Remember, you're in a alternate reality, but there are no aliens or mutants here. just us.
In the early Spring of 2001 several Earth governments finally accepted the facts of global overpopulation and natural resource misuse. They realized humanity had started mass recycling and other environmental sustaining programs much too late. The planet was dying. It was only a matter of time.
A plan was born to use the nearest planetary body as a lifeboat, until the planet could recover and humanity could return home. Mars had been the first choice. But, due to distance and resource drain for preparations, everything needed to relocate a genetically viable human base population, it was decided it was too far away to be a real option with the time remaining. This left the moon.
It was much smaller and it's gravity was only about a quarter of the Earth's, but it was close and easily reachable. It was the best evacuation option with the time they had remaining. It was decided to build some of the base on the surface, but most of it would be built into the interior. Specially designed drilling and mining equipment was built for use in zero atmosphere and the cold of space, then sent up with long term work crews. These volunteers came from nearly every country. Eden became a true global effort. The first in human history.
Knowing not everyone would be able to go to Eden, some governments began work on biodome colonies. Places on Earth where people who would not be accepted to Eden could seek shelter, and possibly survive. In an effort to sustain their controlled populations, the biodomes would make use of as many resource friendly options as possible. Genetically sound individuals with needed skills could look forward to a possible placement at Eden in the future.
The biodome builders had a marked advantage, and their work went much faster. They didn't have to deal with the problems of working in space and zero atmosphere. The first of the biodome colonies began taking in personnel in the Spring of 2010. Eden was finally completed in the Winter of that same year, and by 2011 began accepting people.
The first to go to the biodome colonies and Eden were scientists, engineers, medical personnel, teachers, and others who would be the foundation of a new human society. Agricultural engineers and farmers were high on everyone's list. If you can't feed a population, you can't keep it alive and healthy.
Another valued group of people, medical personnel in the genetics and human reproductive fields. If either the biodomes or Eden were to succeed, then population growth would need to be controlled. Too many babies, or genetically unsound babies, could be the downfall of everything. Knowing this, and hoping to give themselves a better chance at a biodome or Eden placement, many people readily agreed to visit free sterilization clinics around the world.
It was decided early into the programs to disallow anyone over sixty, who wasn't essential personnel, from joining the biodome or Eden programs. Those who were disallowed were given the choice of living out their lives outside the biodomes protection, or if they wished a better end, visiting a Crossover clinic in a local hospital. Each community has it's own way of honoring these dead, but most gave them back to the Earth.
Now, humankind tries to follow it's plans and hope for the best.