Humanity gave up the ways of civilization and embraced an animalistic, primitive way of life known as the Call of the Wild. The Call is a force of nature working through the greater spirits of animals. As humanity embraced the call, they physically changed to a greater resemblance of an animal, and through these changes, they became the tribes.
The tribes act as separate races, and though one can produce children between tribes, the child is not guaranteed to follow either parent. A child is always handed over, at the earliest convenience, to the care of the tribe to whom the child was embraced.