When I watched “Skyline”, an alien invasion movie where they were harvesting people for brains, I realized that these islanders might be the only ones left because the aliens were looking for signs of civilization.
There are cave dwelling tribes still in Papua New Guinea, hunter gatherers at the Amazonian base of the Andes, the Pygmies in Africa who would be the ones left behind to restart the species ... unless the aliens reason for sparing pregnant women was to put a genetically altered next generation on the planet. At which point, it becomes akin to Planet of the Apes type struggle.
But any type of worldwide disaster, and these types of people are likely to be the ones left.
There are also tribes in the inner cities that speak their own language and haven’t had contact with civilization for many years as well. They are extremely violent and cruel, and a shoe that any civilized attempts to contact them.