There has never been a reference in the either series or by any producers that indicates that the ancients evolved on Earth They apparently lived on Earth for a time but left after seeding the planet with life that would evolve into humans The series seems to indicate that the ancients were a galactic civilization that had colonies all over the Milky Way Galaxy Then a plague struck them and their population was decimated The remaining ancients set out to repopulate their kind by seeding worlds that could support life Earth was the last of these worlds in the Milky Way galaxy that was seeded before the ancients left for the Pegasus galaxy to continue their work