Monday, September 25, 2006

DNA studies and Eve

DNA Study Yields Clues on First Migration of Early Humans

NY Times, May 13, 2005

By studying the DNA of an ancient people in Malaysia, a team of geneticists
says it has illuminated many aspects of how modern humans migrated from

The geneticists say there was only one migration of modern humans out of
Africa; that it took a southern route to India, Southeast Asia and
Australia; and that it consisted of a single band of hunter-gatherers,
probably just a few hundred people strong.

Because these events occurred in the last Ice Age, when Europe was at first
too cold for human habitation, the researchers say, it was populated only
later, not directly from Africa but as an offshoot of the southern
migration. The people of this offshoot would presumably have trekked back
through the lands that are now India and Iran to reach the Near East and

The findings depend on analysis of mitochondrial DNA, a type of genetic
material inherited solely through the female line. They are reported today
in Science by a team of geneticists led by Dr. 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 woman, the mitochondrial Eve, who lived some 200,000 years

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 occurred 65,000 years ago, pushed along the coasts of India and
Southeast Asia and reached Australia by 50,000 years ago, the date of the
earliest known archaeological site there.

The Malaysian people whom the geneticists studied are the Orang Asli. The
term means "original men" in Malay.

They are probably descended from this first migration, because they have
several ancient mitochondrial DNA lineages that are found nowhere else.

These lineages are 42,000 to 63,000 years old, the geneticists say.
Subgroups of the Orang Asli, like the Semang, have probably been able to
remain intact because they adapted to the harsh existence of living in
forests, said Dr. Stephen Oppenheimer, the member of the geneticists' team
who collected blood samples in Malaysia.

Some archaeologists theorize that Europe was colonized by a second migration
that traveled north out of Africa. This fits with the earliest known modern
human sites, dating from 45,000 years ago in the Levant and 40,000 years ago
in Europe.

Dr. Macaulay's team says there could have been just 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 just one group succeeded in leaving
Africa. One possibility is that because the migration occurred by continuous
population expansion, leaving people in place at each site, the first
emigrants may have blocked others from leaving. Another is that the terrain
was so difficult for hunter-gatherers, who 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 about its a timing. Archaeologists tend to
view the genetic data as providing invaluable information about the
interrelationship between groups, 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, said Dr. Richard Klein, an archaeologist at Stanford. 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 that in the
archaeological record in Africa, Dr. Klein said. Yet signs of modern human
behavior do not appear in Africa until 50,000 years ago, the transition
between the Middle and Later Stone Ages, 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, because the sea level has risen more
than 200 feet since the last Ice Age. Dr. Klein expressed reservations about
that argument, noting that people would not wait for the slowly rising sea
levels to overwhelm them but would build new sites farther inland.

Dr. Macaulay said genetic dates had improved in recent years, now that it is
affordable to decode the whole ring of mitochondrial DNA, and not just a
small segment.

But he said he agreed "that archaeological dates are much firmer than the
genetic ones" and that it was possible his 65,000-year date for the African
exodus was too old.

Dr. Macaulay's team has been able to estimate the size of the population in
Africa from which the founders descended. The calculation indicates a
maximum of 550 women. 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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