DNA of ancient Siberian human uncovered

25 March 2010

The DNA of an ancient human from Siberia has been revealed, giving clues to early human evolution, scientists report in the journal Nature today.

An international team, including scientists from the Max Planck Institute in Germany, obtained the complete mitochondrial DNA (mtDNA) sequence from a finger bone uncovered from the Denisova Cave in the Altai Mountains in southern Siberia (mitochondria are the tiny power structures in each human cell that contain their own DNA, inherited through the mother).

The ancient human species represented by this new DNA is unidentified at present. However, its genetic information shows it shared a common ancestor with modern humans and Neanderthals about 1 million years ago.

Professor Chris Stringer, human origins expert at the Natural History Museum, comments on the research. 'The recovery of an ancient mitochondrial DNA lineage in a human fossil from Denisova Cave, Siberia, is a very exciting development.

'I think this DNA work provides an entirely new way of looking at early human evolution in Asia, and this is just the tip of the iceberg in terms of what will emerge.'

Genetic clues
Reconstruction of a female Homo erectus cranium from China

Reconstruction of a female Homo erectus cranium from China, known as Peking Man. The mystery Denisovan human from Siberia may be too young to be a descendant of H. erectus.

Genetic information helps scientists compare the relationships between different species. Until now, apart from modern humans (Homo sapiens), the only other human species for which we have genetic information is the Neanderthals, from work that began in 1997. Their remains have been uncovered in sites across Europe and into Asia. 

Other human species such as Homo erectus, are known from the Far East, and enigmatic fossils have also been discovered from India, China and Mongolia, which are difficult to place into any of the known species.

Who is this ancient human?

This new research suggests that the Denisova human lived between 30-50,000 years ago. Modern humans migrated out of Africa between 55,000-60,000 years ago, but the Denisova human is not a Homo sapiens, judging from its distinctive mtDNA. Its ancestor apparently derived from a much earlier human migration, but which one?

Prof Stringer has his own theories. He says the Denisova human may be too young to be a descendant of the first human migration out of Africa. 'The suggested divergence date of the lineage, at around 1 million years ago, appears too young for a descendant of the first Homo erectus known to have dispersed from Africa about 1.75 million years ago.'

Another human migration involved Homo heidelbergensis. Stringer says, based on these dates, the Denisova human was too old to be a descendant of this species, which is known from about 650,000 years ago, and which subsequently may have given rise to both Neanderthals and modern humans.

However, Stringer says that the species Homo antecessor ('Pioneer Man'), known from sites in Spain, has an age range that closely matches the origin of the mtDNA lineage of the Denisova human, 0.8-1.2 million years ago. But, he adds that 'recent research has suggested that H. antecessor evolved from an early Asian H. erectus, in which case its origins would also be too ancient, compared with the mystery Denisova lineage.'

Accuracy of dates

Stringer highlights that the dates are based on complicated factors and an estimate of 6 million years for the date when humans and chimps split from a common ancestor, so the calculations have a range of several hundred thousand years around the 1-million-year figure.

Stringer says that if the dates were worked out backwards from more recent DNA divergences, the Denisova common ancestor might become somewhat younger, meaning it could perhaps be an early off-shoot of H. heidelbergensis after all. Stringer says there are some Asian fossils already considered as possible Asian derivatives of H heidelbergensis, so these are potential candidates for the mystery non-erectus lineage.

Given that there are various Asian fossils that have been difficult to classify, as well as the Denisova fragments, the Asian evolutionary story may be more complex than previously thought.

Overlap with modern humans?

The new research not only suggests that the Denisova human lived close in time with Neanderthals and modern humans, at around 30-50,000 years ago, but also may have overlapped in location

Stringer says this is an intriguing question but adds, 'The distinctiveness of the mtDNA 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.'

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