Yep, you heard me.
Maybe, at one point, they were snakes (the Potentates being the most obvious example), but I think that the skeletons at Pale Pass and the carvings at Sky Haven Temple aren't exactly as incorrect as I once believed.
There's a line in "http://www.imperial-library.info/content/skyrim-children-sky" which compares Thu'um to the "kiai" of the Akaviri swordsmen. Kiai in the real world is just what they call it when martial artists yell out "hi-ya!" or whatever, but that doesn't sound anything like Thu'um to me. Thu'um has power. It's not just a battlecry, it's a primal, magical ability that bends the fabric of reality to your will. So, it's entirely likely that the Akaviri power of Kiai is exactly the same, or at least similar.
Secondly, it seems to me that the only Akaviri swordsmen that Tamriel has really seen were the dragon hunters that were searching for Reman. And, if my theory is correct, these Kiai-using warriros were human.
The Potentates, who came with them, were however entirely snake-like. Could it be that they came from a different caste to the dragon hunters? Some kind of administrators, maybe?
Their names are completely different, for starters. The human Tsaesci have names with (generally) "X" sounds, while the snakes are hyphenated, and have lots of "S" sounds in their names.
Now, I'd like to draw your attention to "http://www.imperial-library.info/content/tsaesci-creation-myth-and-we-ate-it-become-it", which suggests to me that the Tsaesci (at least the Kiai-using caste) have the ability to become those that they eat. Gods included.
And who have the Tsaesci eaten? http://www.imperial-library.info/content/skyrim-mysterious-akavir Now, if we assume that Kiai is an ability similar to Thu'um, I would like to also propose that "eating" is an ability similar to the Dragonborn's ability to absorb dragon souls. Ysmir was also a Dragonborn, and eventually people couldn't look upon him without seeing a dragon. But, why? Well, perhaps it's all the dragon souls he ate.
So, I believe that in Akavir, or at least in the kingdom of Tsaesci, "you are what you eat" is taken literally. The Tsaesci of the "Manborn Caste" ate so many men, that eventually they became them. It's also my belief that the Tsaesci aren't vampiric in the Tamrielic sense of the word, but rather this vampirism relates to their ability to absorb the life-force (souls) of those they kill. And, of course, perhaps they can use these souls to live forever (until they're murdered by the Dark Brotherhood).
Thoughts?