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?