I know you are wondering: "What would happen to me if I accidentally traveled backwards in time 1000 years?" We all wonder that sometimes, and it's perfectly normal. Luckily, the good folks at Marginal Revolution have an answer, and it involves lots of gold and keeping your mouth shut. [Marginal Revolution]
@Sihanouk-s-Poodle: Not just slavery either: in many tribes (though I don't know which ones), they would sometimes 'adopt' someone from outside the tribe to replace someone lost recently, like a child or someone, and that adopted person would take on the name, status, and everything of the dead tribe member as if he were the original.
But yeah, Native Americans were not peaceful, nor very primitive (they didn't have long swords and guns, true, but they did make corn/maize, one of the most incredible feats of selective breeding, and had very advanced farming methods, among other things, all without the help of aliens or large domestic animals, besides dogs and llamas). Of course, I shouldn't generalize, as there were some that were more peaceful than others, but still, don't underestimate them.