Papua New Guinea has around nine million inhabitants and some 840 living languages. Not dialects, but separate languages that often cannot understand one another. No country on Earth has more.
The obvious explanation is difficult terrain. High mountains, dense rainforest, valleys you do not cross in a day. Groups live apart, their languages drift, and after a few centuries they no longer understand each other. It sounds convincing. It just does not hold as an explanation for the whole country, and the more interesting part is the part that contradicts it.
The unexpected observation
The linguist Donald Laycock, who recorded languages in the country for years, noticed a pattern that did not fit. Linguistic diversity was lowest where isolation was greatest, namely in the highlands. It was highest in regions with passable terrain and lively exchange, in other words where people dealt with each other constantly.
That turns the explanation around. If diversity arises where people meet, remoteness cannot be the cause. Current research therefore traces the diversity largely to social reasons. Language served as a marker of one's own group. People spoke differently from their neighbours because they wanted to be recognisably not their neighbours. Where two groups meet every day, an audible difference is more useful than in a place you rarely visit anyway. On this reading, diversity is not a by-product of distance but an achievement of closeness.
People spoke differently from their neighbours because they wanted to be recognisably not their neighbours.
— TerraTally · Languages desk
Then there is time. Settlement of the island began roughly 50,000 years ago, considerably earlier than in large parts of Europe. Around 3,500 years ago speakers of Austronesian languages arrived, an entirely different language family with its own origin. Together that adds up to a very long stretch in which languages could develop, split and exist side by side. Other parts of the world were written over by larger empires several times during those millennia. Here the process simply kept running.
Scale
~840
living languages
50,000
years of settlement
3,500
years since Austronesian arrival
That this diversity reaches into the present also has to do with what did not happen. For a long time there was no dominant lingua franca that pushed all the others aside. Multilingualism in daily life was the norm: people commanded the languages of their neighbours without giving up their own. Anyone trading in the next village learned its language and kept their own for home. And colonisation began comparatively late, so a single school language only gained weight much later.
The situation is not stable. Research documents a marked decline. Younger generations no longer pass on many of the smaller languages and switch to the lingua francas used across the country, the languages in which school, work and administration operate. With every language, knowledge held only in that language is lost, about plants and their uses for instance.
Papua New Guinea is therefore not only the place with the most languages in the world. It is also the place where it can be watched most closely how quickly something like that can disappear.