What if things had happened differently?

I'm very skeptical about the concept of time travel...at least in the traditional sense.  If someone in the future had actually developed a working time travel system, then where are they?  Should we not expect them to randomly show up at different points in time?   Some people think some UFO sightings may actually be time travelers from the future instead of aliens or classified modern technology.  But if so, why have they never revealed themselves?  Are we to assume that they are a part of some closely guarded organization of time travelers with some sort of prime directive that states they can never reveal their identities to people from the past?  And are we to further assume that none of them ever go rogue and take an unauthorized ride to the past and reveal themselves?

You know, maybe we could just set aside an international time travelers day in the not too distant future and publicly announce it and make sure the media covers it well so that it will be well recorded so that if at any future point in time someone figures out how to physically travel through time, they would have a record of a time and place to visit to announce their discovery.  Just pick a location and date.  If someone shows up and can confirm they are a time traveler, then we know it's possible to do so.  If no one shows, then maybe no one ever figures it out or it just isn't possible.

But anyways, what if it were possible and someone did figure it out?  And what if someone who had access to time travel then looked at the history of the world and thought that maybe they could make things better if they just went back and changed a few things?  Say, for instance (just to pick a random mythological event), Prometheus had not taught man how to build a fire.  What if Prometheus was not a 'god', but a man from the future who thought if he just went back in time and taught men how to build a fire earlier than we would have figured it out on our own, that maybe the progression of technology could get a jump start?  Or what if the assassination of Emperor Caligula had actually been instigated by a time traveler who knew what would happen if Caligula had lived longer?  What if things had once happened differently than history now records simply because future time travelers thought they could make things better by making a few changes?

And if so, could they still be actively changing events?  Could something be one way today, but another way tomorrow, and we would never know that it had ever happened the other way?  Or what if the future time travelers saw their manipulations weren't really helping or had made things worse and then banned time travel, simply leaving things the way they were at the time?  What if some ancient prophecies of the future were actually from time travelers who knew the future and left the stories of the future in the past so people would know what to expect?

Personally, I doubt that any of those scenarios is the case.  As I mentioned in the beginning of the post, I'm skeptical that physical time travel is even possible.  I think if we set aside a day for a future time traveler to officially come visit us, no one would show up (although some crackpots posing as time travelers might make an appearance).  And I think the history of the world is what it is and has always been what it is.  But sometimes you just can't help but wonder...what if?