Overall, it seems that at least 80% of Ashkenazi maternal ancestry is due to the assimilation of mtDNAs indigenous to Europe, most likely through conversion. The phylogenetic nesting patterns suggest that the most frequent of the Ashkenazi mtDNA lineages were assimilated in Western Europe, ~2 ka or slightly earlier. Some in particular, including N1b2, M1a1b, K1a9 and perhaps even the major K1a1b1, point to a north Mediterranean source. It seems likely that the major founders were the result of the earliest and presumably most profound wave of founder effects, from the Mediterranean northwards into central Europe, and that most of the minor founders were assimilated in west/central Europe within the last 1,500 years. The sharing of rarer lineages with Eastern European populations may indicate further assimilation in some cases, but can often be explained by exchange via intermarriage in the reverse direction.
The Ashkenazim therefore resemble Jewish communities in Eastern Africa and India, and possibly also others across the Near East, Caucasus and Central Asia, which also carry a substantial fraction of maternal lineages from their 'host' communities. Despite widely differing interpretations of autosomal data, these results in fact fit well with genome-wide studies, which imply a significant European component, with particularly close relationships to Italians. As might be expected from the autosomal picture, Y-chromosome studies generally show the opposite trend to mtDNA (with a predominantly Near Eastern source) with the exception of the large fraction of European ancestry seen in Ashkenazi Levites.
Evidence for haplotype sharing with non-Ashkenazi Jews for each of the three main haplogroup K founders may imply a partial common ancestry in Mediterranean Europe for Ashkenazi and Spanish-exile Sephardic Jews, but may also, at least in part, be due to subsequent gene flow, especially into Bulgaria and Turkey, both of which witnessed substantial immigration from Ashkenazi communities in the fourteenth and fifteenth centuries. Gene flow could have been substantial in some cases—ongoing intermarriage is likely when these communities began living in closer proximity after the Spanish exile. A partial common ancestry for all European Jews—both Ashkenazi and Sephardic—is again strongly supported by the autosomal results.
Jewish communities were already spread across the Graeco-Roman and Persian world >2,000 years ago. It is thought that a substantial Jewish community was present in Rome from at least the mid-second century BCE, maintaining links to Jerusalem and numbering 30,000-50,000 by the first half of the first century CE. By the end of the first millennium CE, Ashkenazi communities were historically visible along the Rhine valley in Germany. After the wave of expulsions in Western Europe during the fifteenth century, they began to disperse once more, into Eastern Europe.
These analyses suggest that the first major wave of assimilation probably took place in Mediterranean Europe, most likely in the Italian peninsula ~2 ka, with substantial further assimilation of minor founders in west/central Europe. There is less evidence for assimilation in Eastern Europe, and almost none for a source in the North Caucasus/Chuvashia, as would be predicted by the Khazar hypothesis—rather, the results show strong genetic continuities between west and east European Ashkenazi communities, albeit with gradual clines of frequency of founders between east and west.
Costa et al. "A substantial prehistoric European ancestry amongst Ashkenazi maternal lineages". Nature Communications, 2013.
The Jews have been left to the end because they do not as a whole fit into any single racial classification heretofore outlined. Historically the Jews of the Biblical period in Palestine were a Semitic-speaking people composed of various Mediterranean strains which had blended together at the time of the formation of the Jewish nation. These Mediterranean strains must have included a small Mediterranean type comparable to the present Yemeni Arabs; a taller, longer-faced strain with a tendency to nasal convexity, as is found among Irano-Afghan peoples today; and a straight-nosed, presumably Atlanto-Mediterranean element contributed by the Philistines.
The Jews began their expansion from Palestine as early as the time of the Babylonian Captivity; at this time they settled Mesopotamia in large numbers, and from there began an expansion into central Asia of which colonies still remain. In the Hellenistic period they migrated into Asia Minor and the Black Sea region, as well as into Egypt; these emigrants became Hellenistic Jews. Under the Romans they settled in Italy, France, and Spain, with especial concentrations in Spain and in the cities of the Rhineland. The Jews expelled from Spain in 1492 and during previous expulsions became the Sephardim, whose descendants are to be found in various countries bordering on the Mediterranean, especially Morocco, the Salonika region of what is now Greece, and Turkey. The Rhineland Jews, persecuted at the time of the First Crusade, moved eastward into Poland, the Ukraine and other central European countries, and met there and absorbed a group of Hellenistic Jews moving westward, among whom were some who had lived among the Turkish Khazars in the Crimea and elsewhere. The two groups blended and the Germanic speech of the more numerous western element prevailed. The modern Yiddish-speaking Ashkenazim are the descendants of this amalgamated body. Racially they preserve to a large measure their Mediterranean character, altered partly by Alpine admixture which has in many cases produced Dinaricization. This Alpine, as well as some Nordic, admixture was probably obtained largely in France and Germany before their departure eastward. The most persistent Palestinian Mediterranean traits which the Jews preserve is a narrowness of the face. The Jewish facial expression, by which many Jews may be distinguished, is a cultural and not a genetic character.
Carleton Coon. The Races of Europe. New York: MacMillan, 1939.