A. afarensis, dating to at least 3.75 million years ago, may be ancestral to all the other species of this genus, with the exception of A. anamensis, a hominid dating to c.4.1 million years ago, discovered in 1994. A. afarensis is known from fossils found at Hadar and Omo, Ethiopia, and Laetoli, Tanzania. The 3.6-million-year-old footprints, preserved in volcanic ash at Laetoli, are commonly attributed to this species. Postcranial skeletal remains show that A. afarensis was relatively small, standing 3.5 to 5 ft (1 to 1.6 m) tall and weighing 45 to 110 lb (20 to 50 kg).

Remains of an australopithecine of similar size and between 2 to 3 million years old have also been found in S Africa. Known as A. africanus, it had molars slightly larger than A. afarensis, but in other respects it had decidedly more human features than A. afarensis, including a higher forehead, less prominent brow ridges, and a shorter face. Most researchers consider A. africanus to be a distinct species that is descended from A. afarensis.

Two other well-known australopithecines, A. boisei (from E Africa) and A. robustus (from S Africa), featured very large molars and premolars, very thick jaws, and craniums topped by prominent crests. These features probably reflect a relatively specialized diet of rough vegetable matter. In contrast, A. afarensis and A. africanus had cranio-dental features consistent with a more generalized diet. The large-toothed australopithecines also had skeletons indicative of a heavier build than the small-toothed australopithecines; the former are believed to have weighed 25 to 50 lb (10 to 20 kg) more than the latter, even though they were approximately the same height. Based on these pronounced differences, australopithecines are classified into two distinct types: gracile and robust. The robust australopithecines all became extinct between 1.5 and 1 million years ago, while one of the gracile autralophithecines is believed to have given rise to the branch leading to the emergence of the genus Homo c.2.5 million years ago.

The species A. barhelghazali is attributed to a 3.5-million-year-old jaw and tooth remains found in central Chad in 1995. The first remains of an Australopithecus recovered outside of E or S Africa, this surprisng find suggests hominid evolution took place over a much larger portion of Africa than many experts had originally believed. A cranium specimen recovered from W Turkana, Kenya, is attributed to the robust species A. aethiopicus. This fossil is 2.5 million year old and shares certain primitive features with A. afarensis, providing strong evidence that the robust A. aethiopicus descended from the gracile A. afarensis. Many experts believe A. aaethiopicus subsequently gave rise to the two major robust species, A. boisei and A. robustus. Tibia and mandible fragments from Allia Bay, Lake Turkana, are attributed to yet another species, A. amarensis, providing evidence for bipedalism c.4.1 million years ago.

There is no consensus among the experts concerning the evolutionary relationship among the various australopithecines, or between the australopithecines and Homo habilis, which is considered by many to be the earliest species of the genus Homo. One proposal is that A. afarensis gave rise to two distinct lineages c.3 million years ago: One branch became the robust australopithecines (doomed to extinction), while the other branch became the gracile species (one species of which eventually evolved into H. habilis). Many researchers believe that the species that evolved into H. habilis was A. africanus. Other experts reject this model, as well as the claim that A. africanus played any such key role. Increasingly, specialists favor assigning the robust australopithecines to a completely seperate genus, Paranthropus, because of the very significant physical differences between the robust and gracile species. According to this view, A. afarensis was the last common ancestor of these two distinct types of hominids.

See also human evolution.