditioning possible took millions and millions of years to happen. In a way our bodies are only suitable for earth-like conditions. Extended space-flight or exposure to reduced or eliminated gravity has innumerable health consequences. We're talking damage to the muscular, skeletal and vestibular systems at the very least, that's not even mentioning the potential consequences of space-faired pregnancy. The work that has been done to make us an ideal species here on earth, doesn't translate well to harsh and foreign living conditions, we just weren't meant to experience low-gravity. The only potential short-term solution to this problem is artificial gravity, which is the creation of inertial force to mimick that of its gravitational counterpart.
In point of fact, it took 4.25 billion years. i.e.: since the creation of life. Please Register or Log in to view the hidden image!
But strictly speaking, if we restrict ourselves to speaking about bones, these arose in the Silurian (bony fishes), so 0.4bn years.
Life evolving to have bones in the first place, is part of an adaptation to conditions here on Earth.