The post-apocalyptic world takes places millions and billions of years later
This seems plausible, with images of Natsu and the images of evolution from fish to dinosaurs superimposed and just how delapitated the ruins of the modern world are. The post-apocalyptic world takes places shortly after the meteorites struck earth
The opposite of the first idea, the world could be taking places a few years after the apocalypse hit. The OPS video documentation has the main professor talk about how the earth is still frozen solid and uninhabitable, though how long the ice age took place is unknown. It could be that it was a rather short-lived one, taking about 20 years, before earth thawed up. The delapitation of the modern world could be explained by the dramatic climate changes that happened: meteorites hit, which also caused volcanos to become active again, lots of gas and smoke, with the sudden drop of temperature before thawing again. And all that, coupled with seawater... Polygamy VS Monogamy
Eventually, when all teams have been brought together, a discussion will ensue whether they should be polygamous, to increase human population faster, or remain monogamous, which is the overall consensus on how things 'were done' in their time. Particularly Hana and Arashi will be for the monogamous option, where as Team Summer A will more lean towards polygamy, if necessary. Hana and Arashi will reunite
But one of them will die shortly after due to- A fatal injury received shortly before
- Poison in their system, from having touched a harmful plant/animal
- Shot by one of the guns still around likely fired by Mozu
The series' end
Will take place when all the teams are reunited and things have been worked out. The teams decide to work together, building a good home for them and show to decently live in the changed world... then meteorites will hit the earth again, killing these last remaining humans and turning the entire thing into a "Shaggy Dog" Story.