Well i have a theory [spoiler]this is actually a spoiler by the way[spoiler]In xcom enemy unknown the aliens that are invading are using it as a test to see how fast we can advance in science, mechanics and later psionics basically mind crush and other abilities and humans pass the test but other species didnt and they joined the ranks like the fallen, cabal, and the vex are being used to help advance us.[/spoiler] [/spoiler]