Ancient civilizations?

I've started watching Ancient Aliens, a show on the History Channel that discusses the possibility that aliens visited this planet before Egyptian civilization and (possibly) interfered with human evolution. Sure, the subject matter is a little "out there" but the show is interesting.

The latest episode showed that there are a number of what appear to be cities off a whole bunch of coastlines. These cities would have been sunken underwater around the end of the last ice age about nine or ten thousand years ago. The trouble is that as far as we know, advanced civilizations - advanced enough to create cities like these - have only been around for the last four or five thousand years or so.

So either these things that look like ancient cities aren't cities or there were civilizations long before we thought. If humans did live in cities at the end of the last ice age, it makes sense that those cities would have gone underwater as sea levels rose. But if there was a civilization that long ago, what happened between then and the earliest known civilizations? Why wasn't there a continuous record of human civilization?

The idea that civilizations might have existed long before we though is definitely interesting. Until these underwater structures are fully explored inside and out no one will be able to say for sure what's going on there. All we or the show has is speculation which is enough to keep me entertained. I'm looking forward to see what other mysteries and theories Ancient Aliens has in store for me.