well it is sort of like how the guys at star trek hem hawed about when asked how come so many races look like human well they said and keep in mind its true if you have a race exist on a planet with the same conditions of earth chances are they will look the same or close to it IE m class planet
I was listening to the Ronald D. Moore PodCast for the BSG show "Exodus Pt.2" and he stated that the reason they destroyed the Pegasus was because they need the stage sets for the Cylon BaseShip that would be prominent in the rest of Season 3. Then I look at the SciFI BSG forums and and see all these "rationalization" threads about why the Pegasus was destroyed. It's simple people, they cannot afford anymore stage sets so they blew the ship up.
So, in Star Trek they have lots of human-looking alien races, especially in the TOS series, due to the most powerful force in the SciFi universe: the budget.