Paleogenomic study of a single bone fragment from a female hominin found in the Denisova Cave in the Altai mountains of Russia provided "direct evidence for genetic mixture between Neanderthals and Denisovans on at least two occasions: once between her Neanderthal mother and her Denisovan father and at least once in the ancestry of her Denisovan father.”

The authors indicated that finding a 1st generation Neanderthal Denisovan offspring among the small number of archaic specimens sequenced to date suggests that mixing between late Pleistocene hominin groups was common when they met.

Order of authorship in the original publication: Slon, Mafessoni, Vernot...Pääbo. Available from PubMedCentral at this link.

(Thanks to Juan Weiss for this reference and its interpretation.)