Australopithecus afarensis

The species A. afarensis is one of the better known australopithecines, with regards to the number of samples attributed to the species. The species was named by D. Johanson and T. White in 1978.

Australopithecus afarensis | return to species list

Written by C. David Kreger
Introduction

The species A. afarensis is one of the better known australopithecines, merely with regard to the number of samples attributed to the species. The species was named by D. Johanson and T. White in 1978. This lead to a heated debate over the validity of the species (seen in a 1980 issue of Science), with the species eventually being accepted by most researchers as a new species of australopithecine and a likely candidate for a human ancestor.

Possibly the best-known specimen of afarensis is AL 288-1 ("Lucy"), a 3.2 million year old partial skeleton found in November 1974 at Hadar, Ethiopia. The afarensis can be separated into two chronological categories: early (3.9-3.5 myr) and late (3.5-2.96 myr). Early afarensis includes material from Laetoli and Belohdelie (and possibly Sibilot Hill), with later afarensis known mainly from Hadar and Maka. Other important specimens attributed to afarensis include the AL 333 specimens (such as A.L. 333-105), AL 444-2, AL 129-1A + 1B, the Laetoli footprints, and the type specimen for the species A. afarensis, LH 4.

The Hadar sample is relatively extensive, and shows important differences with earlier samples from Laetoli. This species is also extremely important in that there is good evidence (from both the Laetoli footprints and examination of the lower limbs of the afarensis material) that the species was bipedal in a human-like manner (though this view is not shared by all.) This information for early bipedality "shook up" many complacent views about the origins of bipedality, but is less important (with regards to the earliest bipedal hominid) with the findings of earlier ramidus material that is also bipedal, and the idea that the ancestors of chimpanzees and gorillas was also likely bipedal.

Diagnostic Features

One of the earliest sites often attributed to afarensis is the site of Belohdelie, Ethiopia. This sample consists of a large piece of frontal bone and four associated cranial fragments. This material has been described as afarensis by B. Asfaw, and T. White finds the specimens very similar to the juvenile Laetoli vault pieces (LH 21) and Al 288-1. However, with the discovery of the ramidus material, the specimens should be reexamined, and determined to one or the other, as ramidus shares many features with afarensis, yet is earlier, with some major differences. The specimens show a frontal squama enclosing the frontal portion of the brain of a size similar to many chimpanzees and gorillas, however, there are several major differences between the Belohdelie material and one or both African ape species, including:

The lateral corner of the supraorbital torus is vertically thicker than both common chimpanzees (Pan troglodytes) and bonobos (Pan paniscus).
The roof of the supraorbital torus slopes evenly up the frontal squama, rather than being separated by a sulcus, which is common in the African apes.
The bone of the squama is thicker than the African apes.
The outside of the squama that forms the internal wall of the temporal fossa slopes inward toward the midline, rather than being vertical as in the African apes.
The upper border of this sloping internal wall ends in a temporal line that runs parallel to the back of the supraorbital torus and then angles strongly towards the midline rather than swinging backward at the outside corner of the supraorbital torus and not parallel to it, as in the African apes.

The Laetoli material was recovered at the site of Laetoli, Tanzania, about 50 km south of Olduvai Gorge, mostly between 1974 and 1979 by M. Leakey. Most of the material has been described by T. White. The remains mostly date from 3.76-3.56 myr, with LH 15 as young as 3.46 myr, and are mostly jaws and teeth, but also includes part of a child's postcranial skeleton (LH 21). The type specimen was selected from the Laetoli materials at the time of the species designation over the more complete Hadar specimens due to its distinctive diagnostic features. This specimen lacks the ascending ramus and contains nine teeth, with the roots of several others still in the mandible.

The Laetoli anterior dentition are among the most complete known for early hominids. The canines retain the primitive form of marked difference between sexes, with the males distinguished by greater size and higher, more tapered crowns. The anterior teeth of afarensis are quite large, and among the largest known for any hominid, and similar to chimpanzees, although relatively narrower labioloingually. The differ from chimpanzees, however, in that the hominid maxillary incisors reflect the plesiomorphic condition in which the lateral incisor is much smaller than the medial one. The upper canines of Laetoli generally have 2 wear facets, and 3/4 of the remains show a diastema. Two of the Laetoli mandibles show incompletely erupted dentition, allowing a reconstruction of eruption patterns. These remains show an eruption pattern that is more hominid-like versus chimpanzee-like.

