As bizarre as it may seem, I'd probably just chalk it up to otherworldly forces guiding evolution on Suskillon the same way they did with Golarion. After all, in Pathfinder, the humans were created by the Alghollthu for some mysterious purpose, and they were explicitly described as beings of alien origin. So since it's canon in Pathfinder lore that aliens created humans, it's possible that the Alghollthu created more of them on several other planets by using their powers to guide evolution to follow their desired outcomes as they went slithering around through the cosmos.
It at least makes way more sense than "they just evolved there too."
