Five of seven ancient humans from Bacho Kiro Cave in Bulgaria dated 45,930-42,580 years ago. Neanderthal males with modern human females produced viable offspring. Modern human males with Neanderthal females did not produce viable offspring. Lack of Neanderthal Y chromosomes in modern humans proves hybrid male inviability. Allergic reaction to Neanderthal sperm hypothesized. Male hybrid offspring naturally aborted or miscarried. Neanderthals properly designated Homo sapiens neanderthalensis subspecies. Sapiens and Neanderthals interbred despite co-location. Hybrid fertility directional due to genetic incompatibility. Neanderthals not separate species from humans. Modern humans carry Neanderthal DNA from female ancestors. Interbreeding produced fertile offspring only one way. Hominid hybridization explains genetic traces. Subspecies interbreeding common in Homo genus. No Neanderthal Y in humans confirms reproductive barrier.