The scientists see "evidence for gene flow only from Neanderthals to modern humans." So if you see someone with a heavy brow or receding chin who looks kind of primitive, he may indeed have some Neanderthal genes.
The recent discovery of a new species of human in a cave in Siberia suggests that three species of humans coexisted at the same time--modern humans, Neanderthals, and the new species. The preliminary finding, reported in the NY Times, was based on DNA from a single finger bone.
So the question naturally arises--did either humans or Neanderthals interbreed with the third species? The plot thickens! The hobbit-like Flores Man may have been a fourth species on earth at the same time.
And all this raises the question--have humans ever interbred with chimpanzees, bonobos, orangutans, or gorillas? Again, there is some evidence that the ancestors of modern humans and the ancestors of chimps interbred at some time in the distant past. Today, the birth of a hybrid child would create enormous legal and ethical problems--so much so that the existence of such a child would probably be kept secret. What possible motivations could lead to creation of such a child?
In fact, there's a rumor that famed zoologist George Schaller was called in to consult on such a case.
An organism with genes from more than one species is called a "chimera." A human-mouse chimera was created when a human gene was inserted into a mouse. Imagine the problems if intelligent mice escape from the lab!
Blurred boundaries in literature
The subject of humans breeding with or falling in love with apes sometimes surfaces in literature. In the short story “Reflections of a Kept Ape,” Ian McEwan writes about "a woman who initiates a sexual relationship with a pet monkey—narrated from the point-of-view of the monkey." Source.
In "The Woman and the Ape" by Peter Hoeg, an intelligent ape escapes and makes off with an alcoholic woman, who eventually falls in love with the ape.
The sad story of Lucy
Lucy was a chimpanzee, raised from infancy by a human family--an experiment to see if a chimpanzee could learn human language, given the proper human upbringing. Since Chimps can't form human vocal sounds, Lucy was taught American Sign Language. She eventually learned well over 100 signs, and could engage in simple conversations.
But as Lucy matured, she became extremely disruptive to her adoptive parents. She developed great strength, along with unpredictable and unmanageable behavior. Faced with losing any semblance of a normal life, her parents traveled the world, looking for some place where they could safely and humanely leave Lucy. The problem was, Lucy thought she was human.
The chimp was eventually placed on an island in Gambia, along with other tame chimps being returned to the wild. Unlike the others, Lucy alone thought she was human, and so she refused to learn how to forage as a wild chimp. Janice Carter stayed with Lucy for a long time, living inside a cage on the island, trying to set an example of what Lucy needed to do. Go here to learn about the haunting and tragic end to this story.
Lucy was marooned in the strange dimension between humanity and animals.
* * *More on Neanderthals here.