Characteristics of Australopithecus africanus: 2-2
Characteristics of Australopithecus africanus:
Skull: Taken as a whole, the skull is a combination of small brain case and large jaws. The brain case lacks high vertical forehead of Homo sapiens and the high roundedness of the skull vault. This gives a simian appearance. But in bone feature it contrasts with pongidae and matches with hominidae.
The supraorbital height index shows that the relative high exceeds range of variation in anthropoid ape and actually comes within the range of hominid skull.
The occipital torus and the inion occupy a low level as in hominid skulls. In adult apes this occipital torus forms a crest high up the occipital aspects of the skull, thus extending considerably the nuchal area for attachment of neck muscles. In Australopithecus the nuchal area is restricted as in homo.
In Australopithecus the occipital condyles are forward in position relative to the total length of the skull and the auditory aperture. The occipital condyles of pongidae are behind the midpoint of the cranial length and also behind the auditory apertures.
In all the Australopithecus skulls in which mastoid region are sufficiently well preserved, there is a well marked pyramidal process typical of hominid form.
The brow ridges are poorly developed compared to that of the apes.
The facial skeleton is however, large in relation to the brain case.
The cranial bones of Australopithecus robustus are thicker than the Australopithecus africanus and the sagittal crest is developed. The zygomatic arches are expanded and flared.
The jaws are also relatively large but no simian shelf.
Cranial capacity:
In cranial capacity, the range of variation is quite considerable. The skull of Australopithecus afarensis and Australopithecus africanus ranges from 400 to 500 cc and Australopithecus robustus ranges from 410 to 530 cc.
Thus the general endocranial size of Australopithecus does not differ markedly from those of gorilla and chimpanzee. However in relation to the body size Australopithecus shows larger brain capacity proportion than the apes.
Dentitions:
Australopithecus dentition is essentially of hominid type. In all the adult specimens the dental arcades is evenly curved as in Homo sapiens with no diastimic intervals. The canine is reduced in size and spatulate in forms. Canines and incisors are almost in the same level.
The anterior upper premolars have two roots, the anterior lower premolars are non-sectorial and have one root (sectorial and two roots in apes). There is good evidence for the study of the immature specimens that the order of eruption of permanent teeth agrees with the Hominidae. In Homo sapiens the canine erupts before the second molar. This is reverse in the apes.
Pelvis:
The total morphological pattern of the pelvis of Australopithecus is hominid.
The anterior inferior iliac spine is strongly developed.
The ilium is much relatively broader than in the apes. The broad ilium lengthens the attachment of the gluteus muscles that makes important in maintaining balance of the trunk on the legs. The gluteus maximus becomes a fourfold extension muscle which is needed for erect bipedal locomotion. Whereas in monkeys and apes the gluteus maximus is an abductor muscle.
The posterior extremity of the iliac crest is extended backward and downward in the sacral area.
The angle of the sciatic notch (depression) is more acute in the pelvis of man and Australopithecus than that of the great apes. This is resulted in part due to development of prominent ischial spine.
Ischial tuberosity is relatively high in man and Australopithecus and closer to acetabulum than that of the apes.
Sacrum is shifted upward and closer to the acetabulum forming more or less a basin or funnel shaped pelvis which indicates the character of erect posture.
In all these characters the pelvic bone of Australopithecus contrasts with the apes and shows pattern for erect bipedalism. However the pelvis of Australopithecus is not fully developed like that of the modern man. Ischail tuberosity is not as quite closely approximated to the acetabulum as it is in Homo sapiens. The anterior superior spine extends further forward.
Limb bones:
In addition to the pelvis, the Australopithecus finds include many fossilized bones of leg, hand and feet. One group from Olduvai Gorge includes twelve of the major bones of a single foot and so perfectly preserved that the details of the foot are readily reconstructable.
The foot of Australopithecus is much smaller than a human foot. But the metatarsals are hominid and the big toe points forward and not splayed out. The carpe bones and phalanges found in Olduvai Gorge also show hominid characters.
Two specimens of the lower part of femur found at Stenfontein have been described. They show a combination of features such as the obliquity of the shaft, the alignment of the condyles, the forward prolongation of the intercondyler notches which confirm to the hominid femur.
Upper extremity of one specimen, however shows certain Pongidae features, such as relatively small size of the articular head. A tibia and fibula found in Olduvai have relatively straight shafts conforming to hominid character.
Difference between Australopithecus africanus and Australopithecus robustus:
Character
Australopithecus africanus
Australopithecus robustus
Height
1.1 - 1.4 m.
1.2 - 1.4 m.
Weight
30 - 60 kg.
40 - 80 m.
Physique
Light build, probably relatively long arms, more human features; Probably less sexual dimorphism.
Very heavy build, relatively Long arms; marked sexual dimorphism.
Brain size
400 - 500 cc
410 - 530 cc
Skull form
Low, flat forehead, brow ridges less prominent.
Prominent crest on top and back of skull, long broad flattish face strong facial buttressing.
Jaw teeth
Small incisor like canine, no gap between upper incisors and canines, larger molars.
Very thick jaws, small incisors and canine, incisors and canine, very large molars.
Phylogenetic Position:
Currently there are three main phylogenetic trees each with its own cadre of proponents. The problem of whether Australopithecus africanus represent early grade in homo lineage or a small brain hominid arising as a separate product of Ramapithecus radiation is yet to resolve.
At the same time we also do not know how the two dimorphic forms i.e. gracile and robust Australopithecines are to be classed phylogenetically. Whether they reflect polytypic nature of single taxon or systematic enough to conceive as two taxa is still a question.
