A world with weird selection pressures

Weird alien scientists go from planet to planet, dropping down primitive people who have just enough to survive and prosper. They also drop robots down that do various stuff. In all cases, the population's limiting factor is not malthusian - it's getting through some kind of rite of passage into adulthood.

Basketball Planet

Imagine a planet where the initiation rite was shooting 10 freethrows. If you miss any you don't make it. After 10,000 years what would the people be like?

Twin World

Every birth on the planet is a multiple - but only one of each is fertile. There would be very quick selection for family-feeling in the non-fertile twins. The non-fertile twins would work to support the fertility of the one fertile one - and that one might not have much freedom.

Longevity Planet

Regular humans, but all the food causes them to be infertile until age 50. Once past this, kids can be born. After a few thousand years the people on this planet would live really long lives.

Segment World

The world is divided into 10 continents connected by portals. The first continent has only kids living on it, from age 0 - 9. On their ninth birthday, the robots put them through a portal. This portal leads them to continent two. On this continent, they must present themselves at a particular spot on their 20th birthday - if so, they are transported on to the next continent. And so on. When children are born, they are dropped off at a certain spot on the first world.