Every time I see a news story about early humans mating with Neanderthals or other near-monkey species, I wonder if there were any species that our ancestors didn't try to have sex with. And how did the conversation go just prior to the first pre-human deciding to get some Neanderthal action?
Pre-human 1: "Hey, that creature by the watering hole has two legs. I'd totally do it."
Pre-human 2: "It's all yours. I've got my eye on a tiny horse with a limp."
I'm no Darwin, but I have a few observations of my own. My first observation is that the only species that have survived to modern times are the ones able to fend off unwanted advances from horny pre-humans. Take the giraffe, for example. Its long legs keep its naughty bits well above the pelvic thrusting level of our ancestors. Then you have your cheetahs that can outrun us, your fish that can hide underwater, your birds that can fly away, your zebras that can kick, and so on. But the poor Neanderthals and other slow-moving bipeds all got banged out of existence by our horny ancestors.
I have a hypothesis that several million years ago just about anything could mate and have offspring with anything else. For example, the modern beaver is probably the offspring of an early human and a bear that was slow to snap out of hibernation. That's just a guess. But the next time you see a beaver standing on his back legs eating a fish, try to imagine him as a buck-toothed tourist at a sushi place. It's easier than it should be.
Contrast the open-minded attitude of our ancestors to our picky modern selves. Now humans won't even date someone who cheers for the wrong sports team or goes to the wrong church. And we don't want our mates to be sporting any hair below the chin. Dating outside your species is totally frowned upon. I think maybe we've lost something. On the plus side, your dog appreciates your willingness to have a platonic relationship. But he still gets nervous when you give him a bath. There's a lot of bad history there.