The leg bones as well as the Laetoli fossil trackways suggest A. afarensis was a competent biped, though somewhat less efficient at walking and slower at running than humans.

The arm and shoulder bones have some similar aspects to those of orangutans and gorillas, which has variously been interpreted as either evidence of partial tree-dwelling (arboreality), or basal traits inherited from the chimpanzee–human last common ancestor with no adaptive functionality.

A. afarensis was probably a generalist omnivore of both C3 forest plants and C4 CAM savanna plants—and perhaps creatures which ate such plants—and was able to exploit a variety of different food sources.

[1] The first to identify a human fossil was German explorer Ludwig Kohl-Larsen in 1939 by the headwaters of the Gerusi River (near Laetoli, Tanzania), who encountered a maxilla.

[6] 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.

Beyond Laetoli and the Afar Region, the species has been recorded in Kenya at Koobi Fora and possibly Lothagam; and elsewhere in Ethiopia at Woranso-Mille, Maka, Belohdelie, Ledi-Geraru and Fejej.

[13]: 1–4 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,[14] and a few earlier or contemporary taxa have been described since, including the 4-million-year-old A. anamensis in 1995,[15] the 3.5-million-year-old Kenyanthropus platyops in 2001,[16] the 6-million-year-old Orrorin tugenensis in 2001,[17] and the 7- to 6-million-year-old Sahelanthropus tchadensis in 2002.

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.

[26][8] In 2003, Spanish writer Camilo José Cela Conde and evolutionary biologist Francisco J. Ayala proposed reinstating "Praeanthropus" including A. afarensis alongside Sahelanthropus, A. anamensis, A. bahrelghazali and A.

[27] In 2004, Danish biologist Bjarne Westergaard and geologist Niels Bonde proposed splitting off "Homo hadar" with the 3.2-million-year-old partial skull AL 333–45 as the holotype, because a foot from the First Family was apparently more humanlike than that of Lucy.

[31] 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.

[36] A. afarensis specimens apparently exhibit a wide range of variation, which is generally explained as marked sexual dimorphism with males much bigger than females.

It is also contested if australopiths even exhibited heightened sexual dimorphism at all, which if correct would mean the range of variation is normal body size disparity between different individuals regardless of sex.

This could perhaps speak to advanced motor functions in the hands of A. afarensis and competency at precision tasks compared to non-human apes, possibly implicated in stone tool use or production.

The neck vertebrae of KDS-VP-1/1 indicate that the nuchal ligament, which stabilises the head while distance running in humans and other cursorial creatures, was either not well developed or absent.

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.

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).

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.

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.

[56] 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.

[60][61][62] The platypelloid pelvis may have caused a different birthing mechanism from modern humans, with the neonate entering the inlet facing laterally (the head was transversally orientated) until it exited through the pelvic outlet.

[65] Some tracks feature a 100 mm (3.9 in) long drag mark probably left by the heel, which may indicate the foot was lifted at a low angle to the ground.

Some footprints of S1 either indicate asymmetrical walking where weight was sometimes placed on the anterolateral part (the side of the front half of the foot) before toe-off, or sometimes the upper body was rotated mid-step.

[13]: 95–97  Lucy presents marked thoracic kyphosis (hunchback) and was diagnosed with Scheuermann's disease, probably caused by overstraining her back, which can lead to a hunched posture in modern humans due to irregular curving of the spine.

British archaeologist Paul Pettitt considered natural causes unlikely and, in 2013, speculated that these individuals were purposefully hidden in tall grass by other hominins (funerary caching).

[72] It does not appear to have had a preferred environment and it inhabited a wide range of habitats such as open grasslands or woodlands, shrublands, and lake- or riverside forests.

The Pliocene of East Africa was warm and wet compared to the preceding Miocene, with the dry season lasting about four months based on floral, faunal, and geological evidence.

[73] During the Late Pliocene around 4–3 million years ago, Africa featured a greater diversity of large carnivores than today, and australopithecines likely fell prey to these dangerous creatures, including hyenas, Panthera, cheetahs, and the saber-toothed cats: Megantereon, Dinofelis, Homotherium and Machairodus.