Australopithecus afarensis is an extinct hominid that lived between 3.9 and 2.9 million years ago. A. afarensis was slenderly built, like the younger Australopithecus africanus. It is thought that A. afarensis was more closely related to the genus Homo (which includes the modern human species Homo sapiens), whether as a direct ancestor or a close relative of an unknown ancestor, than any other known primate from the same time.

The most famous fossil is the partial skeleton named Lucy (3.2 million years old) found by Donald Johanson and colleagues, who, in celebration of their find, repeatedly played the Beatles song Lucy in the Sky with Diamonds.