• Pliocene Hominidae;
  • Hadar;
  • Laetoli


Australopithecus afarensis Johanson in Hinrichsen (New Sci. 78(1105):571, 1978) is now accepted by many workers as an East African early hominid species showing marked sexual dimorphism dating to between 2.8 and 3.8 million years before present (my BP) and possibly as early as 5.0 my BP. It is best known in the latter part of this time range, at Hadar, and is here most phenetically similar to the South African A. africanus. In the earlier part of its time range, at Laetoli and Middle Awash, the morphology of the species is less well known. Much debate has surrounded the taxonomy, dating, morphological adaptations, and phylogenetic interpretation of the Laetoli and Hadar hominids. Some questions, such as the geological age of the sites, are close to resolution. Other questions hinge on whether the Laetoli and Hadar fossil samples are drawn from one biological population or species or from more than one. At present the balance of evidence suggests attribution of the Laetoli and Hadar fossils to a single species. Different emphases in functional morphological studies have led to differences in interpretations of taxonomy, phylogeny, function, and behavior. Various lines of evidence document that A. afarensis engaged in humanlike bipedal locomotion, but its postcranial morphology also documents that the species' locomotor adaptation differed from that of Homo, perhaps toward a more climbing and arboreal adaptation. Cladistic relations between A. afarensis and A. robustus/boisei are unresolved. A. afarensis does appear to be an appropriate ancestor for the A. africanus-Homo lineage.