Red Sky, Blue Moon

Ancient aliens seed the primordial oceans of Earth with protoplasmic material found on other worlds, then return millennia later to see what they have wrought. They not only observe the human civilizations they find, they contact some of them, conducting investigative studies. When they leave, they decide to cull large groups of humans and their environs, which they convey across
the galaxy and transplant onto another world to continue their study. Ultimately this great speciation experiment will have unexpected and dire consequences for the experimenters.

Meanwhile, on this new world, a group of Vikings evolve over the centuries into a cutthroat corporate culture of racial purists in an early industrial society. They have designs on the lands of a nearby continent where primitive tribes of Sioux still cling to their old ways. And they may not be the only forced immigrants on this planet.

War ensues, and at the core of the conflict is a native herb which may be the cure for a disease that ravages the corporatocracy, as well as being the secret to longevity.