Australopithecus anamensis essay


February 12, 2010


Australopithecus anamensis

Fossils attributed to Australopithecus anamensis (which means “southern ape of the lake” from “anam,” meaning “lake” in the Turkana language) have been recovered from sediments at Kanapoi and Allia Bay near Lake Turkana in Kenya.  These fossils, which have been dated to between 4.2 and 3.9 million years ago using radioisotopic dating methods applied to volcanic sediments, are significant because they represent the earliest indisputable evidence of obligate bipedality in the human fossil record.  In addition, the morphology of the skull of Au. anamensis provides a glimpse of the evolutionary changes that represent the transition from earlier, more primitive (i.e., ape-like) hominins—such as Ardipithecus ramidus—to later, more derived (i.e., human-like) species—such as Australopithecus afarensis.

Although Au. anamensis is represented by both cranial  and postcranial (ifrom the parts of the skeleton other than the skull) remains, fossils representing the skull outnumber those of the limbs and trunk.  Fossils of the jaws and teeth are particularly well represented.  Au. anamensis possesses some features in the dentition—i.e., relatively large, broad premolars and molars with relatively thick tooth enamel—that are shared with other species in the genus Australopithecus and early fossil representatives of the genus Homo.  Other features found in the teeth of Au. anamensis, however, differ from those found in later species in the genus Australopithecus and more closely resemble the condition found in living apes.  These features include the size of the teeth in the front of the jaws (which are wider than in later australopiths [species in the genera Australopithecus and Paranthropus]), the size and shape of the lower third premolar (which is larger and single-cusped, unlike the smaller and double-cusped condition found in later australopiths), the shape of the upper canine (which is symmetrical when viewed from the side, unlike the asymmetrical profile found in later australopiths), and the shape of the first deciduous (milk) molar (which, unlike later australopiths, does not closely resemble a permanent molar).  Canine size is smaller in Au. anamensis than in the genus Ardipithecus, but these teeth (especially their roots) are larger than in Au. afarensis.

The skull of Au. anamensis is represented by fossils of the mandible (lower jaw), maxilla (the bone that comprises the upper jaw and the majority of the face), and a single temporal bone (the bone that surrounds the ear and forms part of the side of the skull).   Like the teeth, these skull fossils bear many primitive, ape-like features.  The dental arcade is generally U-shaped when viewed from above, with the molars and premolars located directly behind the canines.  This ape-like shape contrasts with the more parabolic-shaped dental arcade found in Au. afarensis and later hominins.  In addition the front of the mandible is ape-like in shape in side view, receding backward from top to bottom.  In later hominins, this region of the mandible is more vertically oriented.  Due to the large size of the upper canine roots (see above), the rims of the nasal opening are rounded; in Au. afarensis, this rim is sharper.  Finally, similar to that found in living apes, the bony ear opening in Au. anamensis is relatively small in size.

The postcranial elements of Au. anamensis include fossils of the hindlimb ) and forelimb, including portions of the wrist and hand.  The tibia fossils are of particular importance because they demonstrate that this species walked bipedally.  Both the knee- and ankle-ends of the tibia (shin bone) are thickened and the tibial plateau, where the tibia connects to the femur (thigh bone), is larger than in living apes.  These features prove that Au. anamensis was a biped because they indicate that more weight was borne on the tibia, a feature requisite for bipedality.  In addition, the shaft of the tibia is straight and the end of this bone that articulates with the ankle is upright in contrast to the angulation found in these regions in living apes.  The configuration of these joints demonstrates that the knee and ankle joints were reorganized to accommodate a bipedal gait.  The single wrist bone of Au. anamensis suggests that this species had limited ability to rotate the bones of the hand on those of the wrist, similar to later australopiths and species in the genus Homo, but unlike living apes.  Finally, estimates of body size suggest that, at roughly 47-55 kilograms, Au. anamensis was slightly larger than Au. afarensis and Ar. ramidus and sexual dimorphism (i.e., size and shape differences between males and females of the species) was similar to that found in Au. afarensis.

The evolutionary relationships between Au. anamensis and Au. afarensis have received a great deal of scholarly interest.  The fossils of Au. anamensis from Kanapoi are geologically older than those from Allia Bay and are more similar to Ar. ramidus and living apes.  In addition, the Au. anamensis sample from Allia Bay is more similar to the older sample of Au. afarensis fossils found at Laetoli, Tanzania than they are to the younger sample of Au. afarensis fossils from Hadar, Ethiopia.  These facts have led some researchers to suggest  Au. anamensis is the direct ancestor of Au. afarensis and the sequence of fossils from Kanapoi, Allia Bay, Laetoli, and Hadar can be considered a single species.  The trends suggesting that this sequence of fossils represents a single species come predominantly from the size and shape of the mandible and lower third premolar.  For clarity and to formalize the differences found in this single species, which would include fossils currently assigned to Au. afarensis and Au. anamensis, however, most scholars continue to regard the fossils as separate species—i.e., the fossils from Allia Bay and Kanapoi are referred to as Au. anamensis and the fossils from Laetoli and Hadar (and a handful of other sites, see essay on Au. afarensis) are referred to as Au. afarensis.

The environments in which Au. anamensis lived have been reconstructed as forested habitats near streams.  Combined with evidence from other early purportedly bipedal hominin species (e.g., Ardipithecus kadabba and Ar. ramidus), these environmental reconstructions argue strongly against the once wide-held idea that bipedalism initially evolved and flourished in open savanna environments.