Since humans developed the ability to study DNA extracted from fossils, we have uncovered a mystery that until now had no answer. In the DNA of some human species, including our own, Homo sapiens, there were "super-archaic" markers, vestiges of older, unknown species with which we had interbred and produced offspring. Unable to determine who these genomic intruders were, some scientists called them ghost populations...Researchers in China have analysed proteins from the tooth enamel of six fossils dating back around 400,000 years -- five men and one woman -- found at sites across much of the country from north to...