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?
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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.