Australopithecus afarensis : overview

The large sample from Hadar overlaps the smaller hominid samples from several sites that are near it in time, including the dental remains from Laetoli and Maka, and isolated finds from many East African localities. The original excavators of the Hadar hominids recognized the similarities with the other important early hominid site known at the time, Laetoli, and promoted the hypothesis that the Hadar and Laetoli hominids belong to a single species (Johanson and White, 1979). They named this putative species Australopithecus afarensis. Australopithecus was the genus name first given to later hominids from South Africa, covered in the next chapter, with which the Hadar and Laetoli samples are broadly similar, but vary in significant aspects. The species name afarensis refers to the geographic location of the Hadar hominids, in the Afar region of Ethiopia, and means simply "from Afar."

The samples assigned to A. afarensis provide the largest source of evidence for early hominid adaptations, and any general discussion about early hominid morphology is really a discussion about the morphology of this species. A few features distinguish this species from other early samples. These include:

  1. greater development of a lingual cusp on the P3 in many individuals
  2. larger postcanine teeth
  3. smaller canines, with smaller diastemata
Other possible comparisons with other sites, such as in cranial and postcranial form, are much less clear because of the small samples from these other sites available for comparison. These dental features stand apart from earlier and contemporary samples in the development of an adaptation to molar chewing with less canine cutting. These changes and even greater ones will be evident in later australopithecines also, and may reflect a link between later species and A. afarensis.

A. afarensis has often been claimed to be a species with a long period of morphological stasis. But recent analyses have shown the existence of temporal trends within the species in both tooth sizes and mandibular dimensions. Charles Lockwood and colleagues (1998) used the ages of different A. afarensis dental fossils to determine whether significant evolution occurred within this species over time. The analysis found that tooth sizes did increase over time, from Laetoli to Maka and Hadar, and significantly within the Hadar sample from the earliest fossils at 3.4 million years to the more recent ones around 3 million years old. Although most aspects of dental form were constant enough to identify the remains as probably a single species, the gradual evolution of size within this lineage appears as a long-term evolutionary trend. Whether the factors that led to this trend may explain earlier or later changes in tooth size is not yet known, but it seems clear that even in a long-lasting, successful hominid species, substantial evolutionary changes may take place.