Neanderthal DNA specialist Svante Pääbo examines the anatomically modern human femur, found near Ust'-Ishim in western Siberia.
Photograph: Bence Viola/MPI EVA
A little more light was shed on the fascinating human story the other day when an international team from America, Asia and Europe led by Svante Pääbo and Janet Kelso of the Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany, published their findings from the DNA sequenced from the oldest human fossil so far. And the picture is emerging of not only several periods of interbreeding between fully modern humans and Neanderthals but that there may have been several waves of modern human migration out of Africa and into Euro-Asia, not all of which were successful in the long run.
The fossil is the anatomically modern human femur found at Ust'-Ishim in Siberia and dated to about 45,000 years old. This fossil was found quite by chance in 2008, sticking out of the bank of the Irtysh river by Nikolai Peristov, a local artist and ivory carver, looking for fossilised mammoth tusks to make jewelry from. Nothing more is known of the remains which appear to have been deposited in the alluvial silt produced by flooding of the Irtysh river.
We present the high-quality genome sequence of a ~45,000-year-old modern human male from Siberia. This individual derives from a population that lived before—or simultaneously with—the separation of the populations in western and eastern Eurasia and carries a similar amount of Neanderthal ancestry as present-day Eurasians. However, the genomic segments of Neanderthal ancestry are substantially longer than those observed in present-day individuals, indicating that Neanderthal gene flow into the ancestors of this individual occurred 7,000–13,000 years before he lived. We estimate an autosomal mutation rate of 0.4 × 10−9 to 0.6 × 10−9 per site per year, a Y chromosomal mutation rate of 0.7 × 10−9 to 0.9 × 10−9 per site per year based on the additional substitutions that have occurred in present-day non-Africans compared to this genome, and a mitochondrial mutation rate of 1.8 × 10−8 to 3.2 × 10−8 per site per year based on the age of the bone.
|Irtysh River, Siberia, Russia.|
But they also found longer sequences than expected, probably from later interbreeding, indicating that interbreeding might have been more frequent especially during periods when there was significant overlap between the two populations.
The other significant finding to emerge from this study is that this fossil was from a population which was as closely related to Europeans as it was to Southeast Asians, but it doesn't appear to be directly ancestral to either.
This suggests this particular population was not successful in the long run and died out, raising the possibility that human colonization of Euro-Asia may he been a hit and miss affair with some populations failing. This view is consistent with the view that the 100,000 year-old remains of early modern human found in a cave in what is now Israel may not have been ancestral to the people of New Guinea and Australia as some had proposed. The fact that they have similar amounts of Neanderthal DNA to other non-African people suggests they picked up their Neanderthal DNA like the rest of us during a migration through Neanderthal territory. The population that left their bones in a cave in the Middle-East might simply have been another failed colonisation from Africa.
And none of this detracts from the view that, at this period in our evolutionary history, the various diversified forms of humans, some of which may have progressed to the status of a distinct biological species such as Neanderthals and Denisovans, acted like a ring-species much as you would expect of an evolving, widely-dispersed species, consisting of incompletely isolated population, each interbreeding with it's neighbours on the edges of its range. At a later stage in our history, these local populations then merged to form a single gene-pool but with distinct local characteristics.
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