So much change in just 97 years?
Would so much change really happen in such a short amount of time? Let's start with the cultural changes. The people on the ark have a distinct culture with new religion, new cultural phrases (ie "May we meet again."), etc. The grounders and reapers have their own separate tribal cultures with a whole new languages, and seemingly no knowledge of the past.
And talk about the physical changes to earth. I know there was nuclear fallout but come on! They keep showing the Lincoln memorial, so we know they're in the middle DC, but absolutely no other signs of man-made structures??? No city ruins anywhere?