By Paul Rincon
Science reporter, BBC News
The finger bone was unearthed in 2008 at Denisova Cave
Scientists have identified a previously unknown type of ancient human
through analysis of DNA from a finger bone unearthed in a Siberian cave.
The extinct "hominin" (humanlike creature) lived in Central Asia between
48,000 and 30,000 years ago.
An international team has sequenced genetic material from the fossil
showing that it is distinct from that of Neanderthals and modern humans.
Details of the find, dubbed "X-woman", have been published in Nature
journal.
Professor Chris Stringer, human origins researcher at London's Natural
History Museum, called the find "a very exciting development".
"This new DNA work provides an entirely new way of looking at the still
poorly-understood evolution of humans in central and eastern Asia."
The discovery raising the intriguing possibility that three forms of
human - Homo sapiens, Neanderthals and the species represented by
X-woman - could have met each other and interacted in southern Siberia.
Origin unknown
The tiny piece of finger bone was uncovered by archaeologists working at
Denisova Cave in Siberia's Altai mountains in 2008. An international
team of researchers extracted mitochondrial DNA from the bone and
compared the genetic code with those from modern humans and Neanderthals.
Mitochondrial DNA comes from the cell's powerhouses and is passed down
the maternal line only. The analysis carried out by Johannes Krause from
the Max Planck Institute for Evolutionary Anthropology in Leipzig,
Germany, and colleagues revealed the human from Denisova last shared a
common ancestor with modern humans and Neanderthals about one million
years ago.
This is known as the divergence date: essentially, when this human's
ancestors split away from the line that eventually led to Neanderthals
and ourselves.
The Neanderthal and modern human evolutionary lines diverged much later,
around 500,000 years ago. This shows that the individual from Denisova
is the representative of a previously unknown human lineage that derives
from a hitherto unrecognised migration out of Africa.
"Whoever carried this mitochondrial genome out of Africa about a million
years ago is some new creature that has not been on our radar screens so
far," said co-author Professor Svante Paabo, also from the Max Planck
Institute for Evolutionary Anthropology.
The divergence date of one million years is too young for the Denisova
hominin to have been a descendent of Homo erectus, which moved out of
Africa into Asia some two million years ago.
And it is too old to be a descendent of Homo heidelbergensis, another
ancient human thought to have originated around 650,000 years ago.
Slice of time
The research contributes to a more complex emerging picture of humankind
during the Late Pleistocene, the period when modern humans left Africa
and started to colonise the rest of the world.
Professor Clive Finlayson, director of the Gibraltar Museum, has
previously argued that "a time slice at a point in the late Pleistocene
would reveal a range of human populations spread across parts of Africa,
Eurasia and Oceania.
"Some would have been genetically linked to each other, behaving as
sub-species, while the more extreme populations may well have behaved as
good species with minimal or no interbreeding."
It was long known that modern humans may have overlapped with
Neanderthals in Europe, apparently for more than 10,000 years. But in
2004, researchers discovered that a dwarf species of human, dubbed "The
Hobbit", was living on the Indonesian island of Flores until 12,000
years ago - long after modern humans had colonised the area.
Neanderthals appear to have been living at Okladnikov Cave in the Altai
mountains some 40,000 years ago. And a team led by Professor Anatoli
Derevianko, from the Russian Academy of Sciences, has also found
evidence of a modern human presence in the region at around the same time.
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, of from the fossils, for
definitive conclusions to be reached."
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