Phylogeny of Australopithecus

This essay will be a report on my phylogeny of the australopithecus. It will be base on data gained from the analysis of the dental morphology of the austraopithecus's, the structure of the head, the dates and brain size which correspond to the each australopithecine all in relation to the chimpanzee and the modern human. The phylogeny I will be supporting is as follows;

4m.y.a 3m.y.a 2m.y.a. 1m.y.a

A. robustus

A. boisei

A. aethiopicus

A. africanus


A. afarensis


The significance of looking at the dental morphology (Jurmain et al. pg. 129) is to see the relationship of each australopithecus to the modern human and place each in accordance to their similarity to the human dental structure.

Modern man has a small rounded or parabolic dental arcade with a dental formula of 2:1:2:3. The dental formula is consistent through out all Old World monkeys, apes and hominids (Cambridge, pg. 56). The incisors are vertical and large along with the canine. The canines are worn at the tip. The molars and jaw are relatively small. The teeth in general have thick enamel.

In respect to dental structure of the australopithecine's none have anything else in common to Homo except for the A. africanus that has the distinct vertical incisors. This gives A. africanus a link directly to Homo.

Australopithecus afarensis have lower premolars that resemble "sectorial premolars"; a tooth used for shearing which is common in monkeys and apes. This form of premolar is not found in any of the other Hominoids. This gives evidence that A. afarensis is closer linked to prosimians then that of Homo.

A. aethiopicus, A. robustus and A. boisei have very thick jaws and enamel. The thick enamel is a characteristic of the modern man, which can be related to the indication that they are part of the line leading to modern man. Also, the semi-rounded dental arcade is another indication. However, evolution of thick enamel is an affect of body size and time that the teeth have to function ( Cambridge, pg. 59). Whereas, modern man probably developed thick enamel due to combat against their tough diet. The large jaw is part of the powerful chewing machinery these australopithecine's had. This trait is not part of modern man; being relatively weak in the chewing muscles and having a relatively small jaw. Therefore, the conclusion is that this set of Australopithecus were not part of the modern mans lineage.

A. africanus has a semi-rounded dental arch however not as much as modern man but closer to that of A. aethiopicus etc. which places it behind them in this area of development. Therefore, the resulting theory is that modern man and A. aethiopicus etc. branched off from A. africanus.

A. afarensis has a dental arcade which is "U-shaped"( Class Notes, table) and denotes the australopithecus with the closest relationship to the prosimian and the first in the phylogeny with respect to dental morphology.

STRUCTURE OF THE HEAD ( Cambridge, pg. 238)

The shape of the head has altered in many different areas, which I will compare in order to get a general idea of where each australopithecus is placed in the phylogeny time line. The areas that I will compare are the face prognostism, the forehead length, and the zygomatic arch.

The face of a chimpanzee is very prognostic and the face of a modern man is basically flat. This will be the scale the australopithecine's will be measured against from archaic to modern. The A. afarensis is the most prognostic, which equals the most archaic placing it in the beginning . The A. africanus has a little shorter face, less prognostic, then the A. afarensis. In comparison to A robustus and A. boisei the protrusion of the face is greater in relation to the proportion of head size and prognostism. This results in the order of 1)A. afarensis, 2)A. africanus, 3)A. atheopithicus, 4)A, robustus and A. boisei. The A. africanus is the probable bridge between africanus and the other australopithecine's due to the shortening of the face.

The present s of a high forehead begins to appear in the A. africanus. The change is advancement to the low, flat forehead of the A. afarensis. The A. aethiopicus and etc. do not possess anything close to the high forehead of the modern man or to the A africanus, which is another indication that they are not part of the Homo lineage but another branching of the A. africanus.

The zygomatic arch on the A. afarensis is thick, angled and ending just beside the eye socket. The arch on the A. africanus is horizontal and ends a little past the eye socket. In the A. aethiopicus and etc the arch ends far forward on the upper jaw. In contrast, modern man has an arch, which basically ends at the same point on the face as the A. africanus. The arch however, is not as level horizontally as the A. africanus. The relevance to the angle being horizontal in both the A. robustus and etc. along with modern man is evidence that they did not branch directly off A. afarensis. Instead they branched off of A. africanus who has the horizontal arch and the arch ending closer to the front of the face that is greater developed in the A. aethiopicus and etc, especially in the A. robustus. The differences again lead to the conclusion that Homo is a branch off A. africanus and another branch is evident in the comparative characteristics of the A. aethiopicus and etc.


A. afarensis A. africanus A. aethiopicus A. boisei A. robustus Homo

Brian Size c.c. 400-500 400-500 410-500 410-530 530 500-1700

Known Date m.y.a >4-2.5 ~3-<2.5 ~2.7-~2.2 2.6-1.2 2.0-1.0 2-present

(Time, pg. 58-60 & Internet site & Cambridge, pg. 236)

The dates also follow my phylogeny and the brain sizes increase according to the placement of the australopithecus with respect to time.


The A. afarensis is most definitely the oldest australopithecus and the closest to the chimpanzee which connects him to the prosimians due to his prognostism, low forehead etc. The Dates signify the phylogeny coincides but points out that A. boisei came about before A. robustus, which would be hard to figure out through the things I have analyzed. The A. africanus is the bridge between the australopithecine's due to the transitional characteristics it displays, for example the shorter face and the horizontal zygomatic arch; this places it after the A. afarensis. There is definitely a branching which occurs and leads to the A. aethiopicus and etc. and and the path leading on the Homo and eventually modern man (Homo sapien sapien).

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