Image caption The debate continues among geneticists on the origins of Ashkenazi Jews

A new genetics study has challenged previous theories about the maternal ancestry of Ashkenazi Jews.

The study suggests that most female ancestors came from southern and western Europe, not the Middle East as some authors assume.

The new work in Nature Communications indicates that Jewish groups could have absorbed European converts about 2,000 years ago.

However, others working in the field are not convinced by the findings.

"The origins of the [Ashkenazi Jews] is one of the big questions that people have pursued again and again and never really come to a conclusive view on," said Martin Richards from the University of Huddersfield, UK, one of the authors on the new paper.

The team studied mitochondrial DNA (mtDNA), the genetic information in the cell's "batteries", which is passed on only via women. It can be used to probe a population's maternal history.

Tracing mitochondrial DNA Mitochondrial DNA is passed down, almost unchanged, from a mother to her children and allows any person's genetic profile to be traced back to a common ancestor

By studying the mutations, or changes, in mtDNA sequences, researchers are able to probe the maternal histories of different human populations

It has enabled them to build a "family tree" of maternal ancestry, and group different mtDNA lineages together based on shared mutation DNA: the 'smartest' molecule in existence?

The researchers used mtDNA lineages from Europe, the Caucasus and the Middle East. Their conclusions were that Ashkenazi maternal lines were most closely related to those from southern and western Europe.

"This suggests that, even though Jewish men may indeed have migrated into Europe from [the Levant] around 2,000 years ago, they seem to have married European women," explained Prof Richards.

He added that women could have been among early European converts to Judaism.

"The great majority of Ashkenazi maternal lineages were not brought from the Levant, as commonly supposed, nor recruited in the Caucasus, as sometimes suggested, but assimilated within Europe.

"It's a very exciting time for the prehistory of Europe. There will be work coming out in the next few years that will really give us a good picture of what happened over the last 30,000-40,000 years."

Ashkenazi Jews are thought to have migrated from the Levant to Italy in the first or second centuries, and then moved to western and then eastern Europe, where populations expanded in number during the 13th Century.

This new study makes the suggestion that more than 80% of Ashkenazi Jews can trace their ultimate maternal ancestry to prehistoric Europe. But studies looking at the Y chromosome (which is passed down from father to son only) have shown that the male line of descent does trace back to the Middle East.

Harry Ostrer at the Albert Einstein College of Medicine, Yeshiva University in New York, US, told BBC News the latest study was an "interesting paper".

He added: "It's not terribly surprising that there are mitochondrial genomes that coalesce to a long time ago in Europe because that is representative of the first founding event of Ashkenazi Jewry in Europe.

"The genetic, historical and the archaeological records are pretty clear. What we see in the genomes of contemporary Ashkenazi Jews is that they are the descendants of people who have gone through a 'bottleneck' of moving from western to eastern Europe."

As for the debate over geographic origins, Prof Ostrer said there was a belief that most of Jewish history happened in the Levant region but, in his view, "it didn't necessarily; it was happening all over the western world".

However, Karl Skorecki, at the Technion - Israel Institute of Technology in Haifa, said that in his view the new work contained serious flaws at the level of phylogenetic analysis - the study of where a lineage sits on a family tree.

"While Costa et al have re-opened the question of the maternal origins of Ashkenazi Jewry, the phylogenetic analysis in the manuscript does not 'settle' the question," he told BBC News.

Doron Behar, a geneticist at Gene by Gene in Houston, US, was also unconvinced by the findings, but said he would prefer "to first address this issue in a scientific journal".

Another point to note, according to David Goldstein at Duke University in North Carolina, US, was that "the genetic composition of populations changes from generation to generation".

He said that the paper "could be right but we have no way of really knowing" because of these changes - what researchers call "genetic drift".

"It really is the fundamental framework for analysing the data that is at issue here. A framework that ignores real population genetics just doesn't work," Prof Goldstein added.