New Guinea (aka 'Papua', and historically as 'Irian'- which means "to rise" or "Rising Spirit", in the Biak language and several others) is the world's second-largest island, behind only Greenland; the largest island in the Southern Hemisphere, and the largest in the equatorial region by a considerable margin. Formerly part of the tectonic continent of Sahul, aka Greater Australia, before being separated when the area now known as the Torres Strait flooded after the end of the last glacial period (echoing the parallel contemporary partitions of the British Isles and Japanese archipelagos from their respective mainlands at roughly the same time), New Guinea can be differentiated from its far drier, flatter and less fertile southern counterpart, Australia, by its much higher rainfall and its active volcanic geology. The New Guinea Highlands dominates the geography of New Guinea, stretching over 1,600 km (1,000 mi) across the island, with many mountains over 4,000 m (13,100 ft). The western half of the island contains the highest mountains in Oceania, with its highest point, Puncak Jaya, reaching an elevation of 4,884 m (16,023 ft). The tree line today is around 4,000 m (13,100 ft) elevation (roughly the same as the ice-cap line back during the last Ice Age), and the tallest peaks still contain the vestiges of retreating equatorial glaciers.
Except in high elevations, most areas possess a warm humid climate throughout the year, with some seasonal variation associated with the northeast monsoon season. And another major habitat feature is the vast southern and northern lowlands. Stretching for hundreds of kilometres, these include lowland rainforests, extensive wetlands, savanna grasslands, and some of the largest expanses of mangrove forest in the world. The northern lowlands are drained principally by the Mamberamo River and its tributaries on the western side, and by the Sepik on the eastern side. The more extensive southern lowlands are drained by a larger number of rivers, principally the Digul in the west and the Fly in the east. The largest island offshore, Dolak, lies near the Digul estuary, separated by a strait so narrow and shallow it has been named a "creek", which frequently seasonally gets bridged by mudbanks. New Guinea contains many of the world's ecosystem types, including glacial, alpine tundra, savanna, montane and lowland rainforest, mangroves, wetlands, lake and river ecosystems, seagrasses, and some of the richest coral reefs on the planet. And New Guinea also has immense biodiversity, containing between 5 and 10 percent of the total species on the planet- about the same amount as those found in the United States, or the entirety of continental Australia. A high percentage of New Guinea's species are endemic, and thousands are still unknown to science; with New Guinea considered 'nearly a continent' in terms of its biological distinctiveness.
Human habitation on the island dates to as early as 50,000 BC, and first settlement possibly dating back to 60,000 years ago, with the island having also been one of the world's oldest and most established centers of agriculture- sugarcane, bananas, taro, breadfruit, coconuts and any number of other now-global crops were first cultivated and grown here. And IOTL, the island is populated by almost a thousand different tribal groups and a near-equivalent number of separate languages, which makes New Guinea the most linguistically diverse area in the world; with a current population estimated at around 15M now (having roughly doubled from around 7.5M at the turn of the century, experiencing a massive ongoing population boom right now IRL), and among the highest rates of food, agricultural land and water abundance per capita of any landmass in the world. Regarding renewable internal freshwater resources alone, the island of New Guinea possesses over 1/40th, 2.5% of all of the total available freshwater in the entire world- Papua New Guinea alone receives more freshwater on an annual basis than both Australia and New Zealand combined, and New Guinea as a whole receives just over half as much freshwater per annum as the entirety of China (including Taiwan) does; roughly on a par with the amount received by the entirety of India excluding the North East Region ('NER', connected to the rest of India via the narrow Siliguri Corridor).
So bearing all of these factors in mind, just how high a population do you feel that the island of New Guinea could conceivably sustain, in an alternate history scenario? For instance, in an New Guinea-wank alternate history series I've been tentatively working on for a couple of years now, in which the Papuans effectively 'do a Japan' extremely early on (with the Austronesian expansion, c.3,000-3,500ya, effectively kicking off the transition from their equivalent of the Jomon period- which the Papuans, for the most part, effectively never left IOTL- to the island's Yayoi period equivalent, at roughly the same time that the Japanese do both ITTL and IOTL, and the levels of societal development there more-or-less keeping pace with those on the Japanese archipelago from then onward), how plausible would it be, in your opinion, for my TL's New Guinea to have a population density comparable to those of OTL's Sri Lanka, Japan and the Philippines (which would translate to a total population, for the main island, of 260-270M)? Would it require any suspension of belief?
Except in high elevations, most areas possess a warm humid climate throughout the year, with some seasonal variation associated with the northeast monsoon season. And another major habitat feature is the vast southern and northern lowlands. Stretching for hundreds of kilometres, these include lowland rainforests, extensive wetlands, savanna grasslands, and some of the largest expanses of mangrove forest in the world. The northern lowlands are drained principally by the Mamberamo River and its tributaries on the western side, and by the Sepik on the eastern side. The more extensive southern lowlands are drained by a larger number of rivers, principally the Digul in the west and the Fly in the east. The largest island offshore, Dolak, lies near the Digul estuary, separated by a strait so narrow and shallow it has been named a "creek", which frequently seasonally gets bridged by mudbanks. New Guinea contains many of the world's ecosystem types, including glacial, alpine tundra, savanna, montane and lowland rainforest, mangroves, wetlands, lake and river ecosystems, seagrasses, and some of the richest coral reefs on the planet. And New Guinea also has immense biodiversity, containing between 5 and 10 percent of the total species on the planet- about the same amount as those found in the United States, or the entirety of continental Australia. A high percentage of New Guinea's species are endemic, and thousands are still unknown to science; with New Guinea considered 'nearly a continent' in terms of its biological distinctiveness.
Human habitation on the island dates to as early as 50,000 BC, and first settlement possibly dating back to 60,000 years ago, with the island having also been one of the world's oldest and most established centers of agriculture- sugarcane, bananas, taro, breadfruit, coconuts and any number of other now-global crops were first cultivated and grown here. And IOTL, the island is populated by almost a thousand different tribal groups and a near-equivalent number of separate languages, which makes New Guinea the most linguistically diverse area in the world; with a current population estimated at around 15M now (having roughly doubled from around 7.5M at the turn of the century, experiencing a massive ongoing population boom right now IRL), and among the highest rates of food, agricultural land and water abundance per capita of any landmass in the world. Regarding renewable internal freshwater resources alone, the island of New Guinea possesses over 1/40th, 2.5% of all of the total available freshwater in the entire world- Papua New Guinea alone receives more freshwater on an annual basis than both Australia and New Zealand combined, and New Guinea as a whole receives just over half as much freshwater per annum as the entirety of China (including Taiwan) does; roughly on a par with the amount received by the entirety of India excluding the North East Region ('NER', connected to the rest of India via the narrow Siliguri Corridor).
So bearing all of these factors in mind, just how high a population do you feel that the island of New Guinea could conceivably sustain, in an alternate history scenario? For instance, in an New Guinea-wank alternate history series I've been tentatively working on for a couple of years now, in which the Papuans effectively 'do a Japan' extremely early on (with the Austronesian expansion, c.3,000-3,500ya, effectively kicking off the transition from their equivalent of the Jomon period- which the Papuans, for the most part, effectively never left IOTL- to the island's Yayoi period equivalent, at roughly the same time that the Japanese do both ITTL and IOTL, and the levels of societal development there more-or-less keeping pace with those on the Japanese archipelago from then onward), how plausible would it be, in your opinion, for my TL's New Guinea to have a population density comparable to those of OTL's Sri Lanka, Japan and the Philippines (which would translate to a total population, for the main island, of 260-270M)? Would it require any suspension of belief?