For hundreds of thousands of years, several species of humans existed at the same time: the Neanderthals, the Denisovans, Homo erectus, Homo floresiensis, and possibly others we don't know about. An essay at io9 compares the speciation of early humans with the different aliens in the world of Star Trek. Now this might seem silly on the surface, but bear with me. The Star Trek franchise began in 1966, before computers made special effects aliens possible, and they had a limited budget anyway, so all the aliens from different galaxies were human-shaped, with only slight differences in prothetic makeup to distinguish them -and they could even interbreed. Six TV series, a dozen films, and five decades later, that trope continues. The series tried to explain it away in an episode of Star Trek: TNG called "The Chase" in which the connections between all the humanoid aliens are explained by a common ancestor, who left a message:
We knew that one day we would be gone, and nothing of us would survive - so we left you. Our scientists seeded the primordial oceans of many worlds, where life was in its infancy. The seed codes directed your evolution toward a physical form resembling ours: this body you see before you, which is of course shaped as yours is shaped, for you are the end result. The seed codes also contain this message, which is scattered in fragments on many different worlds.
The episode was deemed ridiculous and universally hated by fans. But it led Annalee Newitz to consider the possible analogies that could be made about early humans and their separate evolutionary histories. The different species of early humans all arose from Africa, but left the continent for other parts of the world at different times.
When one species splits into two or more, that's called speciation. Usually it happens when two groups of the same species are separated for long enough that they evolve to the point where they are no longer able to produce offspring. The big question is, were groups like erectus and the Neanderthals another species, or were they humans who just had facial and body structures that were different from modern humans? There is now a lot of DNA evidence that Homo sapiens interbred with Neanderthals and Denisovans. So it's likely that all three groups were, in fact, the same species. But we still know almost nothing about Homo erectus, and are similarly in the dark when it comes to the Hobbits and other hominin groups that are still being discovered. It's possible that Homo sapiens interbred with Neanderthals, Vulcan/Romulan style, but couldn't interbreed with Homo erectus.
Whether human evolution is actually anything like a reverse-engineered explanation for economically-designed extraterrestrials is hard to say, but the essay and the points it raises about our ancestors on earth is a "fascinating" read. Link
If I understand the true distance between Earth and any other inhabitable planet correctly, we Earthlings will never know whether it's possible for humans to breed with a species from another planet. Sorry to be a Debbie Downer.
Of course, when I was younger, I would have preferred to physically travel the universe searching for attractive and willing aliens for attempted breeding. Now that I am settled, I applaud such mental explorations.