Fossil and Geologic Evidence: Fossil Evidence

"Lucy", a 3.2 Ma A. afarensis specimen that exhibits definitive bipedal morphology. Redrawn after Johanson and Edey 1981¹¹

"Lucy", a 3.2 Ma A. afarensis specimen that exhibits definitive bipedal morphology. Redrawn after Johanson and Edey 1981

The 3.6 million year old A. afarensis footprints at the Laetoli site, Africa, demonstrate these hominins had a non-opposable big toe and a human like arch. Redrawn from photograph as on PBS:Evolution 2001.

Homo ergaster, specimen KNM ET 15000, is a nearly complete skeleton and exhibits many hallmarks of bipedalism, such as the bicondylar angle and longer legs relative to the arms.

The fossil records can help anthropologists determine the origins of bipedalism, which in turn allows us to understand which species might be direct ancestors to modern humans. One of the most abundant sources for early bipedalism is found in Australopithecus afarensis, a species that lived between 4 and 2.8 million years ago (Ma). The most famous fossil in the world, Lucy, is a member of the A. afarensis species and has provided a great deal of information on early bipedalism. For example, A. afarensis fossils clearly show hip and knee morphology distinctive to habitual bipedalism.

A. afarensis also left behind a 27 meter long set of footprints known as the Laetoli Tracks in Tanzania. Approximately 3.6 Ma, three A. afarensis individuals walked through a muddy layer of volcanic ash that perfectly preserved their foot prints as the ash cemented²⁰. From the Laetoli tracks it is clear that A. afarensis walked with an upright posture, with a strong heal strike and following through to the ball of the foot, with the big toe making last contact with the ground and pushing off. Interestingly, the prints provide evidence of a slight gap between the big toe and the rest of the toes. This gap suggests that even though the big toe was not divergent, it was not yet directly parallel with the rest of the toes like that seen in modern humans.^8-10,21-23

Though australopithecines are among the earliest hominins to have used habitual bipedalism, earlier hominins dating as far back as 7 million years also provide exciting evidence for early bipedalism. The oldest known hominin to definitively exhibit morphological (i.e. physical) adaptations for bipedal behavior is the extinct species Orrorin tugenensis that dates to 6 Ma. A femur and tibia recovered in Kenya assigned to O. tugenensis exhibits a bicondylar angle seen in habitual bipeds.^24-26 However, a recently discovered fossil specimen known as Sahelanthropus tchadensis from Chad, dating to approximately 7 Ma, shows a more inferiorly positioned foramen magnum consistent with bipedalism, rather than a more dorsal placement seen in modern quadrupeds.^27,28 There is no post-cranial material associated with Sahelanthropus, but if proven to be bipedal, Sahelanthropus may substantiate the hypothesis that the evolution of habitually bipedal hominins was initiated by climate trends beginning in the late Miocene (i.e., a geologic epoch that dates between 23 and 5.3 Ma). Analyses of fauna recovered from the same locations that these fossils were found suggests S. tchadensis and O. tugenensis lived on a lake margin, near the edge of woodland and more open country.

About 2 million years younger than O. tugenensis is a hominin known as known as Ardipithecus ramidus that dates to approximately 4.4 Ma. As with S. tchadensis, there is no post-cranial material associated with Ard. ramidus, and bipedalism has been determined based its more inferiorly positioned foramen magnum^8,10.

The oldest evidence of bipedalism in australopithecines is found in the species A. anamensis (4.2 to 3.9 Ma). Found in Kenya, A. anamensis most likely lived in a wooded savanna. Fossils evidence for this species includes a preserved tibia that exhibits bipedal characteristics such as 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 A. afarensis (4 to 2.8 Ma) and A. africanus (3 to 2 Ma) fossils also show clear signs of habitual 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 A. afarensis seems to have originated in Ethiopia and A. africanus is found only in South Africa, both of these species lived in open habitats, possibly wooded savanna areas near a lake^8-10.

Paranthropines are larger and more robust than australopithecines, but have similar post-cranial morphology, including bipedal adaptations similar to Australopithecus. The oldest paranthropine was found in Ethiopia and is known as P. aethiopicus (2.6 - 2.5 Ma). Although mostly cranial material has been recovered for this species, a newly discovered calcaneus that may be associated with this species appears adapted for bipedalism. The younger paranthropine species, P. robustus (1.75 to 1.5 Ma) and P. boisei (2.5 to 1 Ma), exhibit the same bipedal adaptations as A. africanus, which include an inferiorly oriented foramen magnum, modern human-like talus, longer femoral neck, and 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^8-10.

All species included in the genus Homo are habitually bipedal and show evidence of tool use, beginning with the species H. habilis (i.e., "Handy Man") that dates between approximately 2.6 to 1.6 million years ago to the modern species H. sapiens that dates between approximately 190,000 years ago to the present^8-10.