Neanderthals and sapiens are the same species. People built the first cities, and the soils around got depleted, as they sent their waste down the river, instea

Neanderthals and sapiens are the same species. People built the first cities, and the soils around got depleted, as they sent their waste down the river, instead of returning it into soil. These deficiencies turned people into deformed and mentaly ill "sapiens". The "Sapiens" didn't win, the depletion eventually became universal (or it was made so). Animals are also affected, all the megafauna died back then, now everything is dead, as the ability to remove the "toxin" exceeded the ability of life to adapt.