What is the origin and reason for mourners to sit on the floor or low seat?

Question

I have seen a few examples in the Torah of people ripping their clothes as a sign of mourning. But the earliest example of someone being on the floor as a sign of prayer seems to be King David's example in Samuel II, 12:15.

I'm not sure if this example was used as the source for mourners to sit low or on the ground during Shiva. It would be a strange example, since David wasn't mourning for a dead child. As a matter of fact, after the child was dead, he no longer sat on the ground and he stopped his fast.

What, then, is the source or reason for sitting on the ground or a low stool?

Answer 1

This is an excerpt from The Jewish Way in Death and Mourning by Rabbi Maurice Lamm, via Chabad.org:

Until modern times it was the custom to be seated on the earth itself, a procedure which demonstrated the departure from normalcy during the early stages of bereavement. Thus, expression was given to the sense of loneliness and depression one felt after one's relative was interred in the very earth on which he sat. The Bible tells us that when Job suffered a succession of disasters he was comforted by friends who sat with him "to the earth." It is, almost in a literal sense, a physical adjustment to one's emotional state, a lowering of the body to the level of one's feelings, a symbolic enactment of remorse and desolation.