Australopithecus afarensis

Australopithecus afarensis is an extinct hominid that lived between 3.9 and 2.9 million years ago.[1] A. afarensis was slenderly built, like the younger Australopithecus africanus. It is thought that A. afarensis was more closely related to the genus Homo (which includes the modern human species Homo sapiens), whether as a direct ancestor or a close relative of an unknown ancestor, than any other known primate from the same time.[2]

The most famous fossil is the partial skeleton named Lucy (3.2 million years old) found by Donald Johanson and colleagues, who, in celebration of their find, repeatedly played the Beatles song Lucy in the Sky with Diamonds.

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 the species are found in Hadar, Afar Region of Ethiopia, including the above-mentioned "Lucy" partial skeleton and the "First Family" found at the AL 333 locality. Other localities bearing A. afarensis remains include Omo, Maka, Fejej and Belohdelie in Ethiopia, and Koobi Fora and Lothagam in Kenya.

Craniodental features and brain size
Compared to the 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 has a relatively small brain size (~380–430 cm3) and a prognathic face (i.e. a face with forward projecting jaws).

The image of a bipedal hominid 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 because 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 ~800 cm³).

Skeletal morphology and locomotion
Considerable debate surrounds the locomotor behaviour of A. afarensis. Some studies suggest that A. afarensis was almost exclusively bipedal, while others propose that the creatures were partly arboreal. The anatomy of the hands, feet and shoulder joints in many ways favour the latter interpretation. In particular the morphology of scapula appears to be ape-like and very different from modern humans.[6] The curvature of the finger and toe bones (phalanges) approaches that of modern-day apes, and is suggestive of their ability to efficiently grasp branches and climb. Alternatively, the loss of an abductable great toe and therefore the ability to grasp with the foot (a feature of all other primates) suggests A. afarensis was no longer adapted to climbing.[7]

A number of traits in the A. afarensis skeleton strongly reflect bipedalism, to the extent some researchers have suggested bipedality evolved long before A. afarensis.[8] 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 evidence of a strong attachment for the knee extensors is clear. While the pelvis is not wholly human-like (being markedly wide, or flared, with laterally-oriented 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 toward 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.[citation needed] 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 while 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. Bones of the foot (such as the calcaneus) also indicate bipedality.[9][10]

Computer simulations using dynamic modeling of the skeleton's inertial properties and kinematics suggest A. afarensis was able to walk in the same way modern humans walk, with a normal erect gait or with bent hips and knees, but could not walk in the same way as chimpanzees. The upright gait would have been much more efficient than the bent knee and hip walking, which would have taken twice as much energy