Admixture and Genetic Connectivity: Autosomal Insights Into Indo-Aryan Speakers at the Eastern Edge of the Indian Subcontinent.

Assam serves as a melting pot where various ethnicities converge, shaped by its strategic location bordering the six northeastern states and neighboring countries. This study examines the genomic ancestry of Indo-Aryan speakers in Assam, a region at the eastern edge of the Indian subcontinent known for its diverse linguistic and cultural influences. Principal Components Analysis (PCA) revealed that Assamese Indo-Aryans cluster closely with Bangladeshi Indo-Aryans, diverging from the typical South Asian genetic continuum. This group shows significant ancestral influences from East and Southeast Asia. Outgroup f3 statistics indicated that Assamese Indo-Aryans exhibit greater genetic drift in relation to other Indo-Aryan populations, particularly the Harijan and Kol groups from the Ganga plain, rather than with geographically closer Bengali or Nyishi populations. Haplotype-based fine-structure and identity by descent (IBD) analyses confirmed a close genetic affinity with Bangladeshi Indo-Aryans, along with notable connections to Harijan and Kol populations. The low levels of Runs of Homozygosity (RoH) suggest a high degree of genetic diversity, most likely due to a result of admixture and a large effective population size. Admixture events are estimated to have occurred 55 to 61 generations ago, indicating a composition of 76% Indian ancestry and 24% East/Southeast Asian ancestry. These findings, with enhanced resolution, suggest a reconsideration of the previous assumption that Northeast India served as a genetic barrier. Instead, they appear to support its role as a corridor facilitating bidirectional gene flow between South and East/Southeast Asia. Furthermore, the coexistence of diverse linguistic affiliations of ethnic groups in this region reflects in their ancestry patterns. These findings highlight the complex interplay of geography, language, and genetic exchange in the region.