The evolution of Hominins has been through a number of stages which are a major influence on skeletal changes. In all the stages, Hominin’s behavior was adapted to various modes of survival. Before the bipedal walking and the upright posture, the hominins practiced terrestrial quadrupedalism, Orangutan-like climbing, and suspension, monkey-like arboreal, vertical squat jumps and knuckle-walking (Ko, 2015). The bonobos and the chimpanzee have been considered as the closest relatives of man and their anatomy and evolution history have been used as a model to study their features and anatomy. Some of the documented reasons for the change to bipedalism include; sexual dimorphism, climatic and habitual changes, freeing up the hands for carrying loads, protection against threats and food gathering (Niemitz, 2013).
The evolution of habitual bipedalism can be termed as the greatest milestone of the hominins and also a crucial adaptation of the human lineage. It is believed that the earliest hominins were habitual bipedal before the evolution of a fully human-like gait (Hatala et al., 2016). There have been conflicting debates as to the retention of traits related to climbing as well as the bipedalism of the Australopithecus which brought about conflicting interpretations to the similarity between the hominin bipedal traits and the modern man. Archeological hominin samples have shown the abduction angle of the hallux in relation to the weight transfer as well as the footprint shapes to be human-like to a certain extent (Hatala et al., 2016). The nature of various individual taxa’s bipedalism has had a heated debate among archeologists and different kinds of literature based on identified postcranial fossils of taxa. However, evidence shows the presence of a locomotor diversity of the ancient hominins.
Ecological and behavioral reasons for bipedalism
There are several theories that explain why the hominins became dominantly bipedal and with time replaced their ancestors who used other modes of locomotion. Some of the theories include the savanna-based theory, the provisioning model and the postural feeding hypothesis which could be among the supporting causations for the adaptations (Ko, 2015). the partial bipedalism of the early hominins when balancing to reach for food could be explained but eh postural feeding hypothesis (Takuma & Kase, 2017). The Savannah-based theory expounded on the behavioral traits that enabled the increasingly bipedal hominins to sit on the ground. The gathering of food and monogamy which comes after the feeding hypothesis can be explained by the provisioning model (Senut, Pickford, Gommery, & Ségalen, 2017).
Archeologists believe that the principle of natural selection played a major role in the human evolution and it was not a conscious activity. This has been supported by the evidence of observable mutation traits that lasted through natural selection and the evolution timeline (Indjeian et al., 2016). While the term natural selection is vague cause it can be narrowed to a wide range of things aiding the survival of the species. They include adaptation to different environments, protection against threatening predators, search for nourishment or more nutrients, parental care to the offspring among other beneficial factors to the hominins.
The Savannah-based theory suggested that the habitat forced the hominins to live and adapt to the open savannah with the absence of trees. Hence, the adaptation to walking on two feet allowed them to view over the long grass, watch over approaching predators and hunt (Smith, 2016). This hypothesis favored the upright posture. The theory also linked the bipedalism to survival aspects such as sentinel response and endurance running. However, the theory is challenged by the fact that the hominines could still climb trees (Smith, 2016). There was also the throwing hypothesis which was, however, not primarily behavioral but also required other anatomic adaptations such as the shoulder and arm. This hypothesis is supported by the trailing adaptation and use of the evidence of the earliest stone tools by the Homo erectus and other early hominins (Senut, Pickford, Gommery, & Ségalen, 2017).
The postural feeding hypotheses, on the other hand, suggested the maintenance of balance when obtaining food was convenient for the hominins (Wu et al., 2014). This then evolved to become a habit. They could reach out for fruits from trees above them which was also the case even on trees (Wu et al., 2014). It was also observed that the chimpanzee was bipedal when eating. The savannah environment presented numerous triggers for an upright posture such as fresh sprouts and buds on high bushes. The fact that this theory suggests that the hominines utilized the bipedalism to feed only like when harvesting or grabbing food it suggested that the bipedalism evolved as a feeding posture as opposed to a walking posture (Harcourt-Smith & Aiello, 2004). However, this theory only explains the extent to which the hominins were partially bipedal because the evolution involved walking on the ground and not on trees.
