Australopithecus anamensis


HISTORY

Though not recognized as such for 30 years, the first Australopithecus anamensis discovery occurred in the Kanapoi region of East Lake Turkana in 1965 by a Harvard University expedition. The initial find consisted of a partial left humerus [Johanson and Edgar, 1996]. Aside from a solitary molar discovery in 1982, virtually no further A. anamensis specimens were found until the early 1990′s, at which time, Meave Leakey and other affiliates of the National Museums of Kenya organized a research team for the Kanapoi region [Coffing, et al, 1994].

CLASSIFICATION

These 4 million year-old hominid fossils from East Turkana, Kenya, were initially, albeit tentatively, classified as members of Australopithecus afarensis by Kate Coffing, Craig Feibel, Meave Leakey, and Alan Walker in 1994. Meave Leakey and associates, following successful field seasons in the early 1990′s, and taking note of the apparent differences between these early Kenyan hominids and typical members of Australopithecus afarensis, reclassified the specimens in 1995. The new species designation was Australopithecus anamensis (“anam” = “lake” in the Turkana language).

GEOLOGICAL SETTING

All A. anamensis fossils were found within a single region, East of Lake Turkana, which is dominated by Pliocene sedimentary sequences. This area includes Kanapoi and Allia Bay, the two most important locales from which these hominids come. The Kanapoi sequence contains strata which were deposited during the an interval between 4.17 and 3.5 million years ago [Leakey, et al, 1995]. This sequence has produced hominid remains from two different levels. The lower level’s location, between two layers of volcanic ash (dated to 4.17 and 4.12 million years, respectively), allows a secure date of 4.1 million years to be assigned. The lower level was deposited at a time when an ancient lake, Lake Lonyumun, dominated the landscape. Though most of the Australopithecus anamensis specimens were taken from this level, the assemblage does not include any postcranial material, and mainly consists of elements of this hominid’s dentition [Andrews, 1995].

The upper level at Kanapoi can be dated between 4.1 and 3.5 million years ago. Though these chronological boundaries seem to be very secure, it is not clear how closely related the upper and lower levels may be temporally [Leakey, et al, 1995]. Fossils here were found in the context of a small river channel below the locally occurring Kanapoi Tuff, which is about 3.5 million years old. This level has produced important postcranial remains, including the distal humerus from 1965 and a largely complete tibia [Andrews, 1995]. Abundant vertebrate faunas from Kanapoi suggest an open, bushy, or wooded environment [Leakey, et al, 1995], which most-likely varied substantially from place to place.

Adjacent to Kanapoi lies Allia Bay. This locale, too, has produced several representatives of Australopithecus anamensis, consisting of mandibular fragments and isolated teeth [Coffing, et al, 1994]. Fossils from Allia Bay were obtained either within or below the Moiti Tuff, which is 3.9 million years old. Sedimentation occurred within the proximity of an ancient meandering river called the Proto-Omo [Coffing, et al, 1994].

Non-human faunal remains from Allia Bay probably came from a few different habitats and water transport is purported to have been vital to the bone accumulation process [Coffing, et al, 1995]. The presence of both closed-habitat and open-habitat species is indicative of this possibility. Extensive weathering, a potential result of water transport, is apparent upon some of the fossil specimens. Based upon those bones that showed few signs of weathering, Coffing, et al [1995] suggested a transitional environment, composed of elements of grassland, bushland and forest, at the site.

ANATOMY

Australopithecus anamensis displays a range of primitive and advanced morphological features. Though specific comparisons between A. anamensis and Ardipithicus ramidus would be difficult, due to the small skeletal collections that have been obtained for both fossil hominids, a general similarity seems to be clear. Both species have retained ape-like crania and dentition, while also exhibiting rather advanced postcrania, more or less typically hominid-like in form. The anatomical description recorded below is a synthesis of the reports by Coffing, et al, 1994, and Leakey, et al, 1995.

The dental apparatus of A. anamensis is markedly ape-like. This hominid exhibits many primitive features, in particular, relatively large canines, parallel (rather than parabolic) toothrows, and asymmetrical premolars and molars. Mandibular remains display a robust bony shelf posterior to the incisors, and an infero-posteriorly sloping mandibular symphysis. These, too, are probably primitive traits, as they are typical in Miocene and modern apes. A shallow palate is another primitive feature, though this condition is shared with Australopithecus afarensis, a later hominid.
In general, the dentition of A. anamensis is very primitive for a hominid. However, derived features are present, features which are not evident in Ardipithecus ramidus. Tooth enamel is very thick, and molars are buccolingually expanded, producing a smaller length to breadth ratio, which approaches the condition seen in A. afarensis (length: breadth is 1.49 in A. ramidus, 1.4 in A. anamensis, and 1.2 in A. afarensis [Leakey, et al, 1998]).

The postcrania of Australopithecus anamensis tells a different story. These skeletal elements are the ones that truly give away the status of this ancient animal. It was undoubtedly a bipedal hominid. The excavation of a tibia from the upper Kanapoi stratum has provided the best clues concerning A. anamensis’ locomotor abilities. The distal tibial portion is thick in areas that are subject to high forces of stress during bipedal locomotion [Johansen and Edgar, 1996] , and the articular surface (which joins with the talus) faces inferiorly (rather than antero-inferiorly, which occurs in quadrupedal apes). Additionally, the proximal articular condyles (which join with the femur to form the knee joint) are deeply concave, broad in the antero-posterior plane, and equal in size. All of these conditions are still present in modern humans.

Curiously, this tibia, and the aforementioned humerus of A. anamensis may be more similar to those from members of the genus Homo than they are to Australopithecus afarensis [Andrews, 1995]. This has not been decisively shown, but, if true, would bring up a very interesting possibility. It may very well be possible that we are more closely related to this 4 million year-old hominid than we are to the widely successful later hominid – Australopithecus afarensis. Right now, this is purely speculation. On the other hand, the irregularity of the fossil record provides no completely lucid pictures of our evolutionary history, and the likelihood that many Australopithecine (or more broadly, Hominine) sister-branches existed at one time is excellent. A single hominid lineage has persisted during the past few thousand years, but the overall variety of the Hominidae may have been quite extensive at many other times and places during the past five or six million years. The potential relationships between presently known hominid groups are astoundingly large.

 


BIBLIOGRAPHY

Andrews, Peter (1995) Nature, 376:555–556.

Coffing, K, C Feibel, M Leakey, and A Walker (1994) American Journal of Physical Anthropology, 93:55–65.

Johansen, D, and B Edgar (1996) From Lucy to Language. Simon and Schuster Editions: New York, New York.

Leakey, M G, C S Feibel, I McDougall, and A Walker (1995) Nature, 376:565–571.

Leakey, M G, C S Feibel, I McDougall, C Ward, and A Walker (1998) Nature, 393:62–66