Mudcrabs, and the rest of the animals, are the Earth Bones that lost so much of their divinity that they lost their intelligence. This is the basis for white souls, mostly. This also explains why Falmer have white souls instead of black souls as well, as they have fallen far from their divinity.
Man and Mer are descended from Ehlnofey. The Ehlnofey were either (A) et'ada that gave themselves to Nirn, lost their divinity, but not enough to lose themselves or (B) from one of the 12 worlds of creation or (C) from another Dream (could be B and C).
Hist are similar to Ehlnofey except they were never et'ada. They are from another Dream and ride Memory from Dream to Dream in order to continue their existence. Argonians are extentions of the Hist. Their not created by the Hist. They're more like limbs of the Hist. They're more a part of the trees than they are from them. This had always been theory but dialog from ESO has confirmed it.
I'd say this theory holds some weight with one minor problem. Any mortal can be a Shezarrine. In KotN you mantle Pelinal, a Shezarrine, and therefore become Shezarrine. There's also potential for the Last Dragonborn to be Shezarine as well. At the very least, KotN proves that any race can be Shezarrine, not just Man.