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by Nicholas Wade
(NYT) SCIENCE DESK
November 14, 2000, Tuesday
from
NewAgePointToInfinity Website
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From what had seemed like irreversible
oblivion, archaeologists and population geneticists believe they are
on the verge of retrieving a record of human history stretching back
almost 50,000 years.
The record, built on a synthesis of archaeological and genetic data,
would be a bare bones kind of history without individual names or
deeds. But it could create a chronicle of events, however sketchy,
between the dawn of the human species at least 50,000 years ago and
the beginning of recorded history in 3,500 B.C.
The events would be
the dated migrations of people from one region to another, linked
with the archaeological cultures and perhaps with development of the
world's major language groups.
The new element in this synthesis is the increasing power of
geneticists to look back in time and trace the history of past
populations from analysis of the DNA of people alive today.
''It is astonishing how much archaeology is beginning to learn from
genetics,'' Dr. Colin Renfrew, a leading archaeologist at the
University of Cambridge in England, said at a conference on human
origins held last month at the Cold Spring Harbor Laboratory on Long
Island.
In one of the most detailed genetic reconstructions of population
history so far, Dr. Martin Richards of the University of Huddersfield in England and many colleagues have traced the
remarkably ancient ancestry of the present-day population of Europe.
Some 6 percent of Europeans are descended from the continent's first
founders, who entered Europe from the Near East in the Upper
Paleolithic era 45,000 years ago, Dr. Richards calculates. The
descendants of these earliest arrivals are still more numerous in
certain regions of Europe that may have provided them with refuge
from subsequent waves of immigration.
One is the mountainous
Basque
country, where people still speak a language completely different
from all other European languages. Another is in the European
extreme of Scandinavia.
Another 80 percent arrived 30,000 to 20,000
years ago, before the peak of the last glaciation, and 10 percent
came in the Neolithic 10,000 years ago, when the ice age ended and
agriculture was first introduced to Europe from the Near East.
It used to be thought that the most important human dispersals
occurred in the Neolithic, prompted by the population increases made
possible by the invention of agriculture. But it now seems that the
world filled up early and the first inhabitants were quite resistant
to displacement by later arrivals.
Dr. Richards's estimates, reported in the current issue of
The
American Journal of Human Genetics, are based on analysis of
mitochondrial DNA, a genetic element that occurs in both men and
women but that is transmitted only through the mother; thus, they
reflect only the movement of women.
The movement of men can be followed through analysis of the Y
chromosome, but the Y chromosome is harder to work with and data are
only just now becoming available. In an article in the current issue
of Science, Dr. Peter A. Underhill of Stanford University and
colleagues reported the first analysis of the European population in
terms of the Y chromosome. Although this agrees with the
mitochondrial DNA findings in major outline, suggesting that Europe
was populated mostly in the Paleolithic period with additions in the
Neolithic, there are some points of difference.
The earliest migration into Europe according to mitochondrial DNA
took place from the Near East 45,000 years ago, but Dr. Underhill
and his colleagues said they could see no corresponding migration in
the Y chromosome data.
They have found a very ancient Y chromosome mutation that occurs in
Siberia as well as Europe. They boldly link this mutation with the
bearers of the Aurignacian culture who entered Europe 40,000 years
ago. The culture appears in Siberia at about the same time, as if
these early people had spread both east and west.
Dr. Underhill and his colleagues associate another mutation, which
is common in India, Pakistan and Central Asia as well as Europe,
with the people of the
Kurgan culture who, according to one theory,
expanded from southern Ukraine and spread the Indo-European
languages.
Dr. Underhill's report tries to make the grand synthesis between
archaeological and genetic data, but it will probably be some time
before the specialists in each area agree on how the two types of
data should be associated.
''It is very exciting that the
geneticists now have internal dating procedures, but really I
think the dates are very loose indeed,'' Dr. Renfrew said in an
interview.
Geneticists believe that the world
outside Africa was populated by the migration of a very small number
of people who left east Africa about 50,000 years ago.
These modern
humans, with their more advanced and inventive culture, are thought
to have displaced the archaic hominids like the Neanderthals, which
had emigrated from Africa many thousands of years earlier.
These Paleolithic populations created sophisticated stone tools and
left evidence of their advanced culture in the cave paintings of
southern France, dating to at least 30,000 years ago. Although
anatomically modern humans first appear in Africa about 150,000
years ago, their archaeological remains show little sign of modern
human behavior.
Dr. Richard Klein, an archaeologist at Stanford University, has
suggested that some genetic change, perhaps as profound as the
invention of language, occurred in Africa around 50,000 years ago,
and that it was these behaviorally modern humans who both spread
within Africa and populated the rest of the globe.
This thesis was challenged at the Cold Spring Harbor conference by
two archaeologists, Dr. Sally McBrearty of the University of
Connecticut and Dr. Alison Brooks of George Washington University.
They argued that each of the components said to characterize the
Paleolithic revolution in human behavior, like stone blades, long
distance trade and art, can be found in Africa at earlier dates.
''So all the behaviors of the Upper
Paleolithic have an African pedigree,'' Dr. McBrearty said. The
behaviors were gradually assembled as a package and exported,
''which is why it appears suddenly in Europe 40,000 years ago,''
she said.
Dr. Klein said in an interview that he
doubted some of the early dates proposed by Dr. McBrearty and Dr.
Brooks, and that even if the dates were correct, modern behaviors
conferred such an advantage that they should appear in a broad
pattern, not just at the handful of places cited by his critics.
To
understand what happened in the past, it is necessary to look for
patterns and ignore the ''noise,'' he said.
