Arroyo Seco II — 7200 Years Ago

Arroyo Seco II sits within the broad sweep of the mid‑Holocene Pampas, a landscape of rivers, wetlands and grasslands where human groups adapted to post‑glacial environments. Radiocarbon and contextual dating place the documented occupation between roughly 5462 BCE and 5000 BCE (≈7200 BP). Archaeological data indicates a foraging economy tuned to local resources; seasonal movements and flexible camp patterns would have been well suited to shifting riverine and plain ecosystems.

Genetically, the tiny aDNA sample set from Arroyo Seco II offers a faint but evocative signal of deeper population history. Both recovered mitochondrial genomes belong to haplogroup C1c, a maternal lineage widely documented in early and later Native American contexts. On the paternal side, one individual carries haplogroup Q, a lineage commonly associated with Native American male ancestry, while the other is reported as haplogroup P — which may reflect an under‑resolved paternal assignment or a rare branch captured in older classification schemes.

Limited evidence suggests continuity of maternal lineages in the region, but the small sample size (n=2) means these inferences must remain tentative. Archaeological patterns combined with emerging genetic data point toward long‑standing occupation and local adaptation, yet they also highlight the need for broader sampling to resolve migration routes, sex‑biased processes, and population structure during the mid‑Holocene.

Ancient DNA from Arroyo Seco II comprises two individuals — a very small dataset that must be interpreted cautiously. Both individuals share mitochondrial haplogroup C1c, a maternal lineage known across ancient and modern populations in the Americas; this pattern is consistent with regional maternal continuity but cannot on its own prove long‑term demographic stability. On the paternal side the two Y‑chromosome assignments differ: one individual is assigned to haplogroup Q, commonly found in Native American male lineages, while the other is reported as haplogroup P. Haplogroup P sits upstream of Q in global Y‑phylogeny and, in some cases, may reflect limited resolution or rare paternal variants in older datasets.

These genetic signals, read alongside archaeology, are evocative: the shared mtDNA suggests women bearing C1c lineages were present in the Pampas during the mid‑Holocene, while the paternal diversity hints at complex male line histories or classification uncertainty. However, with only two samples the probability of capturing population‑level variation is low. Sex‑biased mobility, founder effects, and small band sizes can produce patterns that look like continuity or change in tiny datasets.

Arroyo Seco II highlights the power of ancient DNA to open a window on past lifeways, but it also demonstrates the necessity of larger, geographically and temporally distributed samples to distinguish local continuity from broader migratory processes.