> Perhaps you should clarify what "stolen" means.
There are two senses of theft at issue:
1. Some government establishes (or expands) its sovereign territory through violent conquest.
2. Private parties obtain a service from said government and the society it ostensibly represents without paying in full for it (specifically: the issuance, recognition, and enforcement of land titles/boundaries, those being definitionally subsets of said government's sovereign territory - which, again, may or may not have been obtained through violent conquest)
Here in the US, both of those sorts of theft are in play:
1. The entirety of US territory was obtained either by conquering it from other entities (e.g. Native American tribes, Mexico, Spain) or by purchasing it from other powers who had previously done so (e.g. France, Spain, Britain, Russia).
2. The US government (or state/local subdivisions thereof) issues and enforces deeds representing subsets of its sovereign territory to landowners, at the exclusion of the rest of its citizenry - yet the landowners do not pay anywhere near the full rental value of the land to said government or said citizenry.
> Am I to assume that every piece of land has an "original owner"? Who is that?
In the case of American land, that'd be the indigenous peoples thereof.
In a more general/legal sense, the "original owner" would be the government of whose sovereign territory that land is a subset. If that government ostensibly represents its citizens, then said "original ownership" would rest with each and every individual citizen equally. As Locke explained in his Second Treatise of Government:
> Nor was this appropriation of any parcel of land, by improving it, any prejudice to any other man, since there was still enough and as good left, and more than the yet unprovided could use. So that, in effect, there was never the less left for others because of his enclosure for himself. For he that leaves as much as another can make use of, does as good as take nothing at all. Nobody could think himself injured by the drinking of another man, though he took a good draught, who had a whole river of the same water left him to quench his thirst. And the case of land and water, where there is enough of both, is perfectly the same.
That is: every member of society is entitled to an equal share (by value) of the land on which that society exists. Those who own more than their equal share of that land value owe compensation to those who own less than their equal share. The lack of said compensation is theft.