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a new hominin from siberia?

The latest edition of Nature carries an item that raises the possibility of another new - & recent - new hominin species, this time from Siberia (Krasuse et al., 2010). A few years ago, when the story about Homo floresiensis first broke, I remember commenting to my classes that it was probably only a matter of time until another recent relative popped up. After all, all the evidence to date shows that our family tree is much bushier than scientists used to think - when I was in high school that tree was presented as essentially linear in nature. But Siberia?

My mother used to say (when asked by importunate students how old she was), 'I'm as old as my little finger & older than my teeth.' In the case of this possible new hominin, that would make her very old indeed. The new find consists of a finger bone - the tip of a pinky, in fact - and the mitochondrial DNA (mDNA) extracted from it. (For 'pinky' read 'distal manual phalanx of the fifth digit'...)

The bone was found in Denisova Cave, which is in the Altai mountains of Russia & which has been occupied (on & off) by hominins for around 125,000 years. The finger bone wsa found in a layer of sediment dated at 48-30,000 years ago, & which has also yielded a range of other artefacts. Krause & his team decided to see if they could extract & sequence mtDNA from the bone; they felt this was at least a possibility as the cool conditions in the cave are better for long-term preservation of DNA than the tropics. They expected that - if their extraction was successful - the bone would be from a Neandertal or a modern H.sapiens individual, on the basis of the tool assemblages from the site and the geographic range of both species.

Using 30mg of powdered finger bone (this sounds like something Macbeth's witches would have liked...), the research t3eam were able to extract & sequence mtDNA. They then made a section extract & compared the two sequences: they turned out to be identical. The team hen checked that their sequences came from a single individual (if one bone yielded evidence of more than individual, then there could be questions about contamination). It did.  The degradation patterns of all the mtDNA fragments were also typical of ancient, not modern DNA: further evidence that the sample was not contaminated.

The next step was to compare the mtDNA from the Denisova cave individual with sequences from  modern human mtDNA, a sample from an individual who lived in late-Pleaistcene Russia, Neandertal mtDNA, and sequences from a chimp and a bonobo. There must have been a certain amount of excitement in the lab when the results of this came out - becaue the Denisova hominin's DNA had nearly twice as many differences from modern DNA as that of Neandertals (385 base-pair differences for Denisova/sapiens compared to 202 for the Neandertal/sapiens comparison). 

This suggests that the most recent common mtDNA ancestor for modern humans, Neandertals, & the Denisova individual lived about a million years ago.There's a certain amount of uncertainty around the dates, but nonetheless this is a long time ago. Krause's team comment that  'the divergence of the Denisova mtDNA lineage on the order of one million years shows that it was distinct from the initial radiation of H.erectus that first left Africa 1.9 million yeers ago.'  Remember, though, that we really need DNA (ideally both mitochondrial & nuclear) from more complete skeletal remains before it's possible to 'place' the Denisova hominin with any degree of confidence.

Getting back to my original comment - it's entirely possible that multiple hominin lineages co-existed in this part of the world as recently as 40,000 years ago. If the Denisova individual is confirmed as a new species, then it would have had both Neandertal & anatomically-modern humans as close neighbours in space & time. The apparent temporal overlap of floresiensis and sapiens in Indonesia may not have been an isolated event, and our family tree will then be even bushier than my teachers ever imagined :-) 

Krause, J., Fu, Q., Good, J., Viola, B., Shunkov, M., Derevianko, A., & Pääbo, S. (2010). The complete mitochondrial DNA genome of an unknown hominin from southern Siberia Nature DOI: 10.1038/nature08976

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