One of the more well-known remains from Laetoli are the Laetoli footprints. There have been many interpretations of the footprints, with different numbers and sex of individuals, and different ideas as to the form of locomotion used to create them. In the end, most people have come to accept the idea that the footprints were made by afarensis approximately 3.56 myr, which were an obligate bipeds. Features which show this include:

Deep impressions showing pronounced heel strike.
Lateral transmission of force from the heel to the base of the lateral metatarsal.
A well-developed medial longitudinal arch.
Adducted big toe, in front of the ball of the foot and parallel to the other digits.
A deep impression for the big toe commensurate with toe-off.

Other important sites where afarensis has been found include the 3.5 myr material from Turwel and the 3.4 myr Maka site. The material from Turkwel includes several wrist bones (WT 22944), which Carol Ward describes as very humanlike, specifically lacking any knucklewalking adaptation. The anatomy of the carpal bones suggest enhanced finger mobility and powerful wrist flexion. This includes a powerful hamate bone indicating a carpal tunnel about twice the size of modern humans. This may indicate a powerful grip showing some arboreal activity still active in the species.

The Maka site has yieldedafarensis remains that include a proximal femur piece, two partial mandibles, a piece of an ulna, and an almost complete humerus. The femur shows a fairly large hominid for this time period (approximately 45 kg), and is usually assumed to be that of a male. The humerus shows a morphology very similar to that of the much smaller humerus from AL 288-1, showing a high degree of sexual dimorphism present in the species. The most complete of the four Maka jaws is VP 1/2, which preserves many of the characters seen in afarensis material, including:

Parallel postcanine tooth rows.
A marked angle of the mandibular condyles long axis, reflecting the angle of the mandibular fossa on the cranial base.
A canine-premolar diastema.
Large canines and incisors.
Molars with ascending size order (i.e., the smallest first).
A triangular third molar crown shape.
Serrated molar roots.

The Maka material is also important in that the proximal femur preserves definite signs of bipedality. These include features such as:

The gluteal tuberosity (attachment locus for the gluteus maximus) is mostly on the back of the shaft like other hominids, rather than on its side like African apes (where it acts as an adductor).
The femoral neck is ling relative to the size of the shaft, a consequence of lateral iliac flare.
The femoral neck is anterior-posteriorly flattened, making it relatively tall, thus, resistant to bending stresses during one-legged support.
The bone thickness on the anterior neck surface is expanded, a response to muscle forces during toe-off and the force transmitted when the leg comes to the ground at the end of its swing.
The neck-shaft angle is low.

The Hadar site in Ethiopia is the largest, latest, and most variable site attributed to the species, with some researchers recognizing only afarensis, and others seeing more than one species. Most seem to have accepted that there is only afarensis, and so the whole sample will be discussed as afarensis. The sample ranges from 3.4 to 2.96 myr, over several different sites, and includes many of the better known afarensis specimens (e.g., AL 288-1, AL 333, AL 444-2, and others.) The sample consists of fossils representing from 40 to 100 individuals.

The paleoecology of the site seems to indicate a forest margin or savanna-woodland environment, a much different environment from the savanna-grassland environment initially assumed by many researchers. There also seems to be several important differences between the dentition of the early afarensis material from Laetoli, and that of the later material from Hadar. These may be related to a different environment, and hence, different foraging opportunities, a change in behavior, a continuation of earlier evolutionary trends, or sample error. The major difference between the two samples includes a mandibular diastema on three out of four of the Laetoli materials, while there is a mandibular diastema on only six of the sixteen Hadar materials; and the fact that the wear patterns on the upper canines differ, with two wear facets present on the Laetoli remains, while the upper canines on the Hadar remains are worn flat.

Various other specimens from other sites have been attributed to afarensis, with these designations less sure. This includes remains from Allia Bay dating much later than the anamensis finds, and from the Omo region, Koobi Fora, and Sterkfontein.

Conclusions

The afarensis material is important in that it is the best known early hominid species (although as earlier anamensis and/or ramidus material becomes better known, it will lose much of its focus as the earliest known hominid material for which much is known). It shows decisive evidence for obligate bipedality and for the presence of evolutionary trends in its dentition and post-crania that seem to have it on the path to the modern human form.

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