*Spoilers* Anyone feel joining Emet would have been better

[Absimiliard]

We’ve rediscovered the god in the gaps arguments folks.

If you want why I believe they were meant to be human, few things.

1. Like mentioned by Slatersev, the new life to be sacrificed was also said to be entrusted with the planet, an odd statement for plants and animals.
2. The Ascian plan was to sacrifice the inhabitants of the Source post rejoining to replace the souls in Zodiark, a plan that is stated multiple times to not be changed from the beginning.
3. For them to be an equivalent exchange for the souls in Zodiark, logically they could not be creations or familiars, as those are mentioned to be less aetherically dense.
4. The shade of Hythlodaeus uses the phrase “life energies” to describe both the sacrifice done to create Hydaelyn, and the planned third sacrifice.
5. If such an option to use other forms of life existed, then it stands to reason that the Ancients would’ve used as much of that life in the initial sacrifices as possible, yet not a single example of such a thing exists.

1.) The lives the Ancients intended to sacrifice to Zodiark were not the lives the Ascians intended to sacrifice. This is an important distinction.
2.) This is only partly accurate.
3.) We learn in Elpis that some of the Ancients' creations gained souls and were in turn able to create more like them via standard reproduction. A lack of aetheric density can easily be made up for with sheer volume.
4.) Can't disagree here. That phrase got tossed around quite a bit, though life energies and souls are two different things. Zodiark quite clearly contains a multitude of actual souls, not just the aetheric energies of their owners. This is mere supposition on my part, but I figure this might explain why Zodiark's aether reserves don't seem capable of being fully depleted.
5.) From what we are lead to understand in Elpis, much of their world is not yet inhabited by their creations. It seems quite probable that they sacrificed themselves for lack of any other viable sacrifices, or perhaps because Zodiark simply had to be so overwhelmingly powerful to perform its intended function. One of the things they did when they had Zodiark restore the world's ability to sustain life was to have him create new life as well, which we are lead to believe stemmed from the Ancients' own creations.