Bipedalism

13-2 Bipedalism

Although many anthropologists have proposed single selection pressures as the cause of bipedalism, such as standing on branches to reach higher branches, [1] John Langdon argued in 2023 that the evolution of bipedalism was complex with several selection pressures, stages, and end points.[2] Langdon stated that bipedalism evolved more than once in different lineages, which indicates that there is a natural selection pressure for bipedalism. It is possible that there were three independent bipedal hominids in the 7 Ma to 4 Ma time range: Sahelanthropus, Orrorin, and Ardipithecus. The current view is that the shift to bipedalism was a long and gradual process that took place in woodlands between 10 and 4 Ma, with hominins maintaining an ability to climb and acquire food in trees during the transition. The second half (@ 21 minutes) of  https://www.hhmi.org/biointeractive/episode-3-your-inner-monkey describes the shift to bipedalism, as does the following PBS video. https://youtu.be/3bFtotU0of4

Ardipithecus ramidus  (Figure 13-3) lived 4.4 Ma also lived in woodlands. It had a pelvis and femoral shaft like a climbing ape, and an opposable big toe for grasping branches, but other bones in the foot indicate that the foot acted like a lever, which would facilitate bipedal walking. Ardipithecus kababa lived 5.6 to 5.2 Ma, but its bipedal status is uncertain. It had extremely long fingers and toes that enabled it to walk along tree branches. It had a skull like a chimpanzee, indicating the evolution of human bipedal ancestors from chimp-like primates.

Figure 13‑3. Ardepithecus ramidus (Ardi, 4.4 Ma). Left. Credit: Jay Matternes. Fair use. Wikipedia. Right. Ardi skeleton. http://www.sciencemag.org/cgi/content/full/326/5949/64/F3 Fair use. Wikipedia

Australopithecus is the most famous early hominin. Australopithecus anamemsis (4.2 – 3.8 Ma) was the first Australopithecus species (Figure 13-3). Its skull looked like more apelike than human, but its joints indicated regular bipedal walking.

Australopithecus afarensis was the next Australopithecus species to inhabit Africa, 3.85 – 2.95 Ma. Its face was also apelike. It had long fingers and strong arms for an arboreal lifestyle. Their normal walking style was upright walking on two legs. The oldest unequivocal evidence of bipedalism is footsteps left behind in mud by two or three Australopithecus aferensis, 3.6 Ma (Figure 13-4). Mary Leakey discovered these footprints at Laetoli in Olduvai Gorge.

Australopithecus afarensis (44 kg) was similar in appearance to Australopithecus anamensis (possibly its ancestor), but it was 20% smaller (56 kg). Although it still had a small brain, its face was on the progression toward human (Figure 13-6).  Both had a height of less than one meter. The face of Australopithecus afarensis was still ape like. The famous Lucy fossil (Figure 13-5) was Australopithecus afarensis. Lucy reconstructions show that Australopithecus afarensis had a nearly modern gait.  This video compares the gait of chimpanzees, Australopithecus afarensis, and modern humans. https://youtu.be/xT8Np0gI1dI

Figure 13‑5. Left. Australopithecus afarensis skeleton (Lucy). Cleveland Natural History Museum. Credit: Andrew. Used here per CC BY-SA 2.0. Right. Restoration (Lucy) at Natural History Museum, Vienna. Credit: Wolfgang Sauber. Used here per CC BY-SA 4.0.

Raymond Dart discovered Australopithecus africanus (3.3 – 2.1 Ma), the Taung Baby, in 1924. Scientists were surprised that even though it had a body more like a human, it had a small brain like an ape (Figure 13-6), 380-450 cc. At first, Australopithecus was rejected as a human ancestor because the model of human evolution at the time was that the brain became large before the body became human; however, Australopithecus showed that human evolution began with a small brain and a bipedal humanlike body (Figure 13-5). For example, Australopithecus had legs with the knees close together as with modern humans. It also lost grasping toes on the feet, which means it was fully bipedal. Australopithecus africanus skull was progressing toward the human skull. It had a larger brain, small teeth, and rounder cranium (Figure 13-6) than Australopithecus afarensis.

