Someone whose job involves manual labor would have noticeably larger and stronger upper body bones because it has adapted to the everyday activities every day. That is to say, that the body has become larger and stronger to perform the tasks efficiently.
Explanation:
The body adapts itself to our activities. If our job does not involve manual labor, our upper body does not need to have noticeably large and stronger upper body bones. Since it is not manipulating heavy things or moving constantly, there is not an external factor that shapes it this way. Besides, the constant physical work will shape the body to do the job more efficiently and without damaging it.
