Australopithecus Afarensis ^NEW^

In 1978, Johanson, Tim D. White and Coppens classified the hundreds of specimens collected thus far from both Hadar and Laetoli into a single new species, A. afarensis, and considered the apparently wide range of variation a result of sexual dimorphism. The species name honours the Afar Region of Ethiopia where the majority of the specimens had been recovered from. They later selected the jawbone LH 4 as the holotype specimen because of its preservation quality and because White had already fully described and illustrated it the year before.[1]

australopithecus afarensis

For a long time, A. afarensis was the oldest known African great ape until the 1994 description of the 4.4-million-year-old Ardipithecus ramidus,[15] and a few earlier or contemporary taxa have been described since, including the 4-million-year-old A. anamensis in 1995,[16] the 3.5-million-year-old Kenyanthropus platyops in 2001,[17] the 6-million-year-old Orrorin tugenensis in 2001,[18] and the 7- to 6-million-year-old Sahelanthropus tchadensis in 2002.[19] Bipedalism was once thought to have evolved in australopithecines, but it is now thought to have begun evolving much earlier in habitually arboreal primates. The earliest claimed date for the beginnings of an upright spine and a primarily vertical body plan is 21.6 million years ago in the Early Miocene with Morotopithecus bishopi.[20]

A. afarensis is now a widely accepted species, and it is now generally thought that Homo and Paranthropus are sister taxa deriving from Australopithecus, but the classification of Australopithecus species is in disarray. Australopithecus is considered a grade taxon whose members are united by their similar physiology rather than close relations with each other over other hominin genera. It is unclear how any Australopithecus species relate to each other,[21] but it is generally thought that a population of A. anamensis evolved into A. afarensis.[11][21][22]

In 1979, Johanson and White proposed that A. afarensis was the last common ancestor between Homo and Paranthropus, supplanting A. africanus in this role.[23] Considerable debate of the validity of this species followed, with proposals for synonymising them with A. africanus or recognising multiple species from the Laetoli and Hadar remains. In 1980, South African palaeoanthropologist Phillip V. Tobias proposed reclassifying the Laetoli specimens as A. africanus afarensis and the Hadar specimens as A. afr. aethiopicus.[24] The skull KNM-ER 1470 (now H. rudolfensis) was at first dated to 2.9 million years ago, which cast doubt on the ancestral position of both A. afarensis or A. africanus, but it has been re-dated to about 2 million years ago.[9] Several Australopithecus species have since been postulated to represent the ancestor to Homo, but the 2013 discovery of the earliest Homo specimen, LD 350-1, 2.8 million years old (older than almost all other Australopithecus species) from the Afar Region could potentially affirm A. afarensis' ancestral position.[25] However, A. afarensis is also argued to have been too derived (too specialised), due to resemblance in jaw anatomy to the robust australopithecines, to have been a human ancestor.[26]

In 1996, a 3.6-million-year-old jaw from Koro Toro, Chad, originally classified as A. afarensis was split off into a new species as A. bahrelghazali.[30] In 2015, some 3.5- to 3.3-million-year-old jaw specimens from the Afar Region (the same time and place as A. afarensis) were classified as a new species as A. deyiremeda, and the recognition of this species would call into question the species designation of fossils currently assigned to A. afarensis.[31] However, the validity of A. bahrelghazali and A. deyiremeda is debated.[32] Wood and Boyle (2016) stated there was "low confidence" that A. afarensis, A. bahrelghazali and A. deyiremeda are distinct species, with Kenyanthropus platyops perhaps being indistinct from the latter two.[2]

For the five makers of the Laetoli fossil trackways (S1, S2, G1, G2 and G3), based on the relationship between footprint length and bodily dimensions in modern humans, S1 was estimated to have been considerably large at about 165 cm (5 ft 5 in) tall and 45 kg (99 lb) in weight, S2 145 cm (4 ft 9 in) and 39.5 kg (87 lb), G1 114 cm (3 ft 9 in) and 30 kg (66 lb), G2 142 cm (4 ft 8 in) and 39 kg (86 lb), and G3 132 cm (4 ft 4 in) and 35 kg (77 lb). Based on these, S1 is interpreted to have been a male, and the rest females (G1 and G3 possibly juveniles), with A. afarensis being a highly dimorphic species.[41]

