In some form, all mammals require exogamous (literally, from the Greek, marriage-outside) mating patterns. Apparently, most mammals send the males to mate outside the local group, and the group is continued by females. For chimpanzees and early hominins, this functioned the opposite way. According to chemical composites in the teeth that scientists analyzed, all the males in the area were local while at least half the female skeletons apparently came from father afield. The article on australopithecines I read came from the New York Times.
Note: I thought originally that hominin was a typo for hominid. Apparently hominid has been expanded as a wider definition and hominin now refers to human ancestors. For reference, see this article from the Australian Museam.