Australia: The Land Where Time Began |
||||||||||||||
Ancient Gene Flow Early Modern Humans to Eastern Neanderthals
It has previously been shown that
Neanderthals contributed genetically to modern humans outside Africa
47-65 ka. In this paper Kuhlwilm et
al. present the results of
their study of the genome of a Neanderthal and a
Denisovan from the
Altai Mountains in Siberia, as well as the sequence of chromosome 21 of
2 Neanderthals from Spain and Croatia. The results of the study show
that a population that diverged early from other modern human
populations in Africa made a contribution to the genetics of
Neanderthals living in the Altai Mountains about 100,000 BP. In contrast
to this, the study failed to find such a contribution to the Denisovan
or the 2 European Neanderthals. Kuhlwilm et
al. concluded that as well as
later interbreeding events, ancestral Neanderthals from the Altai
Mountains and early modern humans met and interbred, possibly in the
Near East, many thousands of years earlier than had previously been
believed.
It is suggested by the fossil
evidence Neanderthals diverged from modern humans at least 430,000 BP
(Arsuaga et al., 2014), though analysis of a Neanderthal genome from a
fossil recovered from a cave in the Altai Mountains, Siberia, suggests
the time of diversion was between 765,000 and 550,000 BP (Prüfer et al.,
2014). When a Denisovan genome was recovered from a fossil in the same
cave in the Altai Mountains analysis suggested that Neanderthals and
Denisovans diverged from each other between 481,000 and 473,000 BP
(Prüfer et al., 2014). Following this divergence admixture among archaic
and modern human populations, which included gene flow from Neanderthals
into modern humans outside Africa (Prüfer et al., 2014; Green et al.,
2010; Fu et al., 2014; Fu et al., 2015), Denisovan gene flow into
ancestral modern humans of the present in Oceania and mainland Asia
(Reich et al., 2010; Meyer et al., 2012), gene flow into the Denisovans
from Neanderthals, and possibly an archaic group that has yet to be
identified, which had diverged from the other lineages more than 1
million BP (Prüfer et al., 2014). What has remained elusive is gene flow
from modern humans into Neanderthals or Denisovans.
Archaic genomes divergence and heterozygosity
The Neanderthal genome from the
Altai Mountains shares more than 5.4 % derived alleles with Africans of
the present than does the Denisovan genome.
For derived alleles found at >0.9
% frequency in Africans, this excess is particularly pronounced. These
observations have been interpreted as evidence that gene flow from an
unknown archaic hominin, that was more deeply diverged, into the
Denisovan lineage (Prüfer et al., 2014). In this study Kuhlwilm et
al. examined whether the gene
flow from modern humans into the ancestral Altai Neanderthal may have
also taken place.
Kuhlwilm et
al. examined the divergence of
these archaic genomes to 504 African genomes (The 1000 Genome Project
Consortium, 2015) in 15,881 sequence windows of 100 kb (supplementary
information section 9), noting that regions in the Denisovans genome
introgressed from a deeply divergent archaic hominin should have
unusually high divergence to Africans of the present, and that in the
Altai Neanderthal genome regions introgressed from modern humans should
have unusually low divergence to them, the use of only derived alleles
at >0.9 frequency in the combined African genomes, archaic alleles that
had been brought by Eurasians to Africa about 3,000 BP Pickrell et al.,
2014; Llorente et al., 2015) were excluded from these windows. Kuhlwilm
et al. calculated their
divergence to Africans using the archaic alleles in each window that
give the minimum number of differences, to allow introgressed segments
from modern humans to be identified more easily, if they exist. Also
noting that in the Denisovan or Altai Neanderthal genomes introgressed
regions should have divergence to the other archaic genome that should
be unusually high, Kuhlwilm et al.
calculated the divergence that existed between the archaic genomes in
the same window by the use of alleles that give the maximum number of
differences.
Windows of the Denisovan genome
that have high divergence to Africans have also a high divergence to the
Altai Neanderthal, whereas in the Altai Neanderthal genome widows that
have high divergence to Africans tend to not have a high divergence to
the Denisovan, which is consistent with gene flow from a hominin that is
deeply diverged into the Denisovan ancestors. On the other hand it was
found that windows of the Altai Neanderthal genome that have low
divergence to Africans have higher divergence to the Denisovan than the
Denisovan windows that have low divergence to Africans. These windows in
the Altai Neanderthal genome have higher heterozygosity than in the
Denisovan genome, and 40.7 % of their heterozygous sites share a derived
allele with Africans, whereas 24.2 % share a derived allele in the
Denisovan. The possibility of gene flow from modern humans into
Neanderthals is raised by these observations. See Source1
Discussion
It is suggested by the integrated
demographic analysis of multiple archaic and present-day human genomes
by Kuhlwilm et al. a scenario
of decline over a long term of the Neanderthal and Denisovan populations
which possibly reflects a long period of isolation in the Altai
Mountains. This study also provides evidence of modern human
introgression into the ancestors of this Neanderthal population, and no
evidence was found of introgression into the European Neanderthals.
According to Kuhlwilm et al.
these modern humans may represent a population that diverged from other
modern humans in Africa and at a later time met the ancestors of the
Altai Neanderthal. Kuhlwilm et al.
suggest that the finding of African haplotypes as young as 100,000
years old in the genome of the Altai Neanderthal is consistent with
interbreeding around that age.
It has been proposed by Hablin
(Hablin, 1998) that Neanderthals expanded to the east from Europe during
an interglacial about 125,000 BP (Mercier et al., 1993; Grün et al.,
2005) (Oxygen Isotope Stage 5e). As early as 120,000 BP the presence of
modern humans at Skhul and Qafzeh and Neanderthals at Tabun in the
Levant provides a place where gene flow from early modern humans to
Neanderthals could possibly have occurred. There is also a place in
Southern Arabia and the area around the Persian Gulf where modern humans
may also have settled early (Armitage et al., 2011) and where
Neanderthals may also have been present (Rose & Marks, 2014). The
suggestion that modern humans may have migrated out of Africa early is
supported by the recent demonstration that modern humans may have been
in China as early as 120,000 BP (Liu et al., 20156). This suggests early
modern humans may have had the opportunity to admix with archaic
hominins prior to the migration of ancestral of modern non-African
humans if the present.
|
|
|||||||||||||
|
||||||||||||||
Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |