The
rise and fall of empires, the march of armies, the flow of trade
routes, the practice of slavery — all these events have led to a mixing
of populations around the world. Such episodes have left a record in the
human genome, but one that has so far been too complex to decipher on a
global scale.
Now,
geneticists applying new statistical approaches have taken a first shot
at both identifying and dating the major population mixture events of
the last 4,000 years, with the goal of providing a new source of
information for historians.
Some
of the hundred or so major mixing events they describe have plausible
historical explanations, while many others remain to be accounted for.
For instance, many populations of the southern Mediterranean and Middle
East have segments of African origin in their genomes that were inserted
at times between A.D. 650 and 1900, according to the geneticists’
calculations. This could reflect the activity of the Arab slave trade,
which originated in the seventh century, and the absorption of slaves
into their host populations.
Genetic Mixing
Researchers have found genetic evidence for hundreds of
examples of the large-scale mixing of human populations in the past
4,000 years.
Lithuanian
Circles show the apparent sources of DNA within the two parent populations, one European and one Asian.
Scottish
Welsh
Lezgin
Kalash
Georgian
Burusho
Palestinian
Yi
Balochi
Pathan
Druse
Bedouin
Makrani
Sindhi
Parents from
two populations
Child
Grand-
child
Modern
population
Indian
Jew
Indian
Children inherit one set of chromosomes from
each parent, and in later generations that DNA is cut into smaller and
smaller chunks. By measuring the average size of the chunks, researchers
are able to estimate how many generations have passed since the
ancestral populations were mixed.
The Kalash people of Pakistan were found to
have chunks of DNA from an ancient European population. Statistical
analysis suggests a mixing event before 210 B.C., possibly from the army
of Alexander the Great.
The
lowest amount of African admixture occurs in the Druse, a religious
group of the Middle East that prohibited slavery and has been closed to
converts since A.D. 1043.
Another
mixing event is the injection of European-type DNA into the Kalash, a
people of Pakistan, at some time between 990 and 210 B.C. This could
reflect the invasion of India by Alexander the Great in 326 B.C. The
Kalash claim to be descended from Alexander’s soldiers, as do several
other groups in the region.
The genetic atlas of human mixing events was published on Thursday in the journal Science
by a team led by Simon Myers of Oxford University, Garrett Hellenthal
of University College London and Daniel Falush of the Max Planck
Institute for Evolutionary Anthropology in Leipzig, Germany. Having
sampled genomes from around the world, they found they could detect
about 95 distinguishable populations.
Though
all humans have the same set of genes, their genomes are studded with
mutations, which are differences in the sequence of DNA units in the
genome. These mutations occur in patterns because whole sets of
mutations are passed down from parent to child and hence will be common
in a particular population. Based on these patterns, geneticists can
scan a person’s genome and assign the ancestry of each segment to a
particular race or population.
The
team led by Dr. Myers has developed a statistical technique for
identifying the chromosomal segments with particular precision. This
enables them to perform a second feat, that of assigning a date to the
one or more mixing events that have affected a population.
The
dating system is based on measuring the length of chromosome segments
of a particular ancestry that occur in a population. When people of two
different populations intermarry, their children’s genomes carry large
chunks of DNA of one parent’s ancestry interspersed with large chunks
from the other’s.
In
each successive generation, the average size of the chunks becomes
smaller because when DNA is swapped between the parents’ genomes in
making the eggs or sperm, the cuts needed to generate the swapped
sections are made in different places. Therefore, from the average size
of the chunks in a person’s genome, the geneticists can calculate the
number of generations since the mixing event.
“We
are among the first to try to date ancestry events, and we have more
ability to determine the source populations,” Dr. Myers said.
One
of the most widespread events his group has detected is the injection
of Mongol ancestry into populations within the Mongol empire, such as
the Hazara of Afghanistan and the Uighur Turks of Central Asia. The
event occurred 22 generations ago, according to genetic dating, which
corresponds to the beginning of the 14th century, fitting well with the
period of the Mongol empire.
In
another example, the European colonization of America is recorded in
the genomes of the Maya and Pima Indians. And Cambodian genomes mark the
fall of the Khmer empire in the form of ancestral DNA from the invading
Tai people.
Dr.
Myers and his colleagues have detected European ancestry that entered
the Tu people of central China between the 11th and 14th centuries;
this, they surmise, could be from traders traveling the Silk Road. They
find among Northern Italians an insertion of Middle Eastern DNA that
occurred between 776 B.C. and A.D. 550, and may represent the Etruscans,
a mysterious people said by the ancient Greek historian Herodotus to
have emigrated from Lydia in Turkey.
The Myers group has posted its results on a web page
that records the degree of admixture in each population. The English,
however, known to be a rich medley of Celts with invaders such as the
Angles, Saxons, Jutes, Danes and Norwegians, carry the notation “No
strong evidence of admixture.” Dr. Myers said his method cannot yet
detect genetic mixing between very similar populations, as was the case
with the English and their invaders from Scandinavia and Northern
Germany. He said he hoped to distinguish all these groups in a separate
project on British ancestry.
Dr. Hellenthal said, “We’re fairly confident that increasing our sample size will help us follow local migrations.”
John
Novembre, a geneticist at the University of Chicago, described the new
genetic atlas as a “landmark study” because of its scale and the fact
that the authors had been able to extract complex signatures from the
data. “The detailed historical interpretations may need further
questioning and testing,” he said.
Dr.
Myers and Dr. Hellenthal said that they hoped historians would find
their work useful, but that they had not collaborated with historians.
“In
some sense we don’t want to talk to historians,” Dr. Falush said.
“There’s a great virtue in being objective: You put the data in and get
the history out. We do think this is a way of reconstructing history by
just using DNA.”
end quote from:
New York Times -
1 comment:
What astounds me is that here is an actual SCIENTIFIC paper on SCIENTIFIC research and there are no comments! Yet, on hokey "question and answer" sites, where anyone can ask or answer a query, all off the top of one's head, or opinion, or long forgotten/half understood classes (now outdated) the entries are unending....vague, halfway (at best) "answers"...B.S.
to be candid. To me an example of American willingness, indeed PRIDE, in not really knowing facts but relishing B.S. Sort of explains the popularity of FOX "news" too.
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