Distinct changes in architecture and tech levels suggest multiple eras and downfalls throughout history, at the very least we can see evidence of two ages before our own. We have: structure Xs which are centrally scattered, and structure Ys scattered north of the Floodlands area.
Comparing the two eras, X were considerably more advanced and established than Y, with a markedly larger network and more advanced industry. The unnatural scattering of constructions indicates a highly destructive (man-made or natural) disaster as a possible reason for downfall. The significant change in 'elaborateness' of ruins shows a heavy divide within castes.
Y, on the other hand, seems to share some of the same technologies but shows to be a far more ramshackle and inferior version of X. Civilisation X was far more long lived than Y, perhaps Y came into being as a more inferior imitation of X frantically trying to rebuild their lost empire.
Theorist Sabina maintains that the surviving skeleton race we see today lived as the dominant inhabitants of X, however this does not explain why the skeletons would not have been able to rebuild their civilisation, based on their advanced knowledge. Either way, based on the tech found in the labs of civilisation X, skeletons definitely existed within this culture. Whether they were merely a creation of X or the sole inhabitants of X we still do not know. Only further surveying will tell.
The Lost Ancients II
Without a doubt, at least two very distinct cultures existed before our own, however, WHEN these civilisations came into being is the key question. On what evidence are we assuming that these civilisations existed at different times?
Y appears to have been intellectually inferior, but the similar tech levels clearly suggest that these two cultures did in fact have an overlap in history. Did these two civilisations war with and destroy each other? Was one isolating itself and fleeing from the other? Judging by the relics and advanced tech from X, its culture was largely skeleton based. If the skeletons from X warred with and wiped out the organic lifeforms from Y, this would at least give some plausibility to the religious scripts found within the various schools of Okranism.
We cannot write off the historical input of an established religion on account of the disciple's irrational interpretation of it. Clouded with irrationality it may be, but I believe we can still salvage some truth from its ancient foundations.
The first book was written by the current leader of the Machinists, Finch. The last paragraph of his book cited that Sabina believes the Skeletons are the dominant inhabitants of X which is from Mass Extinction Series II written by Sabina.