From A Death in the Rainforest: How a Language and a Way of Life Came to an End in Papua New Guinea, by Don Kulick (Algonquin Books, 2019), Kindle pp. 26-28:
As far back as anyone in Gapun has been able to remember, though, Tayap has never had more than, at most, about 150 speakers: the entire population of Tayap speakers, when the language was at its peak, would have fit into a single New York City subway car. Tiny as that count is, such a small language was not unusual for Papua New Guinea. Most languages spoken in the country have fewer than three thousand speakers. And linguists estimate that about 35 percent of the languages (which means about 350 of them) have never had more than about five hundred speakers.
Contrary to received wisdom, and common sense, this constellation of tiny languages was not the result of isolation; it didn’t arise because villages were separated from one another by mountain barriers or impenetrable jungle walls. Quite the opposite: throughout Papua New Guinea, the areas that have the highest degree of linguistic diversity (that is, the most languages) are the ones where people can get around relatively easily, by paddling a canoe along rivers and creeks, for example. The areas where travel is more difficult, for example in the mountains that run like a jagged spine across the center of the country, is where the largest languages are found (the biggest being a language called Enga, with over two hundred thousand speakers).
The conclusion that linguists have drawn from this counterintuitive distribution of languages is that people in Papua New Guinea have used language as a way of differentiating themselves from one another. Whereas other people throughout the world have come to use religion or food habits or clothing styles to distinguish themselves as a specific group of people in relation to outsiders, Papua New Guineans came to achieve similar results through language. People wanted to be different from their neighbors, and the way they made themselves different was to diverge linguistically.
Large swathes of neighboring groups throughout the mainland share similar traditional beliefs about what happens after one dies; they think related things about sorcery, initiation rituals, and ancestor worship; they have roughly similar myths about how they all originated; and before white colonists started coming to the country in the mid-1800s, they all dressed fairly similarly (and they all do still dress similarly, given the severely limited variety of manufactured clothing available to them today—mostly T-shirts and cloth shorts for men, and for women, baggy, Mother Hubbard–style “meri blouses” introduced by missionaries to promote modesty and cover up brazenly exposed breasts). Neighboring peoples hunt the same pigs and cassowaries that inhabit the rainforest; and they all eat sago, or taro or sweet potato—whichever of those staples their land is capable of growing.
In terms of the languages they speak, though, Papua New Guineans are very different from one another.