Modi, A. et al. 2017 just published mito genomes of two 10,000 year old Mesolithic individuals from Sardinia. One belongs to J2b1 and one to I3. Neither belongs to hg U, the lineage which 97% of 100+ other Mesolithic European mtDNA samples belong to.
A J2 and a hg I dating around 10,000 years old have been found in the Near East. Their presence in Mesolithic Sardinia indicate Mesolithic Europe, maybe specifically Mediterranean Europe, exchanged genes with the Near East before Near Eastern farmers migrated en masse into Europe in the Neolithic.