pastoral nomads

Hundreds of generations before women lived along the river, their ancestors herded stronghorn thousands of miles to the west. They brought their herds through a convoluted mountain pass in search of fresh grazing and found themselves in a part of the continent not yet inhabited by humans. The vast plains seemed rich enough and at first they settled happily enough into a land where they didn't have to compete with other tribes, but the environment proved too different from the stronghorns' original habitat. The new pasturage wasn't sufficiently nourishing, and they needed to eat more to consume the same nutrients. The nomads had to wander farther to find enough grazing to keep their herds alive. A period of extended drought exacerbated the problem, and some herds died out entirely.
Bands who suddenly found themselves without their livestock were forced to return to a hunting and gathering lifestyle, leaving the lowlands to chase wild game. They followed rivers and streams into the mountains, supplementing their hunting with fish. One particular band reached a high-altitude lake that they settled as a summer hunting camp. After learning to cultivate certain nutritious plants in tree canopies, they made the settlement permanent. Over the years, exploring over a ridge revealed a stream that led to a large river, and they set up a seasonal fishing camp at the stream's mouth. A second camp established farther down became a permanent settlement, and the first of the women's villages.
Communication between the river villages and the lake village tapered off over the generations, and ended entirely after the first village was destroyed in a landslide. There were only three villages at the time, and the lake residents made the assumption that all of them had suffered a similar fate.

Culture

Common Myths and Legends

At the height of the drought the nomads sometimes sheltered during the day and traveled during moonlit nights.  It was because of this that the figure of Old Old Mother became conflated with the moon, and after they settled at the lake storytellers added to the basic narrative the story of how the excessively bright light damaged the trees until the moon sent it away, saving them.

View from the future

This is kind of embarrassing, actually...  Creator of the world has no idea what happened to these people.