There is no generally accepted phylogenetic tree for human evolution. There are several proposal branching perhaps the major distinction between them is whether homo is perceived as late arrival or as early arrival.
In some phylogenetic, robust is consider to be on the same evolutionary line with Australopithecines and some consider robustus to be separate from the earlier gracile species in which Paranthropus robustus/promethecus is equivalent to Australopithecus robustus and Homo africanus equivalent to Australopithecus africanus.
This Pliocene and Pleistocene fossils have shown both hominid and ape like features. However the hominid features are overwhelming. The presence of ape like features can be accounted for by way of common inheritance from a hominoid or Pongidae ancestor.
The hominid characters however can be accounted for by way of independent acquisition demonstrating and highlighting the fact that these fossils were on a direct line of human evolution and not Pongidae line.
There are many schools of thought prevailing with reference to the course of evolution these australopithecine have taken. The most important of these are discussed below.
According to Donald C Johnson and Timothy white the east African fossils Australopithecus afarensis split into two branches and australopithecine line represented by A. africanus P. robustus, P. boisei and a hominid line represented by homo habilis, homo erectus and homo sapiens.
This split was supposed to have happened 3 million years ago. The australopithecine line progressively became robust. This pattern generally called two branched theory had its variants also. For some it is A. africanus which is the common link between the australopithecine line and homoline. For still others these two branch represent parallel evolution.
According to this two branch theory A. afarensis gave rise to A. africanus 3 million years ago of the same height living up to two million years ago.
Next arrived A. robustus which showed marked increased in the robustness of the body face , jaws and teeth and it had lined up to 2.3 to 1.8 million years ago. Finally the last and the most robust form A. boisei lived in east Africa from roughly 1.8 to 1 million years ago.
The second branch of this model the homo line also shows a shortening of the face but there is marked decrease in the size of both cheek teeth and the front teeth. There is a massive increase in the size of brain also. This line begins with ta transition form A. afarensis to H. habilis the first hominid who made and used tools and lived in Africa from 2-1.5 million years ago. They had human like teeth and larger brain than australopithecine (750 cc). however we can conclude that hominid evolution may not have been so simple, isolated and clear cut in tis operations.
Instead there is a possibility that three or more hominid lineages may have been evolving and interacting with each other.
This two branch theory was widely accepted till the discovery of the new type of hominid skull - Australopithecus aethiopithecus in northern Kenya inn 1985 by Alan Walker. This skull is considered to be the most robust form ever discovered. It had massive teeth and ape like brain.
The dating of the this specimen indicates that the family of a. boisei didn't evolve in the last leg of the australopithecine evolution as indicated by earlier theory but it originated directly from a. afarensis. Thus the revised theory holds a three line evolutionary sequence. One to boisei line second to Homo and third to africanus, robustus line having A. afarensis as the common ancestor.
The discovery of the youngest australopithecine - A. ramidus has added a new dimension to the three branch theory. It holds that A. afarensis is the common ancestor of Homo, P. boisei and P. robustus but it itself evolved from A. ramidus. With overspecialization of diet, competition for food with H. habilis and the latter's predation along with H.erectus led to the extinction of australopithecine group.
Why are the genera Australopithecus divided into two species?
At first, several discoverers of the various Australopithecus fossils named four to five different genera, but now the consensus of scientific opinion recognizes one genus Australopithecus and two species, one gracile form represented by Australopithecus africanus and other robust form represented by Australopithecus robustus, originally called Paranthropus. This division is made on the basis of their morphological feature.
The gracile form are slenderly built and smaller; gracile form have probably relatively long arm, probably less sexual dimorphism, high forehead, shorter face, brow ridges less prominent. The incisors are also small with no gap between upper incisor and canines and have big molars; cranial capacity ranges from 410-530 cc.
The robust form are large and roughly built. They have relatively long arm with moderate sexual dimorphisms, crest on top of skull, flattish face. The robustus have thick jaws, small incisors and canine, very large molars; the premolar is also large like molars. The cranial capacity ranges from 410- 530 cc.
What is the position of Australopithecus in the line of human evolution?
Different scholars have given different opinion about the position of Australopithecus in the line of human evolution. According to Johanson and White Australopithecus afarensis is regarded as the common ancestor of Australopithecine and Homo.
Australopithecus afarensis rise to Australopithecus africanus and to Homo in one hand and directly to Australopithecus robustus in the other hand. Here Australopithecus africanus act as a transition form from Australopithecine to Homo.
According to L.S.B. Leakey, the common ancestor of both homo lineage and Australopithecine divided quite early. He considers Australopithecus afarensis as not the common ancestor. The homo lineage starts separately after 3 million years and Australopithecus a little bit early but also only 3 million years.
What is T. Robinson Dietary hypothesis?
Going by the dental fossil evidence found in East and South Africa, there are two adaptive groups which can be extended to families. These two adaptive groups are gracile and robust. They are quite distinct in morphological, ecological relationship and behavior.
T. Robinson sees a dichotonomy in cranio-dental form separating these two Australopithecines groups. Robust group has the large post canine teeth crowns, thick enamel, larger occlusal surface, well developed root and relatively flat occlusal surface (greater enamel has been damage).
Crowning and size reduction of the front teeth points to a primary dietary function of crushing and grinding. The massiveness of the entire mastigatory apparatus including the musculature and the wear of the teeth indicate use of a diet of rough material which needed much chewing.
Hence robust was herbivorous or vegetarian. Gracile form shows none of these features rather their post canine teeth are smaller and anterior teeth larger. The needs for these in meat eating would be substantial until tool making had reached a fairly advanced level.