The two major races Protoss and Zergs, are creations of the Xel'Naga. Other alien species have existed in the Starcraft-verse, such as the Tagal, but they mostly exist as obscure things in the fluff. I think it make sense that the three major (and playable) races share a common origin. All three are psionics. Two of them,Terran and Protoss, share a similar body shape. Two of the them, Terran and Zerg, are strong in numbers. Terrans, beign Jack of All Stats combine purity of form and purity of essence. Like the Zergs, they are constantly evolving. However its not their bodies but their tactics and technology that is always evolving, while the Protoss are rather stagnant.