Researchers from Italy and the UK have sequenced the mitochondrial genome of the Tyrolean Iceman (also dubbed Ötzi), and have found that Europe's most celebrated mummy belongs to a rare or extinct line of humans. The sequence represents the oldest complete DNA sequence of modern humans' mitochondria, according to a report published online in the journal Current Biology. The Tyrolean Iceman lived in central Europe around 5,000 years ago, during the transition between the Neolithic and Copper Ages. His body was somehow freeze-dried, and his perfectly mummified corpse, recovered from an Alpine glacier near the Austro-Italian border in 1991, showed that he had met an untimely end. Ötzi was defrosted in Italy by scientists in 2000, and samples of DNA were removed from his intestine under sterile conditions. Initial studies, performed by researchers in the UK and Germany, focused on the mummy's mitochondrial DNA (mtDNA), which is passed from mother to child. They determined that Ötzi belonged to haplogroup K, a mitochondrial lineage shared by a large part of the western Eurasian population. Further investigation showed his mtDNA to belong to the subhalplogroup K1, which is shared by about 8% of modern Europeans. The K1 subhaplogroup is at present divided into three known clusters. In the new study, led by Dr Franco Rollo of the University of Camerino in Italy, Ötzi's mtDNA was found to belong to none of these clusters. 'Through the analysis of a complete mitochondrial genome in a particularly well-preserved human, we have obtained evidence of a significant genetic difference between present-day Europeans and a representative prehistoric human,' said Dr Rollo. This, he said, was in spite of the fact that the Iceman is 'not so old-just about 5,000 years'. The study used 'pyrosequencing 454', an advanced sequencing technique that has been used to characterise up to one million base pairs of the Neanderthal nuclear genome, to sequence the mummy's entire mitochondrial genome. That information was compared to 115 published complete mtDNA sequences of modern individuals. The Iceman's DNA was found to belong to a novel branch of K1 defined by transitions at nucleotide positions 3513 and 8137. The researchers have informally named the newly discovered branch on the human family tree 'Ötzi's branch', or K1ö. 'This doesn't simply mean that Ötzi had some 'personal' mutations making him different from the others but that, in the past, there was a group - a branch of the phylogenetic tree - of men and women sharing the same mitochondrial DNA,' Dr Rollo said. 'Apparently, this genetic group is no longer present. We don't know whether it is extinct or it has become extremely rare.' According to the study, it is not clear whether the branch is unidentified in modern European populations because of changes in central European settlement patterns since the late Neolithic period, random genetic drift or simply insufficient sampling of modern populations. The historical reasons that are used to explain how individuals are geographically distributed today leave open many questions concerning the genetic history of human populations. The results of the current study attest to the value of using complete ancient mtDNA studies to address these questions. 'Further investigation by means of complete sequencing of modern and ancient central European mtDNA genomes,' the study concludes, 'should allow researchers to pin down the time and place of the origins of the maternal lineage of the Iceman with considerable precision in the future.'
Italy, United Kingdom