Australopithecus afarensis - Leakey ancestors

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The Skull Replica
This half-scale model is of a composite skull and mandible reconstructed by Tim White on the basis of fragments discovered at Hadar, Ethiopia by Don Johanson and his field crew. Recently an almost complete skull was discovered, and although this has yet to be studied in detail, preliminary descriptions confirm the accuracy of this reconstruction. The reconstruction was of particular importance because it showed for the first time what the cranium of Australopithecus afarensis was like. In particular it showed the very primitive morphology. Since this discovery, an almost complete cranium has been found which confirms the morphologyof the reconstruction.

Overview
Australopithecus afarensis is best known from Hadar in Ethiopia, where over 200 specimens have been recovered. A number of specimens, including the type specimen, are also known from Laetoli in Tanzania. The species was first recognised in 1978 and for many years Australopithecus afarensis was believed to be the earliest australopithecine and the earliest biped. Many thought that it represented the common ancestor of all later hominids. The discovery of an earlier bipedal australopithecine at Kanapoi and Allia Bay in Kenya has shown the early evolution of the Hominidae to go back earlier and to be more complex than originally thought.

The A. afarensis skull has a small cranial vault (brain case), similar in size to that of a modern Common Chimpanzee. Because the brain case was small, and the teeth moderately large, the male has a distinct crest (sagittal crest) along the top of the low crowned skull like robust australopithecines. The crest serves for the attachment of the large chewing muscles (the temporal muscles) which have only a small surface area on which to attach. The sagittal crest is smaller than that of the robust australopithecines and so are the teeth. Sagittal crests are also present on male gorilla and baboon skulls, species which also have small braincases and large chewing muscles. Females are more lightly built and have smaller muscles so generally lack these prominent crests. The facial region of the skull of A. afarensis is rather ape-like and in many ways resembles that of a chimp. The jaw protrudes forwards in contrast to the rather flat lateral profile of a modern human.

The teeth are intermediate in many features between apes and humans. The upper and lower tooth rows of an ape are parallel whereas those of humans diverge posteriorly. Those of A. afarensis are not quite parallel but do not diverge as much as in Homo. In the upper jaw of an ape there is a gap (called a diastema) between the lateral incisors and the canines, humans lack this gap. A. afarensis has a small gap. The upper and lower canine teeth and the tooth behind the lower canine, the lower third premolar, are intermediate between those of apes and humans. The canines of apes are much larger and more conical than those of humans: those of A. afarensis are intermediate in size and shape. The lower premolars of A. afarensis have only a very small second cusp (metaconid) whereas that of apes has a single large cusp (protoconid) and that of Homo has two almost equal sized cusps. This premolar in A. afarensis also resembles apes in its more oblique orientation.

The A. afarensis collection of fossils is probably the largest of any species of early hominid and has been extensively studied. Specimens include not only cranial parts but also many bones of the skeleton. It has thus been possible to reconstruct quite an accurate picture of what this species looked like. The most complete and well known specimen is the skeleton nicknamed "Lucy." The bones of Lucy's skeleton as well as the bones of other individuals have shown us a lot about how A. afarensis walked and how much mobility this species had in its hands. The body proportions are different to those of Homo sapiens and Homo erectus because the length of the arms was quite short in relation to the length of the trunk and the legs. The footprints at Laetoli indicate that the outline of the feet was similar to that of humans, but those who have studied the footbones of A. afarensis have differing interpretations of the foot morphology. All agree that the toes were slightly longer than ours but some researchers believe that the big toe was positioned in line with the other toes as in humans, while others believe that it was opposable like that of modern apes. The hands were rather like those of apes because they had long fingers with strong muscles for closing (flexing) the hand which in apes are essential for holding tightly onto branches. But A. afarensis was not able to cup its hand like we can or to oppose the tip of the thumb with the index finger (the precision grip). Although the thumb was longer than that of a chimpanzee and the hand slightly more mobile, A. afarensis shows only a few of the adaptations that make our own hands so dextrous and manipulative.

Finally, A. afarensis males were much larger than females, and also had much larger canines. Males are though to have stood about 5'2" tall and to have weighed about 60 pounds, whereas Lucy is believed to have been about 3'9" and to have weighed about 60 pounds. Both sexes were strong like chimps and the males must have been particularly so. These differences between the sexes are known as sexual dimorphism. A afarensis lived between 3 and 4 million years ago.

Important Finds
The type specimen, given the accession number LH-4 was discovered in Laetoli, Tanzania by Maundu Muluila, a member of Mary leakey's team in 1974.

The Hadar finds by Don Johanson, et al are amazingly rich. AL288-1 ("Lucy"), AL 333 ("the First Family"), AL 444-2 (a fairly complete skull), among many others. There is more information at the Institute for Human Origins website.

Where it is found?
The Afar region of Ethiopia is the most rich with over 200 specimens. Finds have also been made in Tanzania.

Classification
The collection of fossils from Hadar, was found to differ from all other early hominids, but of them it resembled Australopithecus africanus most closely. The differences between the Hadar and A. africanus specimens were such that the Hadar collection was recognised as belonging to the genus, Australopithecus, but to a new species. The species name afarensis was given because the majority of the specimens came from the Afar region in Ethiopia.

It is normal to designate the most complete specimen as the type specimen, and the obvious choice for the type specimen in this case was the Lucy skeleton. For unknown reasons a mandible from Laetoli was designated the type so that the species name is not reflective of the type site.

The following shows the taxonomic sequence for Australopithecus afarensis
Kingdom Animalia
Phylum Chordata
Subphylum Vertebrata
Class Mammalia
Order Primates
Superfamily Hominoidea
Family Hominidae
Subfamily Australopithecinae
Genus Australopithecus
Species afarensis

Notice that the word endings of the superfamily is always "oidea", of the family is "idae" and of the subfamily is "inae".

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