The gracile australopithecines is divided into two subgroups: A. africanus and A. afarensis. Both were small in stature (1- 1.5 meters), had similar brain size (400- 500 ml), and rather light body structures. However, A. afarensis was more ape-like, as it predate A. africanus by 1.5 million years, or so. Scientists still wonder how A. afarensis got around. Most theorize that is used both bipedalism and quadrupedalism, as its body had not yet fully developed to handle absolute bipedalism. A. africanus was probably bipedal, but its muscle structure is arranged in a way that it probably moved like the baboons of today, only with more use of bipedalism.
About 4.4 millions years ago, a different type of primate emerged. Paleontologists classify the first bipedal primates as hominids. These first hominids had not yet developed the large brain, teeth structure, and skeletal features we identify as Homo. Instead, they predate, and sometimes overlap the first Homo species. They are known as the australopithecines. There are two types of australopithecine: gracile and robust.
It is not known how these species directly affected human evolution. However, there are three theories: the first has A. afarensis as a common ancestor for all Homo's. One fork of the tree leads to Homo, the other fork leads to A. africanus, A. boiseiand A. robustus. Another theory has A. afarensis being the common ancestor of A. africanusat one fork, and the robust australopithecine at another.
This theory has modern humans evolving from A. africanus. The last theory has A. africanus and A. afarensis branching off from a common ancestor. A. africanus spawned modern humans, and A. afarensis became the robust australopithecines. Either way, it is known that humans evolved from the gracile australopithecine, and the robust died off along the way.