I just realized that I've assumed that Earth was the planet of the Ancients since at least the beginning of season eight and maybe as far back as the episode Frozen in season seven without any significant evidence to support this. When I thought about it the only evidence that suggests Earth was the Ancients planet of origin is the comments made by Daniel and Dr. Beckett during Lost City, which if my memory serves me were something along the lines of "as near as we can tell we are the second evolution of this form on Earth, the Ancients were the first." I'm going completely from memory since I don't have the episode anymore and I think I might be imaging the specification of it being on Earth. Even if I am remembering correctly, the quote is less than positive proof that Earth was the palnet of origin for the Ancients.
Of course once I realized that there wasn't absolute proof that Earth si the Ancients homeworld I started thinking about the case that could be made for it not being their homeworld. The strongest evidence for it not being their homeworld is that the outpost found in Antartica is no different than at least one other one (and probably more) that was used as a landing point for Atlantis. Having multiple landing platforms on different worlds suggests that the Ancients travelled, with their city, from planet to planet. The fact that they left stargates behind on everyworld they visited with their city and all the worlds nearby seems to imply that they only visited a region of space once with their city. based on this logic, if the Ancients left for Pegasus from Earth, then it is not their homeworld.
Feel free to critique this theory but please express your point of view as to where the Ancients originated.
P.S.: If Earth isn't the home of the Ancients they may have seeded human life in many galaxies and be far older and more prolific.