Board Thread:General Discussion/@comment-26006155-20190617222636/@comment-26006155-20190620232326

Ness and Andonuts probably visited several worlds. They make traveling to a world to investigate an anomaly sound as routine as an episode of Star Trek. So adopting disguises was probably part of their First Contact Protocol. Since showing up and yelling "Hey people! We're alien time travelers!" is bound to have unexpected and negative results.

So I believe that Ness and Andonuts became Sans and Gaster after observing the Alpha Timeline residents remotely, then joining in once they had a starting knowledge of the area and disguises that worked.

They would have arrived in the Alpha Timeline before Chara arrived, since in the Beta timeline Gaster has already done his groundbreaking research on Determination Extraction, and died, before Chara even arrived at the end of 2019. Which is another reason Chara wasn't actually The First Human to visit, because without access to a human soul, Gaster could have never done any DE research.

Gaster either needed to be human himself, or have a human test subject, like Sans. So this is proof at least one human was hiding in the Underground before Chara. Although I believe there were two.

To stand where Alphys did, working on the same project and creating Flowey in the Alpha Timeline as the Soul Vessel to make combining the souls possible, Gaster would have had to have already seen The Six Humans that arrived after Chara get collected by Asgore. At this point in time the Kill All Humans mandate was certainly up. And Asgore could have harvested Gaster, or Sans, at any time, to get The Power of Seven.

So the only reason Gaster wasn't dead yet, is because the monsters didn't know he was a human.