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. From analysis it has been thought that A. afarensis was ancestral to both the genus Australopithecus and the genus Homo, which includes the modern human species, Homo sapiens..
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.
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 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 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 ~800cm³).
However, there are also a number of traits in the A. afarensis skeleton which 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 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. 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. The ankle joint of A. afarensis is also markedly human-like.
Computer simulations using dynamic modelling of the skeleton's inertial properties and kinematics have indicated that 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. It appears probable that A. afarensis was quite an efficient bipedal walker over short distances, and the spacing of the footprints at Laetoli indicates that they were walking at 1.0 m/s or above, which matches human small-town walking speeds.
It is commonly thought that upright bipedal walking evolved from knuckle-walking with bent legs, in the manner used by chimpanzees and gorillas to move around on the ground, but fossils such as Orrorin tugenensis indicate bipedalism around 5 to 8 million years ago, in the same general period where genetic studies suggest the lineage of chimpanzees and humans diverged. Modern apes and their fossil ancestors show skeletal adaptations to an upright posture used in tree climbing, and it has been proposed that that upright, straight-legged walking originally evolved as an adaptation to tree-dwelling. Studies of modern orangutans in Sumatra have shown these apes using four legs when walking on large stable branches and when swinging underneath slightly smaller branches, but are bipedal and maintain their legs very straight when using multiple small flexible branches under 4 cm. in diameter while also using their arms for balance and additional support. This enables them to get nearer to the edge of the tree canopy to grasp fruit or cross to another tree.
Climate changes around 11 to 12 million years ago affected forests in East and Central Africa, establishing periods where openings prevented travel through the tree canopy, and during these times ancestral hominids could have adapted the upright walking behaviour for ground travel, while the ancestors of gorillas and chimpanzees became more specialised in climbing vertical tree trunks or lianas with a bent hip and bent knee posture, ultimately leading them to use the related knuckle-walking posture for ground travel. This would lead to A. afarensis usage of upright bipedalism for ground travel, while still having arms well adapted for climbing smaller trees. However, chimpanzees and gorillas are the closest living relatives to humans, and share anatomical features including a fused wrist bone which may also suggest knuckle-walking by human ancestors. Other studies suggest that an upright spine and a primarily vertical body plan in primates dates back to Morotopithecus bishopi in the Early Miocene of 21.6 million years ago
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 million years ago.
The first A. afarensis knee joint was discovered in November 1973 by Donald Johanson as part of a team involving Maurice Taieb, Yves Coppens and Tim White in the Middle Awash of Ethiopia's Afar Depression.
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, was found by White and colleagues in the 1990s. This 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 Ar. ramidus as only small jaw and leg fragments have been discovered thus far.