The threat model suggested the development of bipedalism as a defense mechanism. This was through behavioral and morphological undertakings to exaggerate visual signs as a way of scaring predators. For example, standing upright on two legs and displaying body movements and longer legs (Harcourt-Smith & Aiello, 2004). Another behavioral proposition was the thermoregulatory model. This model suggested that the bipedalism helped in dissipating heat because of the increased exposure of the body surface area. The hominins were also exposed to more favorable temperatures and wind above the ground surface (Harcourt-Smith, 2010). however, the anatomy of the hominins also caused limitations to survival tactics. For instance, their speed was slower as compared to quadrupedal which relatively increased predator threats (Machnicki, Spurlock, Strier, Reno & Lovejoy, 2016).
The provisioning model theory, which was brought about by Owen Lovejoy, gave a supportive explanation of Darwin’s theory. He suggested that bipedalism was an adaptation for pair bonding (Naya, Naya & Lessa, 2016). This is because the use of two hands to carry the loads was effective for food transport. This gave the hands an alternative to locomotion He went ahead to suggest that the sexual dimorphism in food gathering enhanced the chances of infant survival. In this case, the males provided for the females while the females took care of the offspring (Harcourt-Smith & Aiello, 2004). The provisioning male would then mate exclusively with the female it provided for which also brought to an end the fights over females and over time the extinction of the blade-like canine teeth. The infant carrying hypothesis also found it worthy of the hunting hominins to carry their infants on their waist while standing erect as opposed to using one arm (Maslin, Shultz & Trauth, 2015). This hypothesis has, however, been refuted due to the fact that it is more energy costly without the use of a sling
The biological and anatomical argument for bipedalism
Bipedalism was made possible by a number of skeletal changes that can be identified as compromised adaptations. The anatomy and the biological makeup of the hominins and the modern human being is unique and has developed features that enable the firm support of the vertical trunk of the body as well as transferring the human body weight to the legs (Harcourt-Smith & Aiello, 2004). The leg structures are also well developed to coordinate the different activities practiced by man such as running walking, standing and sitting (Harcourt-Smith, 2010). There have been debates about the ancient hominin foot bones and no agreement on the locomotor adaptations. The prevailing suggestions identify that the evolution of bipedalism was in a relatively linear way originating from a hylobatid-like ancestor to a vertical climbing stage then a knuckle-walking stage and finally the obligate bipedalism (Harcourt-Smith & Aiello, 2004). Theories of bipedalism can be categorized based on fossil remains or the anatomical differences of the extant hominid taxa.
The foot
The evolution of the hominin foot has been linked to many leading theories of the development of bipedalism. The hind limbs evolved for the propulsion and holding the body’s weight. The bipeds’ feet are arched in a transverse and longitudinal way while these arches lack in the quadruped (Naya, Naya & Lessa, 2016). The quadrupeds required more energy to stand or walk on two feet since their feet required more muscles to maintain balance. As a sequence, bipedalism locomotion involves the movement of one foot at a time whereby the torso weight moves between the swing and stance phases. It is argued that the preceding ancestors of the hominin had a foot structure resembling that of a hypothetical dryopithecine (Indjeian et al., 2016). Its morphology was postulated and had features of the Pan and Hylobates. It had smaller tarsals than Pan but had elongated and curved digits than those of Hylobates (Harcourt-Smith & Aiello, 2004). It was likened to that of the gorilla. The foot had a longer heel, a less abducted hallux, less torsion between the hallux ant the metatarsals, shorter rays and a less grasping potential. The foot was, however, flexible and could grasp. It is then considered that the modern human foot evolved from the terrestrial gorilloid ancestor.
Other studies argue that the anatomical evolution came from an ape-like foot. This is one that has a human-like opposable hallux, a big toe centered in line with the lateral toes. The tarsal is also stable to act as propelling lever (Harcourt-Smith & Aiello, 2004). Evolution then made the foot more adducted to the joint hence maximizing stability. In relation to this, the modern man’s foot is more closely packed to permanency. To humans, this feature enhances proper weight distribution during take-off while to apes it enhances grip (Hatala et al., 2016). Kidd (1999) argued that the lateral side of the hominin foot was the first to evolve hence stabilizing the midtarsal flexibility which was an adaptation to terrestrial conditions. The medial side evolved second. Evidence of the bipedal locomotion discovered in the Homo erectus and the A. africanus had a major effect on their acceptance as hominins.
