Why does nodding of the head signify yes and shaking of the head no?

Children pick up on everything around them in their life, weather you like them to or not. one the fist acts of shaking you head came from children. when they would be they would turn there head to signify something they did not want. children would also see you moving your head up and down when you do something good, or when someone is happy. this would signify something was good. they would express there emotions by these gestures. Charles Darwin wrote a book on this. I hope this helps!

It is just the way it has been throughout history.

Explanation:

i need an image of the diagram

that is wrong, its not thymus.