While it was previously thought that humans and neanderthals never mixed, Wired reports
that a recent DNA study of both human and neanderthal DNA has revealed the opposite. In non-African humans there is a part of DNA that is neanderthal in origin, which proves that there was human-neanderthal coexistence and reproduction.
Doctor Damian Labuda of the University of Montreal's Department of Pediatrics and the CHU Sainte-Justine Research Center and team report:
"We provide evidence of a notable presence (nine percent overall) of a Neanderthal-derived X chromosome segment among all contemporary human populations outside Africa"