1 Quercus: Of course, Fagalians are completely unrelated to any species from Earth or any other planet.
2 Quercus: Anything else is scientifically implausible to the point of ridiculousness.
3 Iki Piki: Star Trek explained the widespread existence of similar species by invoking a progenitor species who seeded worlds with humanoid DNA.
4 Quercus: I find that unlikely.
4 Serron: In your case they could just use actual seeds.

The original series of Star Trek used humanoid and virtually human-looking aliens because it was by far the easiest way to portray aliens using the television and special effects technology of the time. By the time of The Next Generation, the aliens became more exotic, but still largely "humans in costume". However the audience had grown more sophisticated, and aware of the simple biological fact that it's virtually impossible for life that originated on separate planets to evolve in similar, let alone almost identical fashion - and especially to the point of being able to interbreed, as humans and Vulcans can.

Rather than leave it unexplained, the writers of The Next Generation decided to address the question head on in the season 6 episode, "The Chase". In this episode, the crew of the Enterprise discover that a precursor race explored the Galaxy millions of years earlier, and found no humanoid life anywhere, so they seeded many worlds with their own DNA, thus resulting in the appearance of humanoid life on disparate planets scattered across the Galaxy.

I thought this was a bit of a corny explanation the first time I heard it. But it's at least somewhat plausible - well, more plausible than a lot of things in Star Trek anyway, and definitely more plausible than the alternative.