A Very Brief History
The etymology of the word Australopithecus
is, "southern ape," which harkens back to the name given to the
original specimen. In 1924, Raymond Dart examined the strange, immature
skull of a small-brained creature extracted from the cave breccia of a
South African mine. This was the first discovery of an Australopithecine,
and was named Taung, after the site where it was found; its species
designation Dart gave it was apt-- Australopithecus africanus, or
"southern ape from Africa". The acknowledgement of Taung as a
member of the human lineage remained hotly contested for many years after
the initial finding, primarily because of preconceived notions of which
characteristics embodied the human precursor. Also, many researchers of
Dart's time were convinced that the human line started in Asia and not
Africa, despite the paucity of evidence from both regions. Additionally,
with the previous discoveries of a fossil "hominid" called Eoanthopus
dawsonii, the phylogenetic position of Taung (as well as many other
fossils) remained questionable. It was not until 1953 that E. dawsonii,
more commonly known as the Piltdown Man,
was exposed as a scientific fraud (to read more about the Piltdown hoax,
click here).
The Piltdown hoax did much to reveal preconceptions and underlying motives within the study of human origins. It too perfectly resolved a primary issue - placing the evolution of the brain ahead of locomotory developments - and remains one of the most interesting case studies of the scientific establishment. However, after the discovery of the hoax, once again our origins had to be refigured.
Introducing the Australopithecines
Ardipithecus ramidus
The earliest fossil hominid - while not classified as an Australopithecine - reaches back in time to 4.4 Mya. This specimen, discovered in 1992 by Tim White (University of California at Berkeley) off the Middle Awash River in Ethiopia, has been variably reported as an ancestor to all hominids, or the ancestor to hominids and modern chimpanzees.Australopithecus anamensis
The genus Australopithecus spans a portion of geologic time from approximately 4.2 Mya to about 1.0 Mya. The earliest form, a species called Australopithecus anamensis, was only recently announced by Meave Leakey from the Kenya National Museum (1994). Dated from between 4.2 and 3.5 Mya, these fossils exhibit characteristics of the postcranium which suggest that A. anamensis was bipedal, although dental and mandibular morphology indicates a more apelike physical disposition. These fossils have been found in the western Lake Turkana region in northern Kenya.A. afarensis
The most famous specimen of Australopithecus - "Lucy" - belongs to the species, A. afarensis. Found at sites in East Africa (e.g., Laetoli, Hadar), A. afarensis - which spans a time range between 3.8-2.8 Mya - has provided a great deal of fossil information concerning the morphology of early hominids. Most of the characteristics are continuous, and their condition within A. afarensis lies somewhere between other hominids (relatively lower), and chimpanzees (relatively higher):- subnasal prognathism (degree of facial projection beyond the borders of the braincase whe viewed in profile)
- canine size dimorphism
- diastema
- endocranial capacity (approximately 404 cc, which is very close to modern chimpanzees)
- compound temporal-nuchal crests
- sagittal crest
- V-shaped dental arcade
Later Australopithecines
The remaining groups of Australopithecines can be divided along two lines: geographic distribution, and general morphological features. By geography, these hominids can be subdivided into South African and East African groups; along morphological lines, they can be grouped as either gracile (lightly built), or robust (heavily built).
| Species | Geographic Location | Temporal Distribution | Morphology |
|---|---|---|---|
| A. africanus | South Africa | 3.6 - 2.2 Mya | gracile |
| A. robustus | South Africa | 2.0 - 1.4 Mya | robust |
| A. boisei | East Africa | 2.2 - 1.2 Mya | robust |
| A. aethiopicus | East Africa | 2.5 - 2.2 Mya | robust |
South African Forms
A. africanus
A. africanus includes both Sts 5 (named "Mrs. Ples"), and the previously mentioned Taung child. A. africanus is very similar to A. afarensis in postcranial anatomy, with elements of the pelvis, ribs, and vertebrae exhibiting similar characteristics. The cranium of A. africanus is somewhat different, with a higher degree of cranial flexion and no crests (sagittal or compound T-N), as observed in A. afarensis. These specimens have been found predominantly at the South African sites of Sterkfontein and Makapansgat, in addition to Taung.
A. robustus
A. robustus is also within the genus Australopithecus; however, these fossils differ markedly from the more gracile forms. Their crania exhibit remarkably well-fortified structures and buttresses to counteract increased forces in mastication (chewing). Some of these characteristics include a large sagittal crest, as well as flaring zygomatic arches (cheekbones), both of which provide additional leverage and anchor points for the muscles of mastication. Additionally, the teeth of A. robustus are very large, particularly in comparison to the anterior (front) dentition.
