Sorry but it is incorrect to assume the first snowfall means the first snow to ever happen on earth. With the way the spirits work, they gain spiritual power from people’s souls. They effectively are created through magic and souls. So it’s not that she is the spirit created by the first snowfall, but rather through the collective thought and magic from the time people recognized snow as snow. That would have taken thousands of years before the spirit became mature enough to take a form. So Yuki Origin is more strongly defined by the times before most of the myths in Asia. For Yosh! this goes back pre-history, because the pre-history period of Yosh! is a lost time of magic.

Spirits as beings capable of thought basically can’t exist without creatures capable of complex thought. Those beings mold and shape the spirits till they can take on a life of their own. The fake Amaterasu is the example of a spirit being treated as something it isn’t. The spirit was tied to something and worshipers used that thing to worship it as if it were or was related to Amaterasu. This caused the spirit to think it was and it grew from that power taking on powers related to those beliefs.

Hope this helps clear it up- and saves me from getting beaten up by Yuki from making people think she’s super old! 😛