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September 2011, Week 2

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Fri, 9 Sep 2011 23:34:43 -0400
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Human Ancestors Interbred With Related Species

     Analysis suggests genetic mixing occurred in Africa
     around 35,000 years ago.

Matt Kaplan
Published online 5 September 2011 |
Nature | doi:10.1038/news.2011.518
http://www.nature.com/news/2011/110905/full/news.2011.518.html

Our ancestors bred with other species in the Homo genus,
according to a study published today in the Proceedings
of the National Academy of Sciences[1]. The authors say
that up to 2% of the genomes of some modern African
populations may originally come from a closely related
species.

Palaeontologists have long wondered whether modern
humans came from a single, genetically isolated
population of hominins or whether we are a genetic mix
of various hominin species.

Last year, an analysis comparing the Neanderthal genome
sequence to that of modern H. sapiens showed that some
interbreeding did take place between the two species in
Europe some time between 80 and 30,000 years ago and
that, to a certain extent, Neanderthals 'live on' in the
genes of modern humans[2].

It has been a mystery whether similar genetic mixing
took place among Homo species even earlier, before the
populations that became modern humans left Africa.

To find out, evolutionary biologist Michael Hammer at
the University of Arizona in Tucson and his colleagues
studied DNA from two African hunter-gatherer groups, the
Biaka Pygmies and the San, as well as from a West
African agricultural population known as the Mandenka.

Each of these groups is descended from populations that
are thought to have remained in Africa, meaning they
would have avoided the genetic bottleneck effect that
usually occurs with migration. This means the groups
show particularly high genetic diversity, which makes
their genomes more likely to have retained evidence of
ancient genetic mixing.

To find signs of infiltration from other Homo species,
the researchers looked at 61 non-coding DNA regions in
all three groups. Because direct comparison to archaic
specimens wasn't possible, the authors used computer
models to simulate how infiltration from different
populations might have affected patterns of variation
within modern genomes.

Then they looked for such patterns of variation in the
DNA of the three African populations. On chromosomes 4,
13 and 18, the researchers found genetic regions that
were more divergent on average than known modern
sequences at the same locations, hinting at a different
origin.

Mixing things up

Hammer and his colleagues argue that roughly 2% of the
genetic material found in these modern African
populations was inserted into the human genome some
35,000 years ago. They say these sequences must have
come from a now-extinct member of the Homo genus that
broke away from the modern human lineage around 700,000
years ago.

Hammer says this disproves the conventional view that we
are descended from a single population that arose in
Africa and replaced all other Homo species without
interbreeding. "We need to modify the standard model of
human origins," he says.

Geneticist Sarah Tishkoff, who studies population
genetics and human evolution at the University of
Pennsylvania in Philadelphia, is more cautious. "This
raises the possibility that there may have been ancient
admixture with archaic populations," she says.

But some researchers will require yet more convincing.
"The authors model differences in very small parameters,
such as the difference between no admixture and 1-2%
admixture with an archaic population," says
anthropologist Brenna Henn, a graduate student at
Stanford University in Palo Alto, California. "The
ability to discern complex models of demographic history
with such a small data set, when many of the basic
features of African genomes and history remain unknown,
concerns me."

Tishkoff would also like to see further work. "Analyses
of whole genome sequences of these populations will be
necessary to more definitively test this hypothesis,"
she says.

References

Hammer, M. F. et al. Proc. Natl Acad. Sci. USA
http://dx.doi.org/10.1073/pnas.1109300108 (2011).

Green, R. E. et al. Science 328, 710-722 (2010).

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