The fossil record offers clues as to the origins of bipedalism, which in turn helps us to identify those species ancestral to modern humans. One of the most abundant sources for early bipedalism is found in Australopithecus afarensis, a species that lived between approximately 4 and 2.8 Ma. A. afarensis postcrania clearly shows hip, knee, and foot morphology distinctive to bipedalism.
In addition to the postcranial material, Au. afarensis also left behind a 27 meter long set of footprints known as the Laetoli Tracks in Tanzania. Approximately 3.7 Ma, 3 Au. afarensis individuals walked through a muddy layer of volcanic ash that preserved their foot prints after the ash hardened20. From the Laetoli tracks it is clear that Au. afarensis walked with an upright posture, with a strong heel strike and follow-through to the ball of the foot, with the hallux making last contact with the ground before push-off. Interestingly, the prints provide evidence of a slight gap between the hallux and the other toes. This gap suggests that even though the hallux was not fully divergent, it was alos not yet fully adducted as seen in modern humans8-10,21-23.
Though australopith material offers a strong case for habitual bipedalism, earlier hominins dating as far back as 7 Ma also provide exciting evidence for early bipedalism. The oldest known hominin to show definitive bipedal adapations is the extinct species Orrorin tugenensis that dates to 6 Ma. A femur and tibia recovered in Kenya and assigned to O. tugenensis exhibits features typical of bipeds, including a bicondylar angle24-26. However, a 7 Ma fossil discovered in Chad in 2001, known as Sahelanthropus tchadensis, exhibits a more inferiorly positioned foramen magnum consistent with bipedalism, rather than the relatively dorsal position seen in quadrupeds27,28. No postcranial material has been associated with Sahelanthropus
but if proven to be bipedal, Sahelanthropus may substantiate the hypothesis that bipedal evolution was influenced by climate trends beginning in the late Miocene (i.e., a geologic epoch that dates between 23 and 5.3 Ma). Faunal analyses from these earily hominin sites suggests S. tchadensisand O. tugenensis lived on lake margins, near the edge of woodlands and grasslands.
About 2 million years younger than O. tugenensis is a hominin known as known as Ardipithecus ramidus that dates to approximately 4.4 Ma. Known as "Ardi", Ar. ramidus material exhibits a mosaic of primitive and derived features, including a fully abductable hallux (primitive), relatively inflexible midfoot (derived), arms and legs of similar proportions (primitive), relatively broad iliac ala (derived), and an inferiorly placed foramen magnum8-10,31,32.
The oldest evidence for australopith bipedalism is found in the species Australopithecus anamensis (4.2 to 3.9 Ma). Found in Kenya, Au. anamensis most likely lived in a wooded savanna. Fossil evidence for this species includes a preserved tibia that exhibits bipedal characteristics such as a right angle between the shaft and the proximal surface, and proximal articular condyles of nearly equal size. An abundance of the younger species Au. afarensis (4 to 2.8 Ma) and Australopithecus africanus (3 to 2 Ma) fossils also show clear signs of bipedalism, including a bicondylar angle, an anteriorly placed foramen magnum, laterally flaring iliac blades, longer femoral necks and heads, and the presence of a lumbar curve. Though Au. afarensis seems to have originated in Ethiopia and Au. africanus is found only in South Africa, both of these species lived in open habitats, possibly wooded savanna areas near a lake8-10.
Paranthropines are larger and more robust than australopiths, but have similar postcranial morphology, including bipedal adaptations similar to Australopithecus. The oldest paranthropine was found in Ethiopia and is known as Paranthropus aethiopicus (2.6 – 2.5 Ma). Although postcranial material is scarce, a possible P. aethiopicus calcaneus may exhibit bipedal adaptations. The younger paranthropine species, Paranthropus robustus (1.75 to 1.5 Ma) and Paranthropus boisei (2.5 to 1 Ma), exhibit the same bipedal adaptations as Au. africanus, which include an inferiorly oriented foramen magnum, modern human-like talus, relatively long femoral neck, and a bicondylar angle. In addition, the hand anatomy of P. robustus implies a grip capable of tool use, while the radius of both P. robustus and P. boisei implies Paranthropus retained the ability to effectively climb trees. Paleoecological studies suggest these species were living in open woodland or savanna habitats.
All species included in the genus Homo are obligatatory bipeds and show evidence of tool use, beginning with the species Homo habilis (i.e. “Handy Man”) that dates between approximately 2.6 to 1.6 Ma, and continuing to the modern species Homo sapiens that dates between approximately 190,000 years ago (Ka) to the present8-10.
