Non-Africans harbor 1.8-2.6% Neanderthal DNA with East Asians having more than Europeans. Australo-Melanesians carry up to 6% Denisovan DNA. Africans show introgression from unknown ghost archaics. Widespread purifying selection removes most archaic DNA especially on sex chromosomes and testes genes. Archaic introgression reduces male fertility via meiotic germ cell genes. Neanderthal deserts enrich brain regions like prefrontal cortex and striatum. No Neanderthal ancestry near FOXP2 language gene. Modern humans have multiple AMY1 starch genes unlike single-copy archaic versions under positive selection. Neanderthal alleles downregulate expression in testes and brain. Adaptive introgression aids immunity skin pigmentation hypoxia response and fat distribution in non-Africans. Tibetans acquired high-altitude EPAS1 from Denisovans. Neanderthal variants increase Crohn's celiac and allergy risks in Europeans and Asians. Archaic immune alleles trade off past adaptation for modern autoimmunity. Higher frequency archaic variants drive more gene expression changes under positive selection.