[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


    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
    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.

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