Humans were rare, once
Five years ago, a young girl's tiny finger and a tooth set in place the sometimes-fevered search for how a new human type could fit in. At the moment, the men from the Denisova cave in the Altai Mountains of southern Siberia are almost totally unknown, despite lots of suggestions about what they might have been. We have however been able to extract a whole genome from the bone, so that comparison with the modern human, the Neanderthal and even the Florean "hobbit" can be carried out.
Australasian people have traces of Denisovan hybridisation in their genes while it is puzzling how Asians lack these links. Professors Alan Cooper of the University of Adelaide and Chris Stringer of the London Natural History Museum have simply suggested that a marine voyage of some kind took the Denisovans east, at least 30,000 years ago, while it seems they may have died out in Asia. Wallace's Line consists of the strong Borneo current that acts as a barrier that has kept many placental mammal species such as out of Australasia and restricted many others to our smallest continent. These humans seem to have crossed it.
The evidence we have, though very limited, seems to indicate that female modern humans and male Denisovans interbred in order to leave the traces of this previously unsuspected human group in the genes. It is suggested that the numbers of modern humans reaching the Australasian area were very small. Isolated females could easily have found themselves able to breed with perhaps a more numerous Denisovan population.
Intrepid seamen or lucky colonists, isolated children in a mountain cave or cultured groups of people with great skills of tool-making and hunting? How difficult is that to guess the lifestyle of people who were our relatives in the late Pleistocene? The Neanderthals are certainly a sister group with these Denisovan cave residents. Modern humans diverted earlier from this line and we know they interbred with Denisovan and European Neanderthal relatives. The human species was therefore not then totally separate, as it is now, with many varieties in existence, just as we have many artificial breeds of our domestic animals (but very different in that these were natural variations.)
The differences between Neanderthals and Denisovans are only visible in that lone tooth, which resembles the earliest Homo species. We need to check that hominid remains previously found were not misclassified, hopefully to find more Denisovan bones. This lone tooth is able to separate the two sister groups distinctively, however, perhaps because of their diet. The key interest lies in just how many other "humans" are lying around out there, ready to confuse yet more scientists with their pulled teeth or their pointing fingers?
Alan Cooper and Chris Stringer wrote this perspective in the journal Science.