Board Thread:Questions and Answers/@comment-25958552-20160825130425/@comment-26887879-20160902123054

It has been stated that it took Asgore long time to find a new scientist after Gaster disappeared, but it was also stated that the long time passed between Frisk's arirval and death of last human before Frisk.. Core was obviously there during the time of six humans, Toriel was locked in the ruins trying to protect them and Asgore was in the New Home which is connected to the Core.

But we still don't know when did exactly Gaster's accident happen. We don't even know how many years has passed between Frisk's arrival and Yellow's death. I don't remeber seeing any evidence. In my opinion, Gaster could have been scattered any time before Frisk's arrival.

And, yeah. I don't think Alphys has met a human before. But she knows how to tell monster and human apart most likely because she's a scientist.