This is a time machine... from a delorean? that needs to be dealt with before time travel can be treated as a serious scenario. One possible way around this difficulty is if any race advanced enough to have time travel would also have the wisdom not to leave obvious clues in the past, a kind of prime directive. This might also go the other way, too. If something happens that is completely unexpected, this might (emphasis on might) be because some person in the far future was forced to set up the situation because they learned that they did this in the past. It is even possible that the physics of time travel limit what kind of change to the past can be made.