Figure 1. The extant phylogenetic bracket (EPB) method. In order to infer properties of the soft-tissue anatomy of an animal such as Australopithecus, one can study the soft tissues of two extant relatives with reasonably well established phylogenetic relationships. Human's (Homo) and chimpanzee's (Pan) respective lineages diverged from each other before Homo's lineage diverged from Australopithecus. Therefore, soft-tissue features inherited by Homo and Pan from their most recent common ancestor will have existed ancestrally in Australopithecus as well. Consequently Australopithecus would have also had those features unless they were secondarily lost after Australopithecus diverged from Homo. For instance, it is safe to infer that Australopithecus had a four-chambered heart since both humans and chimpanzees have four-chambered hearts. We can reasonably conclude this even though the fossils of Australopithecus do not provide unambiguous evidence documenting the presence of such a structure.