Australopithecus afarensis

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Australopithecus afarensis ("Lucy")
Fossil range: Pliocene

Picture of Lucy remains, Museo Nacional de Antropología, Mexico City

Scientific classification

Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Primates
Family:	Hominidae
Subfamily:	Homininae
Genus:	Australopithecus
Species:	A. afarensis

Binomial name

†Australopithecus afarensis
Johanson & White, 1978

Australopithecus afarensis is an extinct hominid which lived between 3.9 and 2.9 million years ago. In common with the younger Australopithecus africanus, A. afarensis was slenderly built. It is widely believed that A. afarensis is the ancestor of the genus Homo, which includes the modern human species, Homo sapiens.^[1]

1 Localities
2 Physical characteristics
3 Notable Fossils
4 Related work
5 See Also
6 References
7 External links

[edit] Localities

Australopithecus afarensis fossils have only been discovered within Eastern Africa. Despite Laetoli being the type locality for A. afarensis, the most extensive remains assigned to this species are found in Hadar, Ethiopia, including the famous "Lucy" partial skeleton and the "First Family" found at the A.L. 333 locality. Other localities bearing A. afarensis remains include Omo, Maka, Fejej and Belohdelie in Ethiopia, and Koobi Fora and Lothagam in Kenya.

[edit] Physical characteristics

[edit] Craniodental features and Brain Size

Compared to modern and extinct great apes, A. afarensis has reduced canines and molars, although they are still relatively larger than in modern humans. A. afarensis also had a relatively small brain size (~380-430cm³) and a prognathic (i.e. projecting anteriorly) face.

The image of a bipedal hominin with a small brain and primitive face was quite a revelation to the paleoanthropological world at the time. This was due to the earlier belief that an increase in brain size was the first major hominin adaptive shift. Before the discoveries of A. afarensis in the 1970s, it was widely thought that an increase in brain size preceded the shift to bipedal locomotion. This was mainly due to the fact that the oldest known hominins at the time had relatively large brains (e.g KNM-ER 1470, Homo rudolfensis, which was found just a few years before Lucy and had a cranial capacity of ~800cm³).

[edit] Bipedalism

Reconstruction of A. afarensis from Laetoli (American Museum of Natural History)

There is considerable debate regarding the locomotor behaviour of A. afarensis. Some believe that A. afarensis was almost exclusively bipedal, while others believe that the creatures were partly arboreal. The anatomy of the hands, feet and shoulder joint in many ways favour the latter interpretation. The curvature of the finger and toe bones (Phalanges) approaches that of modern-day apes, and is most likely reflective of their ability to efficiently grasp branches and climb. The presence of a wrist-locking mechanism might suggest that they were knuckle-walkers. The shoulder joint is also orientated more cranially (i.e. towards the skull) than in modern humans. Combined with the relatively long arms A. afarensis are thought to have had, this is thought by many to be reflective of a heightened ability to use the arm above the head in climbing behaviour. Furthermore, scans of the skulls reveal a canal and bony labyrinth morphology that some suggest is not conducive to proper bipedal locomotion.

However, there are also a number of traits in the A. afarensis skeleton that strongly reflect bipedalism. In overall anatomy the pelvis is far more human-like than ape-like. The iliac blades are short and wide, the sacrum is wide and positioned directly behind the hip joint, and there is clear evidence of a strong attachment for the knee extensors. While the pelvis is not wholly human-like (being markedly wide with flared with laterally orientated iliac blades), these features point to a structure that can be considered radically remodeled to accommodate a significant degree of bipedalism in the animals' locomotor repertoire. Importantly, the femur also angles in towards the knee from the hip. This trait would have allowed the foot to have fallen closer to the midline of the body, and is a strong indication of habitual bipedal locomotion. Along with humans, present day orangutans and spider monkeys possess this same feature. The feet also feature adducted big toes, making it difficult if not impossible to grasp branches with the hindlimbs. The loss of a grasping hindlimb also increases the risk of an infant being dropped or falling as primates typically hold onto their mothers as the mother goes about her daily business. Without the second set of grasping limbs the infant cannot maintain as strong a grip and likely had to be held with help from the mother. The problem of holding the infant would be multiplied if the mother also had to climb trees. The ankle joint of A. afarensis is also markedly human-like.^{[citation needed]}

