DNA study sheds light on Aborigines' heritage
When modern humans left Africa as far back as 70,000 years ago, they
dispersed across the world, reaching Australia 50,000 to 40,000 years
ago. From then until the 18th century arrival of European colonists,
aboriginal Australians did not mix their DNA with anyone else in the
world - or so many scientists believed.
Now, a new study has turned up evidence of much more recent
interbreeding between native Australians and people who came from India.
The findings, based on a detailed examination of the DNA of
aboriginal Australians and hundreds of people of other pedigrees, found
that mixing occurred as recently as 4,200 years ago.
Reported in the journal Proceedings of the National Academy of
Sciences, the results dovetail with interesting archaeological and
fossil changes, said study leader Mark Stoneking, a molecular
anthropologist at the Max Planck Institute for Evolutionary Anthropology
in Leipzig, Germany.
Right around that time, new kinds of stone tools called microliths
appeared in Australia, finer than earlier tools discovered there, but
similar to tools already in use elsewhere in the world.
Also at this point in time, Australia's wild dog, the dingo, shows up
for the first time in the fossil records. Scientists know that the dingo
is not native to the Australian continent, where all indigenous mammals
are marsupials that bear immature young and often carry them in pouches.
The dingo, in contrast, is a placental mammal and a subspecies of the
gray wolf, like the domestic dog.
"We don't know for sure that these events are connected, but the fact
that all of these occur at the same time suggests that they may be,"
Stoneking said.
To reach their conclusions, Stoneking's team conducted a detailed
scan of the genomes of 344 people, including Aborigines from the
country's Northern Territories as well as people from Papua New Guinea,
Southeast Asia, India, China and those of Western and Northern European
ancestry.
The scientists looked for places where the DNA code sometimes
differed by a single DNA building block, or nucleotide, between members
of their sample.
Genetically related
By noting to what extent individuals shared the roughly one million
tiny variations that were found, the team could piece together trees
that showed how genetically related each group of people was to the
others and estimate how long ago the groups had become distinct. They
found, for example, that aboriginal Australians, Papua New Guinea
highlanders and the Mamanwa people from the Philippines were genetically
closest to each other and diverged about 36,000 years ago. This fit well
with earlier genetic studies.
However, the team was surprised to find - using four separate
statistical methods - that a much more recent genetic mixing with people
from India had occurred. They estimated that about 11 percent of the DNA
of aboriginal Australians is derived from this event.
Earlier studies had hinted at this, but they were limited to smaller
regions of the genome: the Y chromosome, which is only carried by males,
and a type of DNA called mitochondrial DNA that is passed down only from
mothers to their children, Stoneking said.
The authors of the new study also estimated how far back this genetic
mixing had occurred, via the following reasoning: A child born of an
Aborigine and an Indian would carry in his or her genome an entire,
unbroken stretch of each chromosome, one from each parent.
However, with each generation, those two chromosomes swap bits and
pieces with each other. Down the generations, therefore, the shorter the
stretches pure-Indian or pure-Australian chromosome will become.
Computer simulations
Using the size and number of DNA stretches in people alive today, the
team ran computer simulations to calculate that 141 generations have
passed since the initial interbreeding. If each generation is assumed to
be 30 years, that adds up to 4,230 years.
It isn't clear how the mixing took place. Though it might make sense
that the gene flow occurred in Indonesia, no traces of Indian DNA could
be found among the Indonesians in the sample and no Indonesian DNA could
be found in the Australians, the authors said - perhaps suggesting the
migrants came directly to Australia by water.
Still, a more detailed analysis of Indonesian genomes would be needed
to rule out that connection, Stoneking said.Scientists can now pinpoint
people's movements across the globe and the heady brew of ancestry that
makes them who they are with a precision that would have been
unimaginable not long ago, said Stanford University geneticist Michael
Snyder, who wasn't involved in the new study.
- Los Angeles Times
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