I can't see why you can't go back and kill you own ancestor...
Your ancestor causes you to exist, you go back in time, kill your ancestor, and cease to exist, but continue to 'have existed' for the duration prior to ceasing to exist. The new timeline becomes: ancestor exists, timetraveller appears and kills ancestor then ceases to exist, everyone looks around and says, "oh, right, another temporal suicide" and goes on their merry way. It is a paradox, but the universe can still function...
You go back and kill your ancestor, you spend some time there, and go on existing just fine, you then go to the future, but nobody knows who you are, you have no history, and the world has changed as a result of your ancestor dying. Someone notices you, checks your genes for links to temporally suspicious deaths and you are put on trial for murder... This has the bonus of not having any spontaneous vanishings...
You go back in time and kill your ancestor, you return to your own time and nothing changed, you go back in time and kill your ancestor again, because it was fun the last time. You then proceed to open a chain of entertainment venues where people go back in time and take advantage of the complete lack of sustained consequences... Combined with the multiple parallel universes theory this gets even more disturbing...
Just because you haven't found a paradox yet, doesn't mean it isn't out there...
At this point I would be seriously cautious about getting my hopes up, there is an absolutely phenomenal capacity for this to be dumbed down...