Island In South East Asia May Hold The Key To Humanity's Interbreeding

Replica of the Sangiran 17 Homo erectus cranium from Java - side view. Whether our ancestors met people with skulls like this when they reached Indonesia, any interbreeding did not leave a legacy in our genes. Photo supplied by the Trustees of the Natural History Museum. 

People whose ancestry comes from Southeast Asia's eastern islands carry extensive evidence of interbreeding with Denisovans. There are, however, no such traces of three other members of the human family known to have lived in this area: Homo erectus, Homo floresiensis, and the recently discovered Homo luzonensis. Nevertheless, the presence of these other human species probably left an important legacy in certain large animals' survival. 

The discovery that most modern humans inherited some genes from Neanderthals sent shock waves through anthropology. This was followed by evidence of more localized inheritance from Denisovans, a branch of early humanity known only from a handful of bones and DNA preserved in cave sediments. The obvious question was whether any other ancestral branches of the human family live on in us. Some evidence of this has been found, but we know almost nothing about who they were or what occurred. 

One of the most promising places to seek such a contribution to the human genome is in the islands that now form eastern Indonesia, the Philippines, and New Guinea. Two human species, H. floresiensis (nicknamed "hobbits") and H. luzonensis are only known from the islands that give them their name, and probably survived long enough to encounter the first modern human arrivals. Widely traveled H. erectus was once called Java man after its initial discovery site.

The fact no surviving DNA has been recovered from any of these species' fossils presents an obstacle to detecting their legacy within us. However, Dr João Teixeira of the University of Adelaide compared the genomes of people of Australian and various island heritages with those from the Asian mainland. These were analyzed in search of sequences with traits indicating they might be from a branch of the human family that diverged from modern humans much earlier than the Denisovans, before interbreeding brought it back. 

In Nature Ecology and Evolution, Teixeira and co-authors announce this method revealed no signs of what they call “super-archaic” hominin DNA, no hobbit-DNA in the inhabitants of Flores, for example. On the other hand, the study confirmed previous reports the world's highest proportion of Denisovan genetics is among Indigenous Australians and New Guinea natives, with nearby islands not far behind.

“The levels of Denisovan DNA in contemporary populations indicates that significant interbreeding happened in Island Southeast Asia,” Teixeira said in an emailed statement. “The mystery then remains, why haven’t we found their fossils alongside the other ancient humans in the region? “ 

Co-author Professor Kris Helgen of the Australian Museum Research Institute told IFLScience it is very likely Denisovans crossed the Wallace line, where the plants and animals of Island Southeast Asia radically change. When modern humans reached the area they probably encountered several human species, but only bred successfully with the Denisovans. Perhaps the others looked too alien to be attractive mates, or were too biologically different for mating to produce fertile offspring. 

Island Indonesia with the relative proportion of Denisovan genes of native people of each island, with a full red circle marking the places with the highest proportion. Likely migration routes and surviving megafauna species are marked. Teixeira et al./Nature Ecology and Evolution 

Nevertheless, Helgen thinks the presence of humans before us may have served to inoculate the large animals of these islands against human hunting techniques, giving them a chance to adapt against less sophisticated technology. This would explain why species like the Komodo Dragon and Philippines buffalo survived modern human arrival while their Australian counterparts did not. 

Helgen told IFLScience the anatomical differences make it very unlikely the island humans were breakaways from the Denisovan line. Instead, it seems the islands were a meeting place for many branches of humanity. Many lines of evidence suggest Sulawesi is the most likely place to find evidence of such encounters, he added, making the island among the world's most exciting places for paleoanthropology. 

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