Many scientists think that Australopithecus sediba (Figure 13-7) is a transitional fossil between Australopithecus species and Homo habilis. In Figure 13-7, A. afarensis, Lucy, is between two A. sediba skeletons. The shape of the A. sediba skull is like that of A. africanus, however it is probably more human (more derived). Some researchers think that Australopithecus sediba shares traits with Homo habilis, but this interpretation is controversial.

The shift to bipedalism required structural changes in the spine and pelvis (Figure 13-8).[4] Walking upright puts inconvenient torques on a spine that had been part of four-legged walking for 300 million years.  The typical mammalian spine acts like a cantilever bridge with a central arch. It suspends the internal organs between the front and hind limbs. The spine shifted in bipedal primates to a central column, which necessarily became an S curve to attain balance with the chest in front and to elevate the head. The S shape also acts as a shock absorbing structure. The S curve is initially not in the spine, but it forms in infancy. The spine did not evolve to carry the weight or have this shape, so humans have many back problems. The fact that the weight is no longer supported on four limbs places extra stress on the knees and feet. Because of the stress due to bipedalism, other primates have extremely low incidences of arthritis in comparison to humans. Another problem with bipedalism is that the blood is pulled down toward the feet, which must then be pushed back up by the heart. Development of a heart and circulatory system able to function in a vertical body must have been and additional evolutionary impediment toward bipedalism. Varicose veins and hemorrhoids are a consequence of high blood pressure in the human lower body. The early biped legs needed to realign for bipedal walking. The knees were brought together, and the legs straightened to enable an even gait rather than walking with bent legs and knees apart. 

As with the limbs on tetrapods and the wings on birds, the shift to a vertical spine was guided by the same Hox gene system that was in the original vertebrate fish; however, in this case, it was Hox genes 6 thru 12 (Figure 13-9) rather than Hox-13. As with birds and tetrapods, it would be difficult to ascertain which changes in Hox genes were due to random mutation and which might have been due to divine intervention. Langdon stated that Hox gene activation along the spine is triggered by a retinoic acid gradient and changes in fibroblast growth factor (Figure 13-9). 

Williams and Pilbeam provided a fascinating discussion of the specific changes in Hox gene specification of the vertebral column between early hominoids, chimps, and bipedal humans. [1] It is a complex and contentious field of study. The numbers of vertebrae varied more in apes than in later hominids. Selection for bipedal walking reduced the variation in numbers of vertebrae. Analysis of vertebral numbers indicates the descent of humans from chimps and bonobos rather than other apes.

References

[1] Senut, B., M. Pickford, D. Gommery, and L. Ségalen. "Palaeoenvironments and the origin of hominid bipedalism." Historical Biology 30, no. 1-2 (2018): 284-296.[1] Michael Brunet and others, A New Hominid from the Upper Miocene of Chad, Central Africa, Nature, 418 (2002):145-51.

[2] Langdon, John H. "The Evolution of Bipedality." In Human Evolution: Bones, Cultures, and Genes, pp. 191-247. Cham: Springer International Publishing, 2023.

[3] Brunet, Michael et al. A New Hominid from the Upper Miocene of Chad, Central Africa, Nature, 418 (2002):145-51. 

[4] Dale, Michael T. "The sexual selection of hominin bipedalism." Ideas in Ecology and Evolution 11 (2018). 

[5] Williams, Scott A., and David Pilbeam. "Homeotic change in segment identity derives the human vertebral formula from a chimpanzee‐like one." American Journal of Physical Anthropology 176, no. 2 (2021): 283-294.

Banner

Map of early bipedal homonids. Credit: Kameraad Pjotr & Sting . Used here per CC BY-SA 3.0