The missing link is a hypothetical extinct organism that is a transitional form between modern humans and the anthropoid progenitor. The missing link concept is based on the idea that evolution is a linear process while humans originated from man-ape species. The idea of the missing link was required to explain the theory of evolution of humans in the latter half of the 19th century. However, now, it is accepted that the relationship between humans and apes (i.e., chimpanzees) is not linear and is not related to the direct descent. There should be a species that is a common ancestor of humans and anthropoid apes. During the process of evolution, the species diverged two several species, including direct ancestors of modern humans and other hominids. Although the missing link is a hypothetical concept, several ancestral species have been hypothetically classified as candidates for a missing link. These species include Australopithecus afarensis (also known as Lucy), Australopithecus sediba, Australopithecus africanus, Homo erectus, and Homo habilis.