 The earth was selected as a new home for the exotics because the earth and its primitive Pliocene hominids were the most compatible to the Tanu/Firvulag genetic code.Over time, both the Tanu and Firvulag races have difficulty reproducing on Earth due to the higher levels of terrestrial and solar radiation relative to their homeworld.When the time-traveling humans started to arrive in the Pliocene, the Tanu enslaved many of the humans in an effort to overcome this problem, interbreeding with the humans who have latent metapsychic abilities and soon incorporating humanity into their Pliocene society.