3-rooted lower molar occurs in less than 3.5% of non-Asian Homo sapiens. Frequency exceeds 40% in northeast Asians and Native Americans. Xiahe Denisovan from 160,000 years ago has 3-rooted lower second molar. Penghu 1 archaic mandible from Taiwan also shows 3-rooted lower second molar. Trait absent in early African H. sapiens and early Asian H. sapiens. 3RM originated in pre-sapiens archaic Asian hominins. Modern Asian lineages got 3RM via introgression from Denisovans. Single twin study shows genetic basis for bilateral 3RM. High 3RM frequencies link Native American origins to Asia. Trait rarity outside Asia explained by mutation alone. Selection may retain 3RM in populations with heavy chewing loads. Nepal has 25% 3RM matching high Denisovan introgression areas. Lower 3RM in Australia/New Guinea despite Denisovan DNA due to less robust chewing. Archaic features in recent Asians likely from Denisovan admixture. 3RM provides direct morphological evidence of archaic gene flow into Asians.