May 12, 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 say.
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 further inland.
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.Copyright 2005 The New York Times Company