Originally Posted by
Brinne
I think if we're going to pinpoint a certain event that probably changed their views on "using souls" (because Elpis, Pandaemonium, and Emet's short story with the Phoinix establish that Ancients, and their culture, strongly discourage messing with souls and view it as entirely up to the planet), it was the Final Days, in which they needed to utilize souls in their last resort of creating Zodiark to save the world. They were desperate enough to break what multiple pieces of the text reinforced again and again was, throughout their history, a serious taboo.
As I said, my overall view is that the writers are almost certainly never going to go into detail about the sacrifices for the sake of wanting to keep everyone likable and sympathetic, but for a diegetic conversation, animals do have souls - the requirement of a soul is simply "natural life embraced by the planet." This is reinforced by P9, recently, where the first boss devours a behemoth soul as one of its mechanics and acts accordingly.
This, of course, doesn't disclude the possibility of the souls being "more" than animals, but since the "new life" was stated to be created by Zodiark - aka, Themis - it actually does strike me as a bit weird to suggest that it must have been some form of sapient people. Because the shape of the life would have been guided by Themis, and then the plan was to "cultivate" it over a long period of time until the time was right to offer up "a portion" of it and return to how things were before the apocalypse.