If there are any aliens in our galaxy, I give it quite good odds that they're related to us. Either through panspermia, but also the possibility of them being a sister species to humanity. We've been around for 2 million years (I count
Homo erectus as being human), after all, and it only took us 14000 years to go from hunter-gatherers to spacefarers who have the knowledge (but alas, not the infrastructure) to build starships. Ultraterrestrials is the term used for such "aliens".
I definitely think there were "advanced" ancient civlisations, though probably not much more advanced than the Romans. It would not surprise me if agriculture, pottery, and maybe even metalworking were discovered and lost repeatedly through human history. Feeding, of course, into the next civilisations mythologies.
Perhaps one subspecies were intelligent enough, and survived long enough, to develop spaceflight, before fleeing Terra because of some catastrophe (Toba, perhaps?). There, they began to spread throughout the galaxy, leaving their home world as some backwater planet that few care for...