A human culture develops during the height of global warming. This culture has a coming of age ritual where a young person is put outside during wet bulb temperatures that may kill them.
Those that survive without difficulty, join a privileged class of humans that are allowed to explore outside. Those that barely survive are subjected to a life underground, working in the factories. The two classes are not allowed to interbreed.
Over time, the privileged class spends more time outside eating the fruits, berries and nuts that are available on the surface. The underclass evolves to a life of labor underground.
