This news completely took the world by surprise. They've identified a strand of humanity that is neither on the Neanderthal or the Cromagnon-modern man line.
The paper that describes the finding comes courtesy of the Max Planck Institute's Svante Pääbo, who has been actively pursuing the sequencing of the Neanderthal genome. It seems likely that this particular bone fragment was targeted due to suspicions that it might also provide an additional Neanderthal sequence. The site, called Denisova, is in the Altai Mountains of southern Siberia, an area that has had hominins present as early as 125,000 years ago. The sample itself came from a layer of material that dates from between 30,000 and 50,000 years ago. Neanderthal DNA was found in a sample from the same time period less than 100km away, while artifacts indicate that modern humans were also present in the region by 40,000 years ago.
So, there was no apparent reason to suspect that the bone would yield anything more than a familiar sequence. And in fact, most of the first half of the paper simply describes the methods used to construct a complete sequence of the mitochondrial DNA, including over 150-fold coverage of the genome, and an alignment program designed to account for the errors typical of ancient DNA sequences. About the only surprise here is that Pääbo's group has switched from using 454 sequencing machines to those made by Illumina.
Various checks indicate that the sequence the authors obtained contains damage that's typical of ancient DNA, and that it all comes from a single individual. So far, quite typical.
Things got quite unusual when they attempted to align the sequence to the mitochondrial DNA from the hominin species that were likely to be present at that time and place: human and Neanderthal. Instead of clustering with one or the other, the Denisova mitochondrial genome was a clear outlier, having about 385 differences with the typical human mitochondrial genome. In contrast, Neanderthals only differ from modern humans by an average of 202. The obvious interpretation is that the Denisova lineage split off before modern humans and Neanderthals did. If we accept that Neanderthals are a distinct species of hominin (and we do), then this sample clearly represents yet another one.
Building a tree with the chimpanzee genomes and assuming a divergence time of 6 million years, the data indicates that the Denisova lineage diverged about a million years ago. At that point, Homo Erectus was already a global species, but our human ancestors were still in Africa. That suggests that the Denisova variant probably originated in, or at least near, Africa as well, although there's no way to tell whether it was a distinct species before it started migrating, or whether it became an isolated population because of a migration.
And so the article goes. The most interesting thing about this population is how recently they were living. Keep in mind, 40,000 years ago places it about the time Homo Sapiens is moving into Australia. The Neanderthals don't disappear until 24,000 years ago. The hotly disputed Florensis hobbit population existed between 94,000 years ago through to about 12,000. What we seem to be getting a picture of is a variety of Hominin species cohabiting this planet. The conjecture would lead to the distinct possibility that our ancestors eradicated these populations to become the dominant ape.
Here's another link which suggests that the 'X-woman' aka the Denisova Hominin split off the common line of humanity about 1.04 million years ago.
Professor Stringer commented: "Another intriguing question is whether there might have been overlap and interaction between not only Neanderthals and early moderns in Asia, but also, now, between either of those lineages and this newly-recognised one.
"The distinctiveness of the mitochondrial DNA patterns so far suggests that there was little or no interbreeding, but more extensive data will be needed from other parts of the genome, or from the fossils, for definitive conclusions to be reached."
Experts have been wondering whether X-woman might have links with known fossil humans from Asia, which have controversial classifications.
"Certain enigmatic Asian fossils dated between 250,000-650,000 years ago such as Narmada (in India), and Yunxian, Dali and Jinniushan (in China) have been considered as possible Asian derivatives of Homo heidelbergensis, so they are also potential candidates for this mystery non-erectus lineage," said Prof Stringer.
"However, there are other and younger fragmentary fossils such as the Denisova ones themselves, and partial skulls from Salkhit in Mongolia and Maba in China, which have been difficult to classify, and perhaps they do signal a greater complexity than we have appreciated up to now."
All the same, the world of ancient humans got a whole lot more complicated as a result of this discovery.
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