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 at the Cold Spring Harbor
Laboratory on Long Island.
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.
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, 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 70,000 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
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.
These
genetic findings are important for us in that they prove the all human beings
on earth are descendents of a single set of parents who lived not very long
ago, in terms of the appearance of other species.