His Songs are Stronger Songs, and His Feet are Faster

This post on Tom Bombadil, which was sparked by this excellent musing on the return of the Ring to the Cracks of Doom caught my eye. What is the origin of Tom Bombadil? My own view is to draw an analogy with Ungoliant: she is neither Vala nor, in any obvious sense, Maia, but a spirit who descended to earth before the making of the two trees. She seems to have been an independent power, and she had immense power in her own right, being able to hide herself and Melkor even from the sight of Manwe. Bombadil, I think, is similar, but comes from an even earlier time; hence his name, Eldest. This, I suppse, is a version of the nature spirit theory (as opposed to the other theories of Bombadil: the Maia theory, the Vala theory, the Iluvatar theory). But it isn't quite the standard nature spirit theory, since that posits that some natural feature of the world 'gives rise to Bombadil'. Given how old Bombadil is, however, the only natural feature of the world that could 'give rise to Bombadil' is the earth itself: through all the wars and catastrophes of the ages it is difficult to believe anything else would last that long. This would fit with one or two things, but Bombadil really isn't much like other cases in which a natural feature 'gives rise' to something (e.g., the waking trees). Thus some have suggested that Bombadil is the spirit of Arda itself. But this does not seem at all likely to me. And a common argument put forward for this view, namely, that it would explain Tom's independence from the Ring, is false, I think; at least, I see no reason why it would have no power over him on its own despite the fact that, if Sauron succeeded in finding and using the Ring, Tom too would fall. Tom's independence from the Ring seems to me to be simply an incidental effect of his general carelessness. Power simply does not interest him, one way or another, and therefore he fears and craves nothing. My view is that he is simply some other type of spirit, come to Arda in the very beginning for purposes only he and Eru know.