In comparison, the heel bone is larger in bipeds as compared to quadrupeds especially the calcaneal tuberosity which is the posterior portion. The enlarged calcaneal tuberosity helps in the absorption of the force brought about by the body weight hence offering stability. Additionally, its shape in the bipeds provides the strong ligaments running from the arch to the tibia with a place for attachment (Indjeian et al., 2016). The hindlimbs elongated while the forelimbs shortened as opposed to the quadrupeds that had stronger and longer arms to hang on tree branches. The femur of quadrupeds was vertical and evolved to a slightly angled from the knee. This adaptation enabled the bipeds to stand longer and locomote by keeping their knees together and maintaining the center of gravity (Morimoto, Nakatsukasa, Ponce de León & Zollikofer, 2018). The gluteus muscle which was smaller in the quadrupeds gained more importance in the bipeds by preventing the upper body from falling forward when running.
The hands and fingers of hominins also evolved to adapt to the different roles. The quadruples had relatively curved fingers and toes due to their arboreal behavior and their grasping nature (Almécija, Smaers & Jungers, 2015). The bipeds, on the other hand, possess flat pedal and hand phalanges due to their reduced or no arboreal activities. This evolution increased the precision of hand movements which enabled the bipeds to make and use tools (Almécija, Smaers & Jungers, 2015).
The Evolution of Skeletal Structures
Further skeletal evolutions occurred in order to accommodate the bipedal adaptations and way of life. The changes included the size, shape and arrangement bones, morphological changes to the skeleton. The pelvis evolved from a tall and narrow one in quadrupedal hominins to a short and broad one (Warrener, Lewton, Pontzer & Lieberman, 2015). The iliac form a saddle-like structure around the waist and provides support for the upright posture and locomotion (Harcourt-Smith & Aiello, 2004). On the other hand, in the quadrupeds, they lie flat on the back to aid in the attachment of climbing muscles.
The spine evolved to form an S-shape in biped hominins from the arch-shape in quadrupeds. This ensured maximum mobility as well as maintaining a center of gravity just above their feet (Morimoto, Nakatsukasa, Ponce de León & Zollikofer, 2018). The spine evolved much slower with the earliest hominins having an extra vertebra at the lower back which led to the development of the lumbar curve. The lumbar vertebra is crucial for the maintenance of balance especially during the walking cycle which involves the swinging of the legs one at a time (Dowdeswell et al., 2017). Bipeds also have more and larger lumbar vertebrae than quadrupeds. This enhances the flexibility of the lower back which is fundamentals to the swinging of the hips and trunk when walking. The foramen magnum became centered from its earlier posterior position in quadrupeds (Warrener, Lewton, Pontzer & Lieberman, 2015). This means that the spinal cord runs perpendicularly to the ground unlike in quadrupeds where it is parallel to the ground. The larynx is also lower in bipeds than in quadrupeds.
Conclusion
The evolution of bipedalism can be justified in a number of theories and models. The evolution both had a number of advantages and setbacks. The benefits of the evolution can include; the preservation of energy, ability to carry food items since the hands were freed, improvement of locomotion by being able to travel longer distances and durations, protection against potentially harmful predators as well as improving the hunting methods by the precise use of tools. However, Lovejoy identified some behavioral setbacks such as reduced velocity which increased predatory pressures. Certain morphological problems were also brought about by new adaptations such as the subjection of stress to joints due to a change in locomotion behavior. The anatomical evolution has been proved by archeological findings most of which were discovered before 1960. The foot, spinal cord, knees, arms, legs and the hip joints greatly evolved in order to adapt to bipedalism and the erect posture as well as the activities that accompanied the posture. The female pelvis had an extra adaptation of childbirth too alongside locomotion. These characteristics are evident in the modern man.
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