2011-08-24 07:32:38
By Paul Rincon Science editor, BBC News website
A new study deals a blow to the idea that most European men are descended from
farmers who migrated from the Near East 5,000-10,000 years ago.
The findings challenge previous research showing that the genetic signature of
the farmers displaced that of Europe's indigenous hunters.
The latest research leans towards the idea that most of Europe's males trace a
line of descent to stone-age hunters.
But the authors say more work is needed to answer this question.
The study, by an international team, is published in the journal Proceedings of
the Royal Society B.
Start Quote
I would say that we are putting the ball back in the middle of the field
Dr Cristian Capelli University of Oxford
Archaeological finds show that modern humans first settled in Europe from about
40,000 years ago - during a time known as the Palaeolithic.
These people survived an Ice Age some 20,000 years ago by retreating to
relatively warm refuges in the south of the continent, before expanding into
northern Europe again when the ice melted.
But just a few thousand years after Europe had been resettled by these
hunter-gatherers, the continent underwent momentous cultural change. Farmers
spread westwards from the area that is now Turkey, bringing with them a new
economy and way of life.
The extent to which modern Europeans are descended from these early farmers
versus the indigenous hunter-gatherers who settled the continent thousands of
years previously is a matter of heated debate.
The results vary depending on the genetic markers studied and are subject to
differing interpretations.
Family tree
The latest study focused on the Y chromosome - a package of DNA which is passed
down more or less unchanged from father to son.
The Y chromosomes carried by people today can be classified into different
types, or lineages, which - to some extent - reflect their geographical
origins.
More than 100 million European men carry a type called R-M269, so identifying
when this genetic group spread out is vital to understanding the peopling of
Europe.
R-M269 is most common in western Europe, reaching frequencies of 90% or more in
Spain, Ireland and Wales.
Neolithic pottery The Neolithic was a time of momentous cultural change in
Europe
But while this type reaches its highest distribution on the Atlantic fringe,
Patricia Balaresque and colleagues at the University of Leicester published a
paper in 2010 showing that the genetic diversity of R-M269 increases as one
moves east - reaching a peak in Anatolia (modern Turkey).
Genetic diversity is used as a measure of age; lineages that have been around
for a long time accumulate more diversity. So this principle can be used to
estimate the age of a population.
When the Leicester team estimated how old R-M269 was in different populations
across Europe, they found the age ranges were more compatible with an expansion
in Neolithic times (between 5,000 and 10,000 years ago).
The team's conclusions received support from papers published in August 2010
and in June this year. But one study which appeared last year backed the idea
of a more ancient, Palaeolithic origin for R-M269.
Age estimates
Now, a team including Cristian Capelli and George Busby at Oxford University
have explored the question.
Their results, based on a sample of more than 4,500 men from Europe and western
Asia, showed no geographical trends in the diversity of R-M269. Such trends
would be expected if the lineage had expanded from Anatolia with Neolithic
farmers.
Furthermore, they suggest that some of the markers on the Y chromosome are less
reliable than others for estimating the ages of genetic lineages. On these
grounds, they argue that current analytical tools are unsuitable for dating the
expansion of R-M269.
Neolithic skeletons, the Valdaro lovers from Italy Studies of DNA from ancient
remains could shed more light on European origins
Indeed, Dr Capelli and his team say the problem extends to other studies of
Y-chromosome lineages: dates based on the analysis of conventional DNA markers
may have been "systematically underestimated", they write in Proceedings B.
But Dr Capelli stressed that his study could not answer the question of when
the ubiquitous R-M269 expanded in Europe, although his lab is carrying out more
work on the subject.
"At the moment it's not possible to claim anything about the age of this
lineage," he told BBC News, "I would say that we are putting the ball back in
the middle of the field."
The increasing frequency of R-M269 towards western Europe had long been seen by
some researchers as an indication that Palaeolithic European genes survived in
this region - alongside other clues.
A more recent origin for R-M269 than the Neolithic is still possible. But
researchers point out that after the advent of agriculture, populations in
Europe exploded, meaning that it would have been more difficult for incoming
migrants to displace local people.
Paul.Rincon-INTERNET@bbc.co.uk