At https://www.livescience.com/health/genetics/papua-new-guineans-genetically-isolated-for-50000-years-carry-denisovan-genes-that-help-their-immune-system-study-suggests … Papua New Guineans have been genetically isolated for thousands of years. Virtually all the way back to the Laschamp Event [50 to 40,000 years ago]. They have inherited genes from Denisovans – contemporaries in eastern Asia of Neanderthals in western Asia and Europe. Both Denisovans and Neanderthals died out around the time of Laschamp. It is thought therefore that modern humans arrived from Africa and took the place of the Denisovans. This sort of thinking has become a brick wall in genetics. One problem we may note is that Australian Aborigines, related to the people of New Guinea, are thought to have been living there for 70,000 years. New Guinea and Australia were one land mass so how do some of them turn up 20,000 years before the other lot.
Who were the Denisovans. Go to https://www.livescience.com/denisovans-extinct-human-relative … and this will flesh out what little is known about them. Evidence for their existence came from a finger bone found in the Denisova Cave in the Altai Mountians of southern Siberia. They seem to have lived there alongside Neanderthals. Denisovans were also prominent in SE Asia. They were also endemic in the Melanesian Islands, such as the Solomons, Vanuatu, New Caledonia, Fijis and New Guinea. Australia is not mentioned.