How did our early hominid ancestors begin to utilize bipedal locomotion?

Austrolipthecine Species

Ardipithecus ramidus 

Australopithecus afarensis

Australopithecus anamensis

Australopithecus robustus

Australopithecus aethiopicus

Australopithecus boisei

Australopithecus africanus

Australopithecus garhi

Evidence for Bipedalism: 

Vestibular System, 

Laetoli Footprints, 

Foramen Magnum, 

Metatarsals, & 

Pevlis

Vestibular System

In an effort to better understand the locomotion of living primates and extinct species including several hominids, Spoor, Wood, and Zonneveld used computer tomography to examine the vestibular system (1994). The vestibular system, housed in the petrous portion of the skull, controls for balance and aids in movement. Agile primates were found to have rather large semicircular canals relative to their body size whereas the opposite was found for less agile primates (Spoor, Wood, & Zonneveld, 1994). Two australopithecine crania were included in the study, Australopithecus africanus and Australopithecus robustus. Both species were found to have a less developed vestibular system than that of modern humans implying that the extent of bipedalism was also less developed (Spoor, Wood, & Zonneveld, 1994). The researchers suggested that australopithecines were not fully bipedal hominids but rather used a combination of bipedal and arboreal locomotion (Spoor, Wood, & Zonneveld, 1994). 

Laetoli Footprints

The Laetoli footprints of Tanzania date back to approximately 3.5Ma. There are two sets of parallel prints that pressumably belong to Australopithecus afarensis due to their age. The prints were created when these hominids walked over freshly fallen ash from the Sadiman volcano. Due to its chemical composition, the next rainfall hardened the ash and preserved the footprints (Cowen, 2005). 

There has been much debate as to whether the prints indicate a hominid using a bent-knee, bent-hip biped walking posture like that used by chimpanzees or if they indicate a straight hind limb walking posture like that used by modern humans. Raichlen, Pontzer, and Sockol concluded in their kinesthetic study that either walking posture could have been utilized the by Laetoli hominids to create those particular track patterns (2008). 

Foramen Magnum

Research involving the position and angle of the foramen magnum has produced supporting evidence that australopithecines were indeed bipedal creatures. In modern humans the foramen magnum is oriented anteriorly at an incline whereas in great apes it is oriented posteriorly at an incline (Kimbel & Rak, 2010). The australopithecine foramen magnum data indicates that the orientation of reconstructed crania is similar to that of both modern humans and apes. Kimbel and Rak found that the foramen magnum is situated more anteriorly similar to modern humans but the angle of orientation inclines posteriorly as it does in apes (2010). The position of the foramen magnum indicates only that australopithecines were able to use bipedal locomotion as compared to their quadrupedal relatives; it does not provide much in the way of evidence for cranial capacity or locomotor differences between australopithecines and modern humans (Kimbel & Rak, 2010).

Metatarsals

Full-time bipedalism requires a sturdy base for shock absorption and forward motion. The development of specially adapted foot arches has enabled modern humans to be successful bipeds. Recent research has revealed that australopithecines had a similar metatarsal arrangement allowing them to also walk upright.  According to Ward, Kimbel, and Johanson, the evolution of permanent transverse and logitudinal foot arches is unequivocal evidence for bipedalism (2011). Until very recently the fossil record has been too incomplete to allow for a comprehensive study of australopithecine pedal bone physiology. In 2000, excavations in Hadar, Ethiopia revealed an intact fourth metatarsal bone. There are pronounced differences between humans and apes in the fourth metatarsal, and it is this bone that offers the most definitive pedal evidence for bipedalism (Ward, Kimbel, & Johanson, 2011). The fourth metatarsal is slightly rotated in australopithecines, as it is in modern humans, to produce a walking surface that is strongly supported by a pedal arch (Ward, Kimbel, & Johanson, 2011). The degree of rotation found in the recent Hadar specimen provides further evidence for a transverse pedal arch.  The longitudinal arching of the pedal bones, particularly the fourth metatarsal, and the tarsometatarsal joints provide foot stability and lateral rigidity, two pedal characteristics that are not present in apes (Ward, Kimbel, & Johanson, 2011). It seems likely that Australopithecines had developed human-like pedal structures that were permanently in place for full-time bipedal locomotion.

Pelvis

Australopithecine pelvis reconstructions provide insight into their mode of bipedal locomotion. There has been some variation in the reconstruction of the sacroilliac joint leading to different interpretations of the degree of bipedalism. Berge and Goularas define the major shared characteristics between australopithecines and humans as including a rotational fetal delivery pattern, a similar subpubic angle, and a similar short pubic symphysis (2010). These three similarities provide evidence for an upright posture that is skeletally supported to allow for a more humanlike stance and gait.  

Australopithecus ghari

Australopithecus ghari was found in Ethiopia and dates approximately 2.5Ma. Stone tools were found with A. ghari that date to the same time. Did A. ghari utilize tools?

Australopithecus africanus

Known as the Tuang Child, the skull representing the type specimen for Australopithecus africanus was found in Taung, South Africa and named by Raymond Dart in 1925. It lived approximately 3-2.3Ma. 

Australopithecus boisei

Australopithecus boisei was discovered by Mary Leaky in the Olduvai Gorge, Tanzania. It lived approxiamately 2-1.3Ma and was even more robust than its contemporary, Australopithecus robustus. A. boisei had a large mandible and dentition, and a prominent midsagittal crest that offered attachment support for powerful masseter muscles. 

Australopithecus aethiopicus

Nicknamed "Black Skull," Australopithecus aethiopicus is also considered robust and probably the specie from which the other robust australopithecine species descended. A. aethiopicus had a much higher degree of prognathism than the other robust species but shared in common the midsagittal crest, large mandible and dentition. A. aethiopicus lived approximately 2.5Ma.

Australopithecus robustus

Sometimes assigned to a separate genus, Pronthropus, Australopithecus robustus, is called so due to its features, particularly the larger, more powerful mandible and dentition. A. robustus lived approximately 2-1.3Ma. 

Australopithecus anamensis

Australopithecus anamensis, the oldest specie of australopithecine, lived approximately 4.2-3.9Ma. 

"Lucy"

The famous Lucy was nicknamed by Don Johanson who discovered her in Hadar, Ethiopia. Lucy is grouped among the gracile australopithecines, including A. afarensis, A. ghari, and A. anamensis, indicating a less robust and smaller framed skeletal structure. Australopithecus afarensis approximately 3.2Ma.

Sources Cited

Berge, C. & Goularas, D. (2010). A new reconstruction of Sts 14 pelvis (Australopithecus africanus) from computer tomography and three dimensional modeling techniques. Journal of Human Evolution, 58(3), 262-272.

Cowen, R. (2005). History of Life. Malden, MA: Blackwell Publishing. 

Kimbel, W. & Rak, Y. (2010). The cranial base of australopithecus afarensis: New insights from the female skull. Philosophical Transactions of the Royal Society B-biological Sciences, 365(1556), 3365-3376. 

Raichle, D., Pontzer, H., & Sockol, M. (2008). The laetoli footprints and early hominin locomotor kinematics. Journal of Human Evolution, 54(1), 112-117. 

Spoor, F., Wood, B., & Zonneveld, F. (1994). Implications of early hominin labyrinthine morphology for evolution of human bipedal locomotion. Nature, 369(6482), 645-648.

Ward, C., Kimbel, W., & Johanson, D. (2011). Complete fourth metatarsal and arches in the foot of australopithecus afarensis. Science, 311(6018), 750-573.