No, the reason it occurred in the Markham Valley is because of the bit immediately after that sentence:

> a 1,000 metre climb over about 30 km takes you straight up onto the edge of the eastern highlands

i.e. because the Markham River provides a pathway from the Lowlands into the Highlands.

The paragraph you quote concerns a different proposition, as may be clearer if some of the subordinate clauses are elided:

> The reason why I believe that […] the highlands were functionally cut off, is that […] they themselves did not ever speak of or show knowledge of the highlanders. Similarly, when the highlanders first encountered Australians, they had no concept of the coastal people, or where the shells came from, or any further concept of the broader world.

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walrus01 2 hours ago | parent [-]

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.

Google "language isolates in papua new guinea".

https://www.google.com/search?client=firefox-b-d&q=language+...

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bradrn 2 hours ago | parent | next [-]

> Google "language isolates in papua new guinea".

My preferred source for these things is Palmer’s Language families of the New Guinea Area [0] (part of the extremely comprehensive book, The Languages and Linguistics of the New Guinea Area). Page 10 has a nice map of language families and isolates.

[0] https://www.academia.edu/86071361/The_languages_and_linguist...

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walrus01 2 hours ago | parent [-]

I am the furthest possible thing from a linguist but looking at a hastily searched language map of PNG, my first thought is "wow, that's a lot. And some of those aren't even in the same family of language?"

https://upload.wikimedia.org/wikipedia/commons/f/f6/TNG_map....

	▲	bradrn 42 minutes ago \| parent [-]
		> wow, that's a lot. And some of those aren't even in the same family of language? Indeed! Palmer counts ‘862 languages comprising 43 distinct families […] and 37 isolates’, (where an isolate is a language with no known relationship to any other family). I should add that the map you linked shows the branches of only a single language family, namely Trans–New Guinea — most of the other 42 families are in the grey areas. (Though due to uncertainties in historical linguistics, that map includes some groups as part of Trans–New Guinea which Palmer considers separate families, for instance Timor—Alor–Pantar [there part of ‘West Trans–New Guinea’] and South Bird’s Head.)

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roncesvalles an hour ago | parent | prev | next [-]

TFA answered it though. It's because neighboring tribes were always hostile, and they never developed food that survived long expeditions.

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cwmoore 2 hours ago | parent | prev [-]

Not an expert view, but I have also heard this explanation for the many hundreds of languages in Africa and India.