Centuries into the future, a joint venture by the space programs of various countries culminates in the launch of a massive space shuttle sent to explore the outer reaches of the solar system, not expected to return for centuries.  The ship is populated with 10000 people, all screened to approximate a random sampling of human genetics, 10000 being the calculated minimum number of people required to keep birth defects to a minimum.

The ship launches without a problem, and for several generations everything goes according to plan.  Somehow, through cosmic radiation, or isolated, unexpected and unintentional instances of inbreeding, each generation is dumber, and weaker than the last, leading to a brain drain of sorts, with fewer and fewer births, and more and more interpersonal conflicts, until eventually the ship is a small band of ethnically and genetically undiverse simpletons who’ve forgone their scientific research.

Upon the ships return to earth, the governments now controlling the planet attempt to sequester the ship-borne hicks from the world, which has largely worked past its own issues with race and religion.  The governments fail, and the hicks manage to set off race wars effectively setting the global clock back to the 16th century at the start of the European slave trade.