Australia: The Land Where Time Began

A biography of the Australian continent 

Fossil Ape hints how Bipedal Walking May Have Evolved

An ape with arms that were suited to hanging in trees, though it had legs that were human-like, was revealed by an about 11.6 Ma fossil, which suggests a form of locomotion that might push back for when bipedal walking evolved. There have been longstanding questions in regard to when, why and how ancestral early humans became bipedal, ever since the basis for understanding human evolution was provided by the work of Charles Darwin. A defining feature that enables assignment of fossils to the hominin lineage, which includes all species that are more closely related to humans than are chimpanzees (Pan troglodytes) and bonobos (Pan paniscus), which are the species that is closest to humans, is the commitment to terrestrial bipedalism, which is characterised by skeletal adaptations for walking regularly on 2 feet. The answer to the “when” question has been believed to be between 7 Ma and 5 Ma at the end of the Miocene Epoch (23 Ma-5Ma), on the basis of fossil findings, some of which are more controversial than others (1,2).

A lot depends on the method of locomotion that was being used prior to the evolution of terrestrial bipedalism, when it comes to answering the questions of when and how bipedalism evolved in humans. Did it evolve from an ancestor that spent most of its time in the trees, or were these ancestral forms that were still walking on 4 feet on the ground, and subsequently evolved the ability to stand on 2 feet? It was reported by Böhme et al. in Nature that they had discovered an ape species, (Danuvius guggenmosi) from the Middle Miocene. Previously, the method of walking used by this species was unknown, which locomotion from which human bipedalism evolved. Addressing the questions about the origin of hominin bipedalism, how the last common ancestor of humans and chimpanzees and bonobos has conventionally been by using an approach of either top down or bottom up. Darwin, as well as many paleoanthropologists, favoured the top down approach, in which they examine the living primates, the great apes in particular, for clues as to how bipedalism evolved.

The African apes, chimpanzees, bonobos and gorillas, of the genus Gorilla, spend time in the trees to sleep, eat and when they need protection, but spend most of their time on the ground, where they walk on their knuckles. Given the close relationship between humans and the apes, the similarities between our hands and feet and those of those of apes, some authors have suggested that bipedalism evolved from a knuckle-walking ancestor (5)       or a more generalised quadruped that didn’t have knuckle-walking specialisations (7), that divided their time between the ground and the trees. Others have noticed that the bipedal way that orang-utans (genus Pongo) move through the trees, and the mechanical similarities between the way apes use their legs for climbing and the way humans use their legs in walking, suggest that bipedalism evolved from an ancestral ape that was committed to life in the trees (6,8). A genus of fossil ape, e.g., Nacholapithecus had a body that was similar to that of a monkey, though they had large forelimbs and long toes, whereas another genus of ape, Sivapithecus, had a face like that of an orang-utan, and shoulders that were ape-like, and a pelvis and elbow that were like those of  a monkey (10,11). It is suggested by such an odd combination characteristics that indicate arboreal suspension (hanging from tree branches), quadrupedal movements and body postures that are difficult to imagine at the present, as well as making it hard to determine their probable patterns of locomotion. Böhme et al. have added to this amazing diversity from the Miocene by presenting fossils of D. guggenmosi that are about 11.6 Myr old. Böhme et al. interpreted the shape of the fossils of D, guggenmosi as indicating a type of locomotion that was previously unknown that they term extended limb clambering which combines adaptations for both suspension in the trees and bipedal locomotion. According to Böhme et al. this makes it a good possible model for the last common ancestor.

According to Kivell D guggenmosi was identified by its teeth as belonging to a group of fossil ape species, the dryopithocins that have been recovered from sites dating to the Late Miocene in Europe, which some considered to be ancestral to African Apes (9). Living species of African Apes inhabit equatorial Africa, though at times in the Miocene, many ancestral great apes were living throughout Europe and Asia (Euroasia), a time when they were migrating both to and from Africa. It has been suggested by some researchers that features found in chimpanzees and gorillas at the present have also been found to be exhibited by dryopithocins and therefore make good candidates for ancestral species of living African apes (9). When compared with the other dryopithocin species both in its preservation of 2 limb bones that are, an ulna (a forearm bone) and a tibia (leg bone) and in the characteristics it displays. Böhme et al. focused their attention on partial skeletons that are baboon-sized and probably male. The skeleton includes some vertebrae, a partial thighbone (femur), and hand and foot bones, as well as the ulna and tibia. The forearm of D. guggenmosi was shown by the ulna length to that of the tibia, was long relative to the leg, which is similar to the form in bonobos. The forelimb has telltale signs of an arboreal suspension, as found in all extant great apes, combined with a flexible elbow and hand bones that indicate a powerful grasping thumb and curved fingers.

A different story is told, however, by the lower limb of D. guggenmosi, and one that is more reminiscent of human lower limbs than those of great apes. The shapes of the femur and tibia suggest the use of extended upright hip and knee postures, which differ from the bent hips and knees used by extant African apes when they occasionally walk bipedally on the ground or in the trees. Reinforcing of the top of the tibia and the increased stability of the ankle joint, are both properties that are adaptations for resisting the higher load that is placed on the lower leg when moving on 2 limbs instead of 4. But there is a long robust big toe on the foot that would be good for grasping, which suggests that D. guggenmosi might have walked flatfooted on branches. It is not so clear if it walked bipedally on the ground.

Taken together, this mosaic of features of D. guggenmosi provides, arguably, the best model yet of what a common ancestor of humans and African apes might have looked like. It offers something for everyone:

1)    The forelimbs that are suited to life in the trees, that all living apes, including humans, still have;

2)    Lower limbs suited to extended postures, such as those used  by orang-utans, during bipedalism in the trees (8);

3)    And further specialisation of such features of the lower limbs in humans to enable habitual terrestrial bipedalism.

If it has been accepted that the locomotor behaviour that has been observed in extant great apes and humans evolved from an ancestral form that used extended clambering, then this would answer the question of what kind of early locomotion was the basis of the origins of bipedalism. And that would answer the question of why and how ancestral humans became less dependent on an arboreal life and embraced fully bipedal locomotion. Until more evidence is uncovered of how the evolution is found, a bottom-up approach from the Miocene is probably the best approach to deciphering the evolution of one of our most defining features.

Sources & Further reading

Kivell, T. L. (2019). "Fossil ape hints at how walking on two feet evolved." Nature 575(7783): 445-446.