As I understand it one of the reasons why PNG developed so many unique languages is the extreme geographic complexity of the place. This resulted in language developing in isolation.
I have a personal theory that people living hunter/gatherer or very small scale subsistence agriculture lifestyles historically did not have a ton of surplus calories to spend on hiking over multiple ridges and mountain ranges to discover who's living in the valley 45 km away. The complexity of the jungle environment also makes it very difficult to 'bush whack' through somewhere that no trail exists. And if a foot trail isn't frequently used it will become overgrown very quickly.
I have a personal theory that people living hunter/gatherer or very small scale subsistence agriculture lifestyles historically did not have a ton of surplus calories to spend on hiking over multiple ridges and mountain ranges to discover who's living in the valley 45 km away. The complexity of the jungle environment also makes it very difficult to 'bush whack' through somewhere that no trail exists. And if a foot trail isn't frequently used it will become overgrown very quickly.
Google "language isolates in papua new guinea".
https://www.google.com/search?client=firefox-b-d&q=language+...