The whole tons of races look like humans except for the head irks me in general. We're supposed to believe that on each of the respective home planets of these species that their evolution was directed by occurrences that lead to a perfectly human looking body (same number of digits, proportions, etc.) but something happened in their environment that made everything from the neck up (or in Zabrak case the forehead up) be radically different..... Sure there are some races that have a radically different body in the Star Wars universe but you don't see multiple races with say a body like a hutt but with tentacles for hair.
Actually thats not necessarily true at least about the Zabrak. Zabrak may look like humans on the out side but have internally different systems including a second heart. I'm not entirely sure about some of the others such as twi'leks though i know their lekku are more than just decorative danglies. As far as why the Miraluka look so much like human's is according to the lore, they were human at one point and because of their colony being where it was for so long and their isolation they lost the ability to see through evolution basically. Wookieepedia doesn't have enough information on it about the origins of the Mirialans for me to say anything about them. Sith pure blood on the other hand are technically just Sith/human hybrids that have enough of the original Sith dna in them to present with physical characteristics that show their heritage so inreality more or less aren't even a distinct species. Thats just a couple examples, Rattatki are listed as a near-human species as well, which (im not sure on their terminology with the phrase) may or may not mean that at once point were true humans as well, once again there isn't enough information on their history and origins.
What you do have to remember with the Star wars universe is that there is plenty of history that has not been talked about or even written up yet that could explain the origin of many of the species of the galaxy. For those of you that probably don't read the books, the Rakata aren't even the only species that have been come up with that have transplanted other species to planets that they didn't come from. The entire Corellian system was artificially created by an extremely technologically advanced species that no one in the Star Wars corporation has even begun to explain or flesh out with information. So in responce to the person's post that I have copied, it may well be that in the beginning these species were all the same thing till they were transplanted around the far reaches of space and then were subject to changes in morphology brought on by the environments that they lived in for millenia upon millenia.