My hypothesis is that dark earths started from privy pits. People used charcoal from where they cooked a meal and dumped it in the pit to kill the smell. Once the pit filled they dug another. Eventually they found out that the former pits made great farm soil. The bacteria in the human microbiome bound those minerals onto organic molecules which then clung to charcoal instead of rinsing out of sandy soils.
That seems likely as a factor. Slash and burn, leveling of the ground, continuing compost, new field every year in rotation, old field used for the outhouse, and circling ‘round for generations.