Australopithecus anamensis: overview

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Note: I wrote this post in 2004. Much additional evidence about Au. anamensis has come to light since that time. Later posts present more up-to-date information on this species.

The sample from Kanapoi, along with other smaller samples predating Laetoli, at 3.7 million years, exhibit several differences from the later A. afarensis sites. Since jaws and teeth are best known for these early hominids, these dental features have been used extensively to define both samples and species. The major differences involve morphology of the mandible, including:

  1. long mandibular symphysis, angled posteriorly
  2. long mandibular surface behind the incisors, called a postincisive plane
  3. long parallel tooth rows

There are also several dental differences, including:

  1. more projecting canines
  2. more asymmetric P3
  3. small P4

Postcranially, the Kanapoi sample and other samples from the same time exhibit no specific morphological differences from the much later Hadar hominids. However, the three postcranial fragments from which body mass can be estimated, including a tibia, a distal humerus, and a radius from Sibilot Hill, all result in a body mass estimate larger than the mean size for male remains at Hadar. These findings may imply a larger body size for these earlier hominids, or they may merely represent the chance discovery of three large specimens from these early sites (Ward et al., 2001).

The dental and mandibular differences in the earlier sample support to some extent the hypothesis that the earlier sites represent a distinct species. This species was named Australopithecus anamensis by Leakey and colleagues, where "anam" means "lake" in the local Turkana language. Despite the differences, however, these samples represent a hominid that is clearly similar in dental adaptation to the later A. afarensis sample, including:

  1. large molars with thick enamel
  2. tall mandible, over twice the height of the molar roots
  3. relatively small canines compared to Miocene apes, with no diastema

Most dental differences between the Kanapoi dental sample and later samples are differences of degree, with the Kanapoi hominids exhibiting a form that is clearly like later hominids, but slightly in the direction of Miocene apes, or in some cases in the direction of the Aramis sample. The overall appearance of the Kanapoi dental sample is that it is a primitive form of the later Laetoli, Maka, and Hadar samples. No significant postcranial differences---other than possibly body size---separate the samples. It is therefore reasonable to support the hypothesis that the major sites of Kanapoi, Laetoli, Maka, and Hadar represent a single evolving lineage, developing an adaptation to greater chewing over time. This hypothesis has been supported by analysis of temporal change among the A. afarensis samples (Lockwood et al., 1999), and would imply that the hominids from around 4.0 million years ago in fact represent an early segment of the evolutionary species A. afarensis. Naturally, if it were shown that a significant speciation had taken place between the early and later samples in this lineage, perhaps involving the appearance of hominids in South Africa or Central Africa, then the species name A. anamensis would be justified for Kanapoi and other contemporary samples.