Like other australopiths, the A. afarensis skeleton exhibits a mosaic anatomy with some aspects similar to modern humans and others to non-human great apes. The pelvis and leg bones clearly indicate weight-bearing ability, equating to habitual bipedal, but the upper limbs are reminiscent of orangutans, which would indicate arboreal locomotion. However, this is much debated, as tree-climbing adaptations could simply be basal traits inherited from the great ape last common ancestor in the absence of major selective pressures at this stage to adopt a more humanlike arm anatomy.[46]

The shoulder joint is somewhat in a shrugging position, closer to the head, like in non-human apes.[47] Juvenile modern humans have a somewhat similar configuration, but this changes to the normal human condition with age; such a change does not appear to have occurred in A. afarensis development. It was once argued that this was simply a byproduct of being a small-bodied species, but the discovery of the similarly sized H. floresiensis with a more or less human shoulder configuration and larger A. afarensis specimens retaining the shrugging shoulders show this to not have been the case. The scapular spine (reflecting the strength of the back muscles) is closer to the range of gorillas.[47]

The australopith pelvis is platypelloid and maintains a relatively wider distance between the hip sockets and a more oval shape. Despite being much smaller, Lucy's pelvic inlet is 132 mm (5.2 in) wide, about the same breadth as that of a modern human woman. These were likely adaptations to minimise how far the centre of mass drops while walking upright in order to compensate for the short legs (rotating the hips may have been more important for A. afarensis). Likewise, later Homo could reduce relative pelvic inlet size probably due to the elongation of the legs. Pelvic inlet size may not have been due to fetal head size (which would have increased birth canal and thus pelvic inlet width) as an A. afarensis newborn would have had a similar or smaller head size compared to that of a newborn chimpanzee.[51][52] It is debated if the platypelloid pelvis provided poorer leverage for the hamstrings or not.[51]

The heel bone of A. afarensis adults and modern humans have the same adaptations for bipedality, indicating a developed grade of walking. The big toe is not dextrous as is in non-human apes (it is adducted), which would make walking more energy efficient at the expense of arboreal locomotion, no longer able to grasp onto tree branches with the feet.[53] However, the foot of the infantile specimen DIK-1-1 indicates some mobility of the big toe, though not to the degree in non-human primates. This would have reduced walking efficiency, but a partially dextrous foot in the juvenile stage may have been important in climbing activities for food or safety, or made it easier for the infant to cling onto and be carried by an adult.[54]

In 2009 at Dikika, Ethiopia, a rib fragment belonging to a cow-sized hoofed animal and a partial femur of a goat-sized juvenile bovid was found to exhibit cut marks, and the former some crushing, which were initially interpreted as the oldest evidence of butchering with stone tools. If correct, this would make it the oldest evidence of sharp-edged stone tool use at 3.4 million years old, and would be attributable to A. afarensis as it is the only species known within the time and place.[57] However, because the fossils were found in a sandstone unit (and were modified by abrasive sand and gravel particles during the fossilisation process), the attribution to hominin activity is weak.[58]

In 2016, palaeoanthropologist John Kappelman argued that the fracturing exhibited by Lucy was consistent with a proximal humerus fracture, which is most often caused by falling in humans. He then concluded she died from falling out of a tree, and that A. afarensis slept in trees or climbed trees to escape predators. However, similar fracturing is exhibited in many other creatures in the area, including the bones of antelope, elephants, giraffes and rhinos, and may well simply be taphonomic bias (fracturing was caused by fossilisation).[67] Lucy may also have been killed in an animal attack or a mudslide.[68]

Facial reconstructions of what the early hominin (human relative) Australopithecus afarensis may have looked like left image by Ellywa [CC BY-SA 4.0], right image by Daderot [CC0 1.0], both from Wikimedia Commons

Au. afarensis belongs to the genus Australopithecus, a group of small-bodied and small-brained early hominin species (human relatives) that were capable of upright walking but not well adapted for travelling long distances on the ground. Species in the australopith group - which also includes Au. africanus, Au. sediba, Au. anamensis and Kenyanthropus platyops - probably gave rise to two more recent hominin groups, Homo and Paranthropus, before 2.5 million years ago. Au. afarensis wasn't the first member of the group discovered - that was the Au. africanus from South Africa - but its discovery confirmed our ancient relatives habitually walked upright, and that this feature of the human lineage occurred long before the evolution of bigger brains. 041b061a72