H3H6

Origins and Evolution

mtDNA haplogroup H3H6 is a sublineage nested within the H3H branch of haplogroup H3. H3H as a whole is widely interpreted in population genetics studies as part of a post‑glacial maternal re‑expansion from southwestern European refugia, particularly along the Atlantic/Iberian margin. Given that context and the phylogenetic position of H3H6 below H3H, the most parsimonious inference is that H3H6 originated on the Iberian/Atlantic fringe during the Early Holocene (approximately 9 kya), carrying the genetic signature of populations expanding northward and along coastal Europe after the Last Glacial Maximum.

The clade is currently represented in a small number of modern samples and in a limited set of ancient DNA (aDNA) contexts (three reported aDNA occurrences in the referenced database), which supports an Early Holocene time depth but also indicates a relatively low overall frequency and a patchy preservation in the archaeological record.

Subclades (if applicable)

H3H6 sits as a downstream branch of H3H and may itself contain further minor sub-branches identifiable by additional private mutations in full mitochondrial genomes. At present, available data indicate H3H6 is a low-frequency terminal or near-terminal subclade (few known downstream splits), but future deeper mitogenome sequencing and expanded aDNA sampling could reveal further diversification. Because H3H6 is relatively rare in both modern and ancient samples, detailed internal substructure remains incompletely resolved.

Geographical Distribution

Observed occurrences of H3H6 are concentrated on the Atlantic/Iberian margin, with the highest representation in Iberian populations (including the Basque region) and in Atlantic‑fringe western Europe. Lower-frequency occurrences are documented in parts of southern Europe (including limited findings in Italy and Sardinia), northwest Africa (Maghreb) likely reflecting prehistoric and historic cross‑Mediterranean contact, and sporadically in Near Eastern/Anatolian samples reflecting the wide dispersal of H haplogroups across Eurasia. Modern diaspora communities derived from Atlantic European populations can also carry H3H6 at variable low frequencies.

Because H3H6 is uncommon, regional frequency estimates are modest and often based on limited sample sizes; its presence in ancient contexts, though limited, is consistent with a long‑standing local presence in Atlantic Iberia since the Early Holocene.

Historical and Cultural Significance

H3H6 should be interpreted primarily as a marker of maternal continuity and local re‑expansion from southwestern European refugia rather than as a hallmark of a single later archaeological culture. Its origin predates the Neolithic agricultural expansions and therefore reflects Mesolithic/Early Holocene population dynamics. That said, H3H6 lineages persisted into later periods and are compatible with being carried by populations involved in the Neolithic coastal expansions and later cultural phenomena linked to the Atlantic façade.

• Bell Beaker and other Bronze Age movements across Europe were large demographic events, but given the low frequency and localized nature of H3H6, it appears to have been a minor contributor to those later expansions rather than a defining marker.
• The occasional detection of H3H6 in the Maghreb and in eastern Mediterranean contexts is consistent with known prehistoric and historic gene flow across the western Mediterranean.

Overall, H3H6 is useful for studies focused on regional maternal continuity in Atlantic Iberia and neighboring regions, and for tracing micro‑scale demographic histories along the Atlantic margin.

Conclusion

mtDNA H3H6 is a low‑frequency, regionally informative subclade of H3H with an Early Holocene origin on the Iberian/Atlantic margin. It documents part of the maternal legacy of post‑glacial re‑expansion from southwestern refugia and persists at low to moderate frequencies in Iberian and Atlantic European populations, with sporadic occurrences beyond that core area. Broader mitogenome sampling and additional aDNA recovery will be required to refine its internal topology, precise age estimate, and the full extent of its prehistoric and historic dispersals.