Australopithecus afarensis was a type of hominid which belonged to the genus Australopithecus and lived between 3.7 and 2.9 million years ago. Australopithecus afarensis was slenderly built, like the younger Australopithecus africanus. It is thought that A. afarensis was ancestral to both the genus Australopithecus and the genus Homo, which includes the modern human species, Homo sapiens. The most famous fossil is the partial skeleton named Lucy (3.2 million years old) by Donald Johanson and colleagues, after they played the Beatles song Lucy in the Sky with Diamonds over and over in celebration of their find.
Compared to the modern and extinct great apes, A. afarensis has reduced canines and molars, although they are still relatively larger than in modern humans. A. afarensis also has a relatively small brain size (~380–430 cm³). The image of a bipedal hominin with a small brain and primitive face was quite a revelation to the paleoanthropological world at the time. This was due to the earlier belief that an increase in brain size was the first major hominin adaptive shift.
Australopithecus afarensis fossils have only been discovered within northern Africa. Despite Laetoli being the type locality for A. afarensis, the most extensive remains assigned to this species are found in Hadar, Afar Region of Ethiopia, including the above-mentioned "Lucy" partial skeleton and the "First Family" found at the A.L. 333 locality. Other localities bearing A. afarensis remains include Omo, Maka, Fejej and Belohdelie in Ethiopia, and Koobi Fora and Lothagam in Kenya.