How can things become someone's property?

Well, now-a-days, everything that is inside the boundaries of a country belongs to the country (except people, duh!). Even animals. But my question is, why? In the beggining of times you found something and it was yours, but when did it happen that something that doesn't have an owner or a explicit and verifiable thing that says that that something belongs to someone, belongs to the government? To rephrase that, when and who said that? Is it on the constitution? (I could answer that question, but it would take me a bit of time and my will is not in a good mood today), if so, then who owns the piece of paper that says that I own something?
today, if you want to own something, you have to meet the conditions the previous owner says (ussualy to pay money), but, who is the very first owner of that thing? in fact, who is the very first owner of everything? God? (oh dear lord in the otherworld illuminate us with your wisdom and help your children in Earth to understand your will, OH LOORD!)
no, seriously, who was the first owner of anything?
I hope I can go on with this post, but I missed the class our teacher sent us this homework, so all I said started from scratch, not like my other posts where I actually did think a lot to say what its there. Well, this is the E.N.D. (the extreme point of something, or Energy Never Dies? did Black Eyed Peas answer what they meant with the name of their last album?)