Yavuzer, (2020) argues that the main functions that distinguish humans from all different primates are the bipedal gait’s continuous use. This available function is visible as this type of definition function, with which skeletal variations similar to bipedalism are mostly used to pick out our extinct hominid progenitors. Given the lack of fossil stocks, the fragmentation of fossil remains, and the problem of inferring behavior from fossils, questions remain widespread unanswered regarding the development of human bipedalism (Yavuzer, 2020). However, experiments over the last thirty years have been conducted to study locomotion in humans. Various primates have accomplished a great deal in expanding our expertise in human locomotion mechanics and providing insight into human bipedalism’s evolution. Of all existing primates, are humans the best obligatory bipeds?
Holowka & Lieberman (2018) argue that humans are not the only creatures to possess the bipedalism trait. Humans stride, whereas most mammalian bipeds waddle or hop. Primates are unique from most mammals because they possess a type of locomotion that is hind-limbed. Accordingly, bipedalism in humans has evolved from the basic structure of the primate body plan, where the hind limbs have been structured for upright posture when feeding, locomotion, and resting.
Theories of Bipedalism
Highly specialized post–cranial adaptations, particularly within the waning limb, represent this specific form of locomotion. The foot is best suited to the fact that the locomotion of the biped increases. Since the biped locomotion increases, the foot becomes the best form, directly in contact with the ground, and sooner or later finds itself under a heavy load of selection with every stability and every drive in an exact greenway (Holowka, O’Neill, Thompson, & Demes, 2017). Even within the mainly arboreal, grandiose monkeys, the declining limb is continuously the essential locomotion limb. For existing primates, this is important for our specialist knowledge of the origins and development of bipedalism.
Many theories have been developed, attempting to explain why humans are bipedal. However, none of them have been convincing enough. Firstly, speed can be disqualified since humans are generally not fast. Like Darwin, ancient scientists linked the hands, which are left free due to the bipedal gait, to tool use. However, this theory raises a lot of questions since 3.3 mya is when the earliest stones have been dated, which is way long after the hominins had already become bipedal (Holowka, O’Neill, Thompson & Demes, 2017).
Several other theories have been proposed in the twentieth-century over the evolution of hominin bipedalism. These theories present factors like energy conservation, carrying objects, sexual displays, and standing in all grass vigilantly as key to bipedalism’s evolution. However, some scientists argue that pre-bipedal primates must have been terrestrial quadrupeds, just like the current bonobos, gorillas, and chimpanzees. Conversely, it can also be argued that the ancient habitual walkers were already well equipped for tasks like running or terrestrial bipedalism, upright posture to forage overhead, climbing vertical vines, and tree trunks. This layout is similar to the studies conducted on the gibbons, which are known to have the ability to run bipedally but only when pushed to do so (Landi, Profico, Veneziano, De Groote & Manzi, 2020).
Gibbons have robust and long hind limbs. During locomotion, they stand more uprightly than chimpanzees and exert less energy when running than when climbing vertically or jumping along the branches; hence acquiring a bipedal stance would have been an easy task. The lower joints, ligaments, and bones would have undergone some alterations as well as the foot. A humanoid hip, foot structure, and knee would evolve due to the erect standing using the hind limbs and foraging.
Why Bipedalism Evolved So Rapidly
Yavuzer (2020) argues that bipedalism is correlated to the upright posture displayed by most primates. Apes branchiate with a vertical suspension of the body, monkeys sit semi-upright, and almost all gibbons nurse their young in an upright posture. However, humans are unique gibbons in the sense that they walk on two legs.
As Africa became drier and warmer, bipedal locomotion aided in thermoregulation (Landi, Profico, Veneziano, De Groote & Manzi, 2020). This is also why selection chose to reduce body hair and increase sweat glands. However, gorillas and infant chimps cling to their mothers’ long hair using their prehensile feet or hands. Human babies, though, have to be carried by their mothers since they don’t have prehensile feet. This fact may have propelled the evolution of bipedalism. Partially bipedal hominins, with a large toe facing forward slightly, would have less grasping feet meaning that infants would have found it challenging to grasp their mothers, who would have to be compelled to use their arms to hold them. Since their arms would be less accessible for locomotion, they would rely a lot on their legs, increasing the advantage of getting a forward-facing big toe leading to a positive feedback cycle.
The mother’s dependency may have influenced some of the social behaviors practiced by our ancestors to carry the bairn. Primates are not known to share food among themselves. However, this may have been a disadvantage to the females if they had been forced to carry their infants since it would have been impossible for them to forage for food. Thus males may have been more involved in that role. Later, the infants’ development would allow the freeing of hands allowing females to participate in the development of tools leading to better diets. This was another positive feedback cycle that also resulted in the brain’s development (Landi, Profico, Veneziano, De Groote & Manzi, 2020).
Significance of Bipedalism
The vast advantages that came along with bipedalism meant that this trait would be passed on to future hominid species. Bipedalism made it possible for hominids to free their arms completely. This meant that they could use their tools and engage in tasks like plucking fruits from trees and defense from predators. They could use their hands for communication and social display (Farris, Kelly, Cresswell, & Lichtwark, 2019). Bipedal hominids had less surface area exposed to sunlight, meaning that they could spend much time out in the savannah scavenging and foraging. Bipedal locomotion also played a pivotal role in ensuring energy was conserved. This energy could later be used in reproduction hence increasing the chances of passing on the trait to their offspring (Farris, Kelly, Cresswell & Lichtwark, 2019).
Early hominid bipedalism through experimental data is suggested to have evolved in a climbing primate in an arboreal environment. A significant form of locomotion displayed in current humans is said to have been the earliest form of bipedalism, excluding the reversed pendulum-like mechanics. Unstabilized, small, and gracile hind limbs present in our ancient ancestors seem to be the significant differences in movement styles between modern humans and early hominids. The adoption of a stiff-legged style observed in the genus Homo reflected the shift to a modern skeleton, which changes our complete understanding of locomotion and its adaptations in the genus Homo, suggesting that these features may have evolved recently.
Holowka, N. B., & Lieberman, D. E. (2018). Rethinking the evolution of the human foot: insights from experimental research. Journal of experimental biology, 221(17).
Yavuzer, M. G. (2020). Evolution of bipedalism. In Comparative Kinesiology of the Human Body (pp. 489-497). Academic Press.
Holowka, N. B., O’Neill, M. C., Thompson, N. E., & Demes, B. (2017). Chimpanzee and human midfoot motion during bipedal walking and the evolution of the longitudinal arch of the foot. Journal of Human Evolution, 104, 23-31.
Farris, D. J., Kelly, L. A., Cresswell, A. G., & Lichtwark, G. A. (2019). The functional importance of human foot muscles for bipedal locomotion. Proceedings of the National Academy of Sciences, 116(5), 1645-1650.
Landi, F., Profico, A., Veneziano, A., De Groote, I., & Manzi, G. (2020). Locomotion, posture, and the foramen magnum in primates: Reliability of indices and insights into hominin bipedalism. American Journal of Primatology, 82(9), e23170.