Sans and Papyrus were old, very old. They were already part of the sea when dinosaurs still roamed the world.
However, after a while it got lonely, just with the two of them. They could only talk to each other and eventually they ran out of topics to talk about. So they had decided to separate for some time to experience something on their own.
Sans settled down off a shore and began to sleep. And when he woke up again, he had a city on his back. He couldn't just dive down since it would kill the people of this city. And Sans wouldn't hurt any innocent people. So they came to an agreement. Sans would take their city in his shell, keep it safe forever, and for that these people join him in the sea. They became the first Mers to ever live in the sea.
Unknown to both parties, the legend of the lost city of Atlantis and the myth of the world turtles were born.
When Sans then reunited with Papyrus, he excitedly told him about the people who were now like them. However, Papyrus's action surprised him much more. He spread his pectoral fins, revealing five little creatures that Sans knew were like him and Papyrus. And he couldn't say he wasn't looking forward to taking care of them.