The introduction to this series can be found here.
Summary: Genetic evidence shows that various Pleistocene populations interbred, including humans, Neanderthals and Denisovans. Is it possible to know whether these could have occurred in pair-bonded relationships? What were the evolutionary origins of pair-bonds in hominins, or are single explanations too simple?
Paleoanthropologist John Hawks created this great infographic that summarizes what we know about ancient human interbreeding, based on recent genetic discoveries (this link to his site has a larger version). The graphic shows that several archaic populations mated with each other, although likely at low rates, including modern humans with Neanderthals and Denisovans. There are also a couple of mystery populations in the mix, whose existence is known solely based on the DNA they left behind. Very cool stuff.