East African Forms
A. aethiopicus
A. aethiopicus, discovered by Alan Walker (Penn State University), stirred quite a controversy over the phylogenetic arrangement of the hominid lineage. Until this discovery, most phylogenies had the Australopithecines evolving down one exclusive lineage, while Homo evolved separately down another (although both shared a common ancestor, presumably A. afarensis). A major portion of this puzzle included a tight relatioship between the robust Australopithecines, an arrangement which required rethinking because of the A. aethiopicus specimen, KNM-ER 17000 (aptly named the Black Skull). Because it bore a number of similarities with A. afarensis which suggest its placement as an intermediate form between A. afarensis and A. boisei, the previous arrangement with the South African A. robustus became questionable. Were the South African A. robustus fossils still part of a common lineage including A. boisei, or could South and East African Australopithecines have evolved independently of each other?
A. boisei
A. boisei, the East African robust form, is thought to have descended from A. aethiopicus. This species exhibits a hyper-robust morphology, with exaggerated features similar in kind to those of A. aethiopicus, but far greater in degree. The massive dentition is coupled with a sagittal crest; increased dishing of the face, or orthognathism, characterizes these forms. This reduction in postnasal facial projection probably came as a result of increased demands to combat masticatory forces. Specimens include OH 5 and KNM-ER 406, among others. Whereas earlier interpretations of their dental specialization offered dietary shifts (e.g., consuming a tougher, lower quality diet), recent analyses of dental microwear suggest that A. boisei incorporated a diverse range of foods in its diet.
Early Homo
The first specimens of a different type of hominid emerged approximately 2.3 Mya in East Africa. These hominids exhibited a different suite of characteristics, including:
- higher cranial capacity
- reduction in tooth size (particularly molars and premolars)
- reduction in masticatory apparatus
- parabolic dental arcade
- thinner tooth enamel than Australopithecine species
- less prognathic facial profile
 |
These specimens have been
traditionally attributed to Homo habilis, although recent
investivators have alleged that perhaps two (or more) species are
represented in the assemblage commonly called H. habilis.
In addition to the above characteristics, early Homo also appears concurrently with a stone tool technology known as the Oldowon Industry. |
Bipedal Locomotion
The principle feature which binds all hominids together into a unique group is their choice of locomotion. All hominids exhibit features of their anatomy which indicate habitual bipedalism. There are a great many features which provide direct or subtle evidence for this behavior; a some of the main features of the skeletal system include:
- shape and proportioning of the pelvic girdle
- positioning of the head and neck of the femur
- position of the big toe (abducted, as opposed to divergent)
- position of the foramen magnum, the hole on the underside of the cranium which transmits the brainstem and spinal cord
Other features include asymmetry of the
distal femoral condyles and proximal tibial condyles; relative positioning
of the ischium and the iliac crest; angle of the femoral neck.
Additionally, reconfiguration of muscular attachments, especially the hip abductors,
is required to maintain proper
balance while moving in a bipedal gait. These characteristics, when
observed in fossilized skeletal elements, argue for an organism which
utilized a highly specialized mode of locomotion shared with all human
beings today.
Theories on the Origin of Bipedality
| Locomotor efficiency | Two researchers from UC Davis - Peter Rodman and Henry McHenry - examined the locomotor efficiency of bipedalism. They found bouth bipedalism and quadrupedalism to be similar in energy costs; however, examining out closest living relatives (the Great Apes) suggests that perhaps early hominids adopted a similar suspensory posture, from which the move to bipedalism could be facilitated. |
|---|---|
| Thermoregulation | Peter Wheeler (Liverpool John Moores university) suggested that an erect posture was adopted by early hominids to combat high temperatures, especially when travelling exposed to direct sunlight (as in open savanna). The benefits of bipedalism would be twofold: reduction of exposed area of the body to sunlight, along with an increased exposure to cooling winds (Click here to see a drawing of this). |
| Freeing hands for other uses | The benefits of being able to transport things manually are obvious. Some species of primates have adapted mechanisms to perform these tasks (e.g., cheek pouches in Cercopithecines); however, the ability to carry a resource either to save for later, or to consume in a safer location, would provide allow for flexibility in planning for early hominids. |
| Postural feeding | Kevin Hunt (Indiana University) recently proposed that the emergence of bipedalism came as a postural feeding adaptation. Observing common chimpanzees in the wild, he determined that, although they rarely walk bipedally, when they choose to adopt a bipedal position it is almost always in the context of feeding on fruit from small trees. This increases foraging efficiency, particularly when fruiting trees are spread about; feeding chimpanzees can shuffle over to the next tree, rather than get down on all fours, got to the next tree, and get back up. |
A new member of the family - A. garhi
Found at and around a site called Bouri, in Ethiopia, this species was only recently named. The general morphology of this hominid is similar to the other Australopithecines, though A. garhi has a sagittal crest and their canines are generally larger. And surprisingly, there is evidence that this species of Australopithecine used stone tools, a trait generally associated only with the genus Homo. The discovery of this species prompts researchers to ask new questions about the lineage leading to Homo sapiens sapiens, since it is dated at just about the time to be the ancestor of the first Homo species.
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