Last week the discovery of a lower jaw bone of a Denisovan hit the headlines all over the world.
Denisovans are a now-extinct species of humans that were closely related to us as well as to Neanderthals. To an outside observer, the excitement of researchers about this find might have appeared a bit over the top. After all, Denisovans have been discovered nearly 10 years ago, their genome has been sequenced and their relationship to our own species analysed. So, what is all this fuss about?
The relatively large amount of recent, high-profile research on Denisovans means that they have become a rather well-known entity in our family tree, if not a household name like Neanderthals. This familiarity hides one crucial fact that distinguishes Denisovans from all our other relatives and ancestors: They are the only hominin (humans and their close relatives) that has ever been described based on a DNA sequence rather than a morphologically-distinctive fossil. That DNA sequence came from a tiny fragment of a finger bone, found in Denisova cave in the Altai mountains, which was indistinguishable from that of modern humans. So, until now, we knew how Denisovan DNA looked, but did not have a clue how Denisovans themselves looked (other than that they had rather big teeth). Up until now, the Denisovans were therefore a set of faceless DNA sequences in a database.
That is a rather big issue if you want to find further Denisovan remains. No matter how well preserved and how informative a fossil looks, as long as it is not well enough preserved to yield DNA data, there is no way of telling whether it belongs to a Denisovan or not. Or, to be more precise, there was no way of telling.
The newly-published study changed this in two ways. First of all, it presents an actual part of a Denisovan’s face. Thus, for the first time, Denisovans have become more than an abstract series of letters representing a DNA sequence. Second, the study introduces another means of identifying Denisovans: Amino acid sequences of proteins rather than DNA sequences. At first that may seem surprising, seeing as our body assembles amino acid sequences by reading and translating a DNA template. They are simply the second step in the production line that turns a DNA sequence into a human being. DNA and amino acids are therefore closely linked. However, it appears that amino acid sequences of proteins are longer lived than the template they are built from. This means fossils too poorly preserved for DNA survival can now be identified as Denisovans from characteristic amino acid sequences.
Physical evidence of the presence of a species is essential if we want to understand its distribution, as the distribution of genes alone is not a proxy for where a species lived. For example, native North Americans have the same percentage of Neanderthal genes in their genomes as Europeans and Asians. Yet we can be fairly confident that Neanderthals never reached the New World and that the Neanderthal genes have reached the New World with modern humans. Similarly, the significant amount of Denisovan ancestry in Melanesian and Australian populations does by no means indicate that Denisovans crossed oceans to reach New Guinea or Australia. It is more likely that the ancestors of Melanesians and Australians carried the Denisovan genes to these regions after encountering them somewhere in Asia on their journey out of Africa.
The new study presents the first physical evidence of Denisovan presence outside of Denisova cave and it is particularly interesting that this evidence (the jaw bone) comes from the Tibetan plateau. Genetic research has shown that modern human populations living at high attitudes on the Tibetan plateau have inherited some of their high-altitude adaptations from Denisovans. But does that mean that Denisovans lived on the Tibetan plateau, or did the modern humans settling the area carry the advantageous mutations with them from wherever they might have encountered Denisovans? And how did Denisovans develop high altitude adaptations in the first place? Denisova cave itself is at a moderate altitude of only about 700 meters above sea level. The discovery of Denisovan remains on the Tibetan plateau clearly suggests that Denisovans lived at high altitudes long before modern humans, that they adapted to this environment and that Tibetan modern human populations have acquired the Denisovan’s high altitude adaptations as they moved into the alpine environment.
So, the fossil jaw bone described in the journal Nature a bit more than a week ago was really quite an important piece of bone. It may actually hold the key to understanding a lot more about Denisovan distribution and critically, about their role in our own evolution.
Dr Michael Knapp is a Rutherford Discovery Fellow and senior lecturer in biological anthropology at the University of Otago.