Written by
C. David Kreger
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.
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
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