Relatedly, do we know what were the first claims made on already inhabited land? Did those events set precedent for dealing with native inhabitants as well?
There may have been earlier claims of newly discovered lands, but for the Americas there existed the legal construct of the requerimiento. It was read to native people declaring them to be subjects of the Spanish Crown, and goes back to Christian realms' campaigns against Muslims in medieval Iberia (aka the reconquista). For a good discussion of the requerimiento and its contents see this question in the AMA Native American Revolt, Rebellion and Resistance.