[edit] Social characteristics

It is difficult to predict the social behaviour of extinct fossil species. However, the social structure of modern apes and monkeys can be anticipated to some extent by the average range of body size between males and females (known as sexual dimorphism). Although there is considerable debate over how large the degree of sexual dimorphism was between males and females of A. afarensis, it is likely that males were relatively larger than females. If observations on the relationship between sexual dimorphism and social group structure from modern great apes are applied to A. afarensis then these creatures most likely lived in small family groups containing a single dominant male and a number of breeding females.^[1]

There are no known stone-tools associated with A. afarensis, and the present archeological record of stone artifacts only dates back to approximately 2.5 Mya.^[1]

[edit] Notable Fossils

[edit] Type specimen

The type specimen for A. afarensis is LH 4, an adult mandible from the site of Laetoli, Tanzania.

[edit] Lucy

Main article: Lucy (Australopithecus)

The first A. afarensis skeleton was discovered on November 24, 1974 by Donald Johanson, Maurice Taieb, Yves Coppens and Tim White in the Middle Awash of Ethiopia's Afar Depression.

[edit] Site 333

Michael Bush, one of Don Johanson's students, made another major discovery in 1975: near Lucy, on the other side of the hill, he found the "First Family", including 200 fragments of A. afarensis. The site of the findings is now known as "site 333", by a count of fossil fragments uncovered, such as teeth and pieces of jaw. 13 individuals were uncovered and all were adults, with no injuries caused by carnivores. All 13 individuals seemed to have died at the same time, thus Johanson concluded that they might have been killed instantly from a flash flood.

[edit] Selam

Main article: Selam (Australopithecus)

On September 20, 2006, Scientific American magazine presented the findings of a dig in Dikika, Ethiopia, a few miles from the place where Lucy was found. The recovered skeleton of a 3-year-old A. afarensis girl comprises almost the entire skull and torso, and most parts of the limbs. The features of the skeleton suggest adaptation to walking upright (bipedalism) as well as tree-climbing, features that match the skeletal features of Lucy and fall midway between human and humanoid ape anatomy. "Baby Lucy" has officially been named Selam (meaning peace in most Ethiopian languages). [1]

[edit] Others

[edit] Related work

Further findings at Afar, including the many hominin bones in site 333, produced more bones of concurrent date, and led to Johanson and White's eventual argument that the Koobi Fora hominins were concurrent with the Afar hominins. In other words, Lucy was not unique in evolving bipedalism and a flat face.

Recently, an entirely new species has been discovered, called Kenyanthropus platyops, however the cranium KNM WT 40000 has a much distorted matrix making it hard to distinguish (however a flat face is present). This had many of the same characteristics as Lucy, but is possibly an entirely different genus.

Another species, called Ardipithecus ramidus, has been found, which was fully bipedal, yet appears to have been contemporaneous with a woodland environment, and, more importantly, contemporaneous with Australopithecus afarensis. Scientists have not yet been able to draw an estimation of the cranial capacity of A. ramidus as only small jaw and leg fragments have been discovered thus far.

[edit] See Also

List of human fossils
List of fossil sites (with link directory)
List of hominina (hominid) fossils (with images)

[edit] References

^ ^a ^b ^c Jones, S. Martin; & R. Pilbeam (ed.) (2004). The Cambridge Encyclopedia of Human Evolution (8th ed.).Cambridge University Press. ISBN 0-521-46786-1

BBC - Dawn of Man (2000) by Robin Mckie| ISBN 0-7894-6262-1
Barraclough, G. (1989). in Stone, N. (ed.): Atlas of World History, 3rd edition, Times Books Limited. ISBN 0-7230-0304-1.
Australopithecus afarensis from The Human Origins Program at the Smithsonian Institution
(2000) in Delson, E., I. Tattersall, J.A. Van Couvering & A.S. Brooks (eds.): Encyclopedia of human evolution and prehistory, 2nd Edition, Garland Publishing, New York. ISBN 0-8153-1696-8.