"Hey, don't look down on flowers. At least they're just way better than wicked cats like you. Did you know? Before history, only machines and plants existed in the tower. I'm not interested in that sort of thing. There's a saying that the first humans were born from plants. That's why they're called the great plants (origin). These plants, so to speak, are like our parents."Irregulars are an invasive species to the tower, with inherent advantages allowing them to displace and wipe out indigenous life.
Ancients have become an endangered species in their own natural habitat.
With irregulars introducing something not unlike -climate change- to the lowest 134 floors.
That bit yamas mother said gives away the plot on this angle. The plant kingdom probably evolved into human like creatures in the tower and meanwhile the empire favored those and wiped out ancient species which were a threat to them.