Mmh. The Hist have the power to shape the Argonians. I don't know how quick this change is supposed to be, or if it's still even supposed to continue. They started out as some kind of sea reptiles, with simplistic minds. Every day the same day, hunting, hiding from danger, the world consisting only of light, darkness, nothing beyond that. From simple animal minds into intelligent agents of the Hist. Argonians still like to simplify their minds, which they can do by touching the Hist.
I can't bother to search for exact quotes, but that's all from the novel Lord of Souls, and thus I think considered canon. It's nice to think that their evolution is still continuing, thus the minor/major changes between the games.
I can't help but think that the Hist (and/or Um-Hist) are the "thing" that have achieved Amaranth. Has it been confirmed they aren't? After all, they do hallucinate and create new on their own. I get lost when MK's numbers kick in, though.