User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-4674269-20120208030247/@comment-1011548-20120209080417

According to the Elder Dragon Observationist Guy in MHFU, Yama is descended from a jellyfish-like creature that lived on or near the surface of the sea. Sounds plausible enough to me (as long as you don't think about how such a change would've taken place over millions of years, and that the humans probably haven't been around for millions of years).