Interbreeding was a practice used primarily by the Tanu with Ramapithecus and later humans to strengthen their reproductive possibilities, but later was adopted by the Firvulag using less draconian methods. Brede chose the earth primarily for the germ plasm compatibility with the exotics. However, this assay was based on the proto-human Ramapithecus hominids and not humans.
By the Tanu Edit
The Tanu practice was largely a form of reproductive slavery, especially with the Ramapithecus. The compatibility between Ramas and Tanu was such that the Ramas were used mostly to carry Tanu babies to partial term. Later when human exiles started arriving through the Time-gate, the Tanu basically enslaved them and attempted to convince the best candidates whether by benefits or coercion to serve as surrogates for hybrid babies. Initially, human hosts could only serve as carriers similar, but superior to the ramas, but a skilled, reproductive, exile surgeon, Tasha Bybar
By the Firvulag Edit
Despite the demonstrated advantages over the a long period of time demonstrated by the Tanu, the Firvulage were actually more prejudiced against the practice and tended to restrict the practice to only between mutant Howler members and human in order to breed out mutations. They also only started the practice after the Howlers reunited with the normal Firvulags.