This is fictional, obviously, but I see it as very likely and even perhaps inevitable.
In the future (goes this story idea) the surface of the Earth has been divided into two kinds of zones or areas: "Inside" and "Outside".
Inside: Artificial, Safe, ImmortalThere is a single city but it's discontinuous, not connected. It's like islands and archipelagos of built area embedded in a "sea" of wild land and water. The city is ultra-tech, nano and beyond. People there do not age, or age and rejuvenate as they please. All the transhumanist techno-utopian dreams are come to fruition within the City. Barring accidents you live as long as you want and it is Christmas every single day.
Outside: Real, Dangerous, ReproductiveThe rest of the world is one giant Nature reserve. All of the oceans and the great majority of the land masses are kept in a Natural state and evolution is permitted to continue without interference from our human institutions within the City. People live Outside too, in tribes and homesteads, and although there is first aid and basic medicine and surgery, they voluntarily endure the "slings and barbs of outrageous fortune". Transhuman modifications and forms are not "worn" Outside. This is also where all new people are conceived, gestated, and born. If you want to have a baby and raise a child you have to go Outside.
This is Humanity's grand compromise with our technology. In order to maintain a normative baseline, a "control group", for our wild forays into the Transhuman realms Inside we have to permit our own natural evolution to proceed Outside.