The synthesis of archaeology with population genetics may provide a
basis into which a third discipline can join, that of historical
linguistics. Most linguists insist that languages change so rapidly
that their roots cannot reliably be traced further back than 5,000
years. Only a few, like Dr. Joseph Greenberg of Stanford, believe
that some elements of language remain constant, enough to
reconstruct all the world's languages into just 14 superfamilies of
a much great antiquity.
The signature of these ancient super-families can be seen in the
geographic distribution of languages, Dr. Renfrew said.
In some
areas of the world, like the Caucasus, New Guinea and South America,
there are many language families packed into a small area, which he
called a mosaic zone. In other areas, a single language family
covers a broad area or spread zone. The Indo-European languages,
which stretch from Europe to India, are one such example.
Another is
Afro-Asiatic, the super-family that includes the languages of
Ethiopia and Somalia and Semitic languages like Arabic and Hebrew.
The spread zones, Dr. Renfrew said, are mostly the result of recent
dispersals caused by agricultural inventions. The mosaic zones ''may
be those of the first humans to occupy those areas, at least in
Australia and America,'' he said.
The language spoken by the ancestral human population may never be
known, though Dr. Greenberg has tried to reconstruct a few words of
it.
But some linguists who study the click languages of southern
Africa feel they are very ancient. This belief is supported by
genetic evidence showing that the
Khoisan peoples, the principal
speakers of click languages, belong to the most ancient of all the
human lineages, based on
mitochondrial DNA.
Dr. Anthony Traill, a click language expert at the University of the
Witwatersrand in Johannesburg, said that linguistically the
languages fell into three separate groups whose relationship, aside
from the clicks, was hard to establish.
The clicks must be ancient,
he said, because,
''the chances of clicks being invented after being
lost is zero.''
The only use of clicks outside of Africa is in an
Australian aboriginal initiation languages in which the clicks are
used as meaningless sounds.
''The idea that clicks were lost
from all languages other than Khoisan,'' Dr. Traill said, ''is
stimulating, but I don't know what to make of it.''
Of the three disciplines that bear on
human origins - historical linguistics, population genetics and
archaeology - only archaeology has a rock-solid method of dating,
based on radiocarbon and other kinds of radioactive decay.
But geneticists are now improving their dating methods, even though
the dates are still very approximate, to the point that they can
begin to correlate their findings with the archaeologists.
The
geneticists' first foray into human prehistory was the famous
''mitochondrial Eve'' article of 1987 by the late Allan Wilson
(Mitochondrial
DNA and Human Evolution),
showing that when people around the world were placed on a family
tree constructed from their mitochondrial DNA, the tree was rooted
in African populations, in an individual who lived about 200,000
years ago.
Though the methodology of the paper was imperfect, its result was
unchanged after the method had been corrected, and geneticists have
developed a growing confidence in mitochondrial DNA dates. The
mitochondrial DNA trees trace back to a single individual, not
because there was only one Eve - the ancestral human population is
thought to have contained about 10,000 people - but because the
lineages of all the other Eves have gone extinct.
The process is
easy to visualize by thinking of an island population with 10
surnames. In each generation, some men will have no children or only
daughters and their surnames will disappear until only one is left;
the Y chromosome and mitochondrial DNA follow the same pattern.
The first major branch points in the mitochondrial Eve tree have
been called the daughters of Eve and they fall in a geographic
pattern with some daughters of Eve being characteristic of Africa,
some of Asia and the Americas and some of Europe and the Near East.
Dr. Richards and his colleagues have analyzed the ancestry of the
present European population by looking within the major daughter of
Eve branches for sub-branches that occur both in Europe and the Near
East, from western Iran through Turkey and Arabia to Egypt, because
the Near East is the probable source of most of the ancestral
populations that entered Europe.
The sub-branches from each region were then dated by counting the
number of mutations that had occurred in the mitochondrial DNA
sequence from the beginning of the sub-branch until today. If the
sub-branch was older in the Near East than Europe, it indicated a
migration into Europe. By this method Dr. Richards's team was able
to date the migrations into Europe.
They also picked up a sizable
back-migration from Europe to the Near East.
The geneticists working on the Y chromosome may eventually be able
to date migrations with similar precision. The major class of
mutation on the Y is so rare that the ticks of the mutation clock
are too many thousands of years apart to be reliably averaged. But a
second kind of mutation occurs more rapidly and the combination of
the two may make a reasonable clock.
Analysis of the Y chromosome has already yielded interesting
results. Dr. Ariella Oppenheim of the Hebrew University in Jerusalem
said she had found considerable similarity between Jews and Israeli
and Palestinian Arabs, as if the Y chromosomes of both groups had
been drawn from a common population that began to expand 7,800 years
ago.
In the middle ages,
the Vikings settled in Greenland but contact
with their colonies was lost at the beginning of the 15th century.
In 1720, by which time the Danes had long become Protestants, there
arose considerable concern that the missing colonists, if they still
existed, would be Roman Catholics and in need of conversion. An
expedition was sent to Greenland but found only ruined houses and
Eskimos.
Did the Vikings perish or intermarry?
An analysis of
Greenlanders' mitochondrial DNA shows only genetic signatures
typical of the New World, and it indicates their unalloyed descent
from Eskimos of Alaska.
''It looks bad for the Vikings,'' said Dr.
Peter Forster of the University of Cambridge, a co-author of the
study.
Dr. Douglas Wallace of Emory University, who pioneered the use of
mitochondrial DNA to analyze human origins, said of the emerging
type of analysis:
''The Y chromosome has a great
future. But it is a very new technology.''
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