Denisovan mandible from 160kya Xiahe China has 3-rooted lower second molar. 3RM frequency under 3.5% in non-Asian Homo sapiens. 3RM exceeds 40% in northeast Asians and Native Americans. Trait absent in early Asian and African H. sapiens. 3RM originated in archaic Asian hominins before H. sapiens arrival. Penghu 1 mandible from Taiwan shows 3RM2 with archaic Denisovan-like features. Modern Asian 3RM traces directly to Denisovan introgression. Twin study shows genetic basis for bilateral 3RM. High 3RM links Native American origins to Asia. Denisovan EPAS1 gene flow explains Tibetan high-altitude adaptation like 3RM. Selection for masticatory robusticity maintains high 3RM in Asians. Lower 3RM in populations with Denisovan DNA but less robust jaws like Australia. Archaic features in recent Asians likely from Denisovan admixture. 3RM provides morphological proof of archaic-modern human hybridization in Asia.