The "Probability" trilogy takes place in a galaxy that has been colonized by humans. This was made possible by the space tunnels, a network of FTL warp gates that were created by a now-lost progenitor race. Humanity is not united under a common government and political system. The Terrans have also discovered a number of alien races, most of them vastly similar in body format, living conditions and even DNA, leading to the hypothesis that the aforementioned progenitor race seeded the galaxy with sentient life, which then evolved according to the conditions on each planet. Of the known alien races, humanity is the only one that has reached space.