European genetic ancestry originates from three main ancestral populations - Western hunter-gatherers, early European farmers and Yamnaya Eurasian herders - whose edges geographically met in present-day France. Despite its central role to our understanding of how the ancestral populations interacted and gave rise to modern population structure, the population history of France has remained largely understudied. Here, we analysed 856 high-coverage whole-genome sequences along with genome-wide genotyping data of 3,234 present-day individuals from the northern half of France and merged them with publicly available present-day and ancient Europe-wide genotype datasets. We also analysed, for the first time, the whole-genome sequences of six medieval individuals (300-1100 CE) from Western France to gain insights into the genetic impact of what is commonly known as the Migration Period in Europe. We found extensive fine-scale population structure across Brittany and the downstream Loire basin, emphasizing the need for investigating local populations to better understand the distribution of rare and putatively deleterious variants across space. Overall, we observed an increased population differentiation between the northern and southern sides of the river Loire, which are characterised by different proportions of steppe vs. Neolithic-related ancestry. Samples from Western Brittany carry the largest levels of steppe ancestry and show high levels of allele sharing with individuals associated with the Bell Beaker complex, levels that are only comparable with those found in populations lying on the northwestern edges of Europe. Together, our results imply that present-day individuals from Western Brittany retain substantial legacy of the genetic changes that occurred in Northwestern Europe following the arrival of the Bell Beaker people c. 2500 BCE. Such genetic legacy may explain the sharing of disease-related alleles with other present-day populations from Western Britain and Ireland.

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This study tries to contribute to the linguistic geography of France by comparing historical records of surnames in the five departments of Brittany with other geolinguistic resources of the same area. In order to map these data, a new software was developed and applied to the respective data sets. The results show interesting concordances between the distributions of patronyms and isoglosses of lexical and phonological variants.

Referring to the traditional "Kurgan theory", the archaeological discoveries of the last thirty years show at an overwhelming majority that there is no evidence of an Indo-European invasion during the 4 th Millennium BC at the European level 1 � Reconsidering the origin of European languages in that new perspective, it appears that the Celts would not have migrated from a territory situated between Austria and southern Germany, as has been generally accepted since the nineteenth century� They could have settled since the end of Upper Palaeolithic and the Mesolithic as groups of fishermen and seafarers along the Atlantic shores from the Iberian Peninsula to the British Isles through ancient Gaul� On the basis of dialectal and geolinguistic data, the present approach tries to better understand this hypothesis� Studying geolinguistic variations, it considers the Celto-Atlantic area and the exchanges that may have been established in the long term with the neighbouring linguistic Germanic and Romance areas� New lexical borrowings from Celtic to Romance and Germanic (and vice versa) open research perspectives and raise real questions about the continuity of language and population areas in the Atlantic zone of Europe