In Star Trek: The Next Generation, Season 6, Episode 20 ("The Chase"), we find out some genetic components were seeded throughout the Alpha Quadrant by an ancient race of beings, known in canon as "ancient humanoids". The holographic message which is played depicts a being which looks strikingly similar to the Founders / Changelings who rule the Gamma Quadrant's Dominion in Deep Space Nine. In fact, the voice was so similar I had to look up the actress and sure enough Salome Jens played both roles.
If memory serves, the ancient alien in "The Chase" said they knew they would not survive forever and seeded the genetic material to leave a legacy. There was no hint the race was actually dying and could have continued for a very long time after the seeding took place. They would have had plenty of time to spread into other quadrants.
In DS9, the female changeling explained their race had once been like the solids but that they'd evolved. It seems they were responsible for the rapid evolution of the Vorta from tree-dwelling mammals to their position with the founders. They also seem to have been responsible for engineering the Jem'Hadar to be reliant on Ketracel White.
Has this link been addressed and I've just missed it? I'm curious if the actress was a purposeful choice or merely a coincidence. I know many instances where an actor played unrelated parts in more than one series (and in DS9 I saw a lot of repeats in the same series!)