[Paleopsych] NYT: Geneticists Link Modern Humans to Single Band Out of Africa
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Fri May 13 15:50:10 UTC 2005
Geneticists Link Modern Humans to Single Band Out of Africa
By NICHOLAS WADE
A team of geneticists believe they have shed light on many aspects of
how modern humans emigrated from Africa by analyzing the DNA of the
Orang Asli, the original inhabitants of Malaysia. Because the Orang
Asli appear to be directly descended from the first emigrants from
Africa, they have provided valuable new clues about that momentous
event in early human history.
The geneticists conclude that there was only one migration of modern
humans out of Africa - that it took a southern route to India,
Southeast Asia and Australasia, and consisted of a single band of
hunter-gatherers, probably just a few hundred people strong.
A further inference is that because these events took place during the
last Ice Age, Europe was at first too cold for human habitation and
was populated only later - not directly from Africa but as an offshoot
of the southern migration which trekked back through the lands that
are now India and Iran to reach the Near East and Europe.
The findings depend on analysis of mitochondrial DNA, a type of
genetic material inherited only through the female line. They are
reported in today's issue of Science by a team of geneticists led by
Vincent Macaulay of the University of Glasgow.
Everyone in the world can be placed on a single family tree, in terms
of their mitochondrial DNA, because everyone has inherited that piece
of DNA from a single female, the mitochondrial Eve, who lived some
200,000 years ago. There were, of course, many other women in that
ancient population, but over the generations one mitochondrial DNA
replaced all the others through the process known as genetic drift.
With the help of mutations that have built up on the one surviving
copy, geneticists can arrange people in lineages and estimate the time
of origin of each lineage.
With this approach, Dr. Macaulay's team calculates that the emigration
from Africa took place about 65,000 years ago, pushed along the
coastlines of India and Southeast Asia, and reached Australia by
50,000 years ago, the date of the earliest known archaeological site.
The Orang Asli - meaning "original men" in Malay - are probably one of
the surviving populations descended from this first migration, since
they have several ancient mitochondrial DNA lineages that are found
nowhere else. These lineages are between 42,000 and 63,000 years old,
the geneticists say.
Groups of Orang Asli like the Semang have probably been able to remain
intact because they are adapted to the harsh life of living in
forests, said Dr. Stephen Oppenheimer, the member of the geneticists'
team who collected blood samples in Malaysia.
Some archaeologists believe that Europe was colonized by a second
migration, which traveled north out of Africa. This fits with the
earliest known modern human sites - which date to 45,000 years ago in
the Levant and 40,000 years ago in Europe.
But Dr. Macaulay's team says there could only have been one migration,
not two, because the mitochondrial lineages of everyone outside Africa
converge at the same time to the same common ancestors. Therefore,
people from the southern migration, probably in India, must have
struck inland to reach the Levant, and later Europe, the geneticists
Dr. Macaulay said it was not clear why only one group had succeeded in
leaving Africa. One possibility is that since the migration occurred
by one population budding into another, leaving people in place at
each site, the first emigrants may have blocked others from leaving.
Another possibility is that the terrain was so difficult for
hunter-gatherers, who must carry all their belongings with them, that
only one group succeeded in the exodus.
Although there is general, but not complete, agreement that modern
humans emigrated from Africa in recent times, there is still a
difference between geneticists and archaeologists as to the timing of
this event. Archaeologists tend to view the genetic data as providing
invaluable information about the interrelationship between groups of
people, but they place less confidence in the dates derived from
genetic family trees.
There is no evidence of modern humans outside Africa earlier than
50,000 years ago, says Dr. Richard Klein, an archaeologist at Stanford
University. Also, if something happened 65,000 years ago to allow
people to leave Africa, as Dr. Macaulay's team suggests, there should
surely be some record of this event in the archaeological record
within Africa, Dr. Klein said. Yet signs of modern human behavior do
not appear in Africa until the transition between the Middle and Later
Stone Age, 50,000 years ago, he said.
"If they want to push such an idea, find me a 65,000-year-old site
with evidence of human occupation outside of Africa," Dr. Klein said.
Geneticists counter that many of the coastline sites occupied by the
first emigrants would now lie under water, since sea level has risen
more than 200 feet since the last Ice Age. Dr. Klein expressed
reservations about this argument, noting that rather than waiting for
the rising sea levels to overwhelm them, people would build new sites
Dr. Macaulay said that genetic dates have improved in recent years now
that it is affordable to decode the whole ring of mitochondrial DNA,
not just a small segment as before. But he said he agreed "that
archaeological dates are much firmer than the genetic ones" and that
it is possible his 65,000-year date for the African exodus is too old.
Dr. Macaulay's team has been able to estimate the size of the
population in Africa from which the founders are descended. The
calculation indicates a maximum of 550 women, but the true size may
have been considerably less. This points to a single group of
hunter-gatherers, perhaps a couple of hundred strong, as the ancestors
of all humans outside of Africa, Dr. Macaulay said.
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