So turns out that Far Cry Primal's map was based on Far Cry 4's, right down to the river layout and paths. Which kind of makes the game seem more like a total conversion than separate entity. The fact that it took fans to reveal this information, rather than Ubisoft being upfront is a little surprising. As is the fact that the Himalayas have the same geographic make-up as ancient Europe. Click through to see a comparison.
The following images are courtesy of Game Pressure
. As you can see the similarities are pretty striking, and Game Pressure notes that where one would find bridges in Far Cry 4, in Primal you'll find water fords, and other natural crossings.
Far Cry 4 Map
Far Cry Primal Map
The question though is if this really fundamentally changes one's enjoyment of the game.