Bipedalism in Human Evolution: Theories and Adaptations

Introduction

Human evolution takes into account both the biological and cultural development of humans. Human understandings of how the evolution of man came to be are shaped by beliefs espoused by scientists and societies dating back some 400 years. The human species, scientifically referred to as Homo sapiens, has evolved enormously over the past several million years. Numerous scientific developments and distinct events gave rise to the ultimate evolution of humankind. One of the key changes that took place through evolution is bipedalism, which encompasses alterations in body features — for instance, an increase in brain capacity. In particular, bipedalism is a form of locomotion conducted on two feet and is the one aspect that distinguishes humans from other kinds of hominoids (Ishida et al., 2006).

The purpose of this paper is to examine and emphasize the origin of bipedalism in terms of the different predominant theories regarding it and the manner in which it has given rise to numerous developments in the origin story of human beings. In addition, the paper undertakes an analysis of the bodily adaptations that took place in order to sustain bipedalism, and the consequences this new means of locomotion brought with it.

Savannah and Mosaic Theories

The very first attempt to explain the evolution of bipedalism was made by Darwin through the Savannah theory. Essentially, this theory argued that an ape living in trees began spending longer periods in the open savannah as its forest home declined owing to climate change. As a result, the ape progressively developed and adapted a bipedal posture so as to free its forelimbs for wielding primitive tools such as stones and sticks. However, since it was eventually acknowledged that all of the aforementioned attributes, including bipedalism, could have been selected against the savannah ecology, many proponents of this theory found it unconvincing. Moreover, primates that presently live in that environment lack such attributes. Another theory lacking merit at the time was the Mosaic theory. This model argued that the aforementioned ape began subsisting in a mixed environment consisting of forest, swamp, and savannah. Its downside is that it failed to offer any convincing reason for the evolution of the ape into a bipedal form of locomotion (Kraak, 1991).

Postural Feeding Theory

Chimpanzee biology together with australopithecine functional morphology suggest a combined terrestrial and arboreal postural feeding origin for bipedalism among hominids. According to Hunt (1996), bipedalism among chimpanzees typically appears while feeding on fruits in the forest — most often by reaching up to pluck fruits while standing erect, or by maintaining balance on tree branches while holding on in a semi-arm-hanging position. Some anatomical features shared by Australopithecus and apes are adapted to reduce muscle exertion and physical fatigue during arm-hanging. Additional ape-like features in hominids — including curved hand bones and metacarpals — are adaptations for both upper-limb hanging and erect climbing. The anatomy of the australopithecines, especially with respect to the hind limb and hip regions, is suggestive of characteristic bipedal locomotion on the ground. However, when compared to modern human beings, their movement was less efficient and created greater strains in the hip. Taking this into account, it is posited that hominid bipedalism may have originated as a feeding posture — with arboreal arm-hanging, bipedalism, and erect climbing as significant means of gathering food — before being further developed into an efficient locomotor adaptation (Hunt, 1996).

This theory is further advanced by Skoyles (2006), who outlines a new model of the distinctiveness, nature, and evolution of human bipedalism in the context of the etiology of equilibrium dysequilibrium syndrome disorder. From a biological perspective, human bipedalism is novel in several noteworthy respects. First, humans are obligate, habitual, and varied in their bipedalism. Second, they hold their body posture spinally vertical in a multi-segmental stance against gravity. Third, they use their forelimbs for purposes other than locomotion. Additionally, body weight is fully supported by vertical balance, and such bipedalism is combined with upper body movements that rapidly shift the body's center of mass. "Due to the situation that humans are obligate erect terrestrial animals, two frameworks — the body-defined and gravity-defined frameworks — are in constant alignment in the vertical z-axis." This allows the human sense of balance to adapt egocentric body perceptions so as to detect deviations from the gravitational vertical. The argument is that cortical regions handling three-dimensional and other perceptions required for vertical balance were a significant driver of brain size development in Homo erectus (Skoyles, 2006).

Behavioral Theory

The behavioral theory of the origin of bipedalism was developed by Owen C. Lovejoy. Its central position is that the social behavior of human beings is precisely what drove them to become bipedal. More specifically, Lovejoy emphasized the social behaviors that influenced human survival and reproduction. He posited that the sexual behaviors of early humans, together with their anatomy, implied a monogamous reproductive arrangement that was seen as a common necessity for the male. The practice of carefully selecting mates on the basis of their locomotion — or the manner in which they walked — was reinforced by the notion that if a male used his upper limbs to gather and bring food to a mate, the mate perceived this as a significant selection criterion for producing offspring, owing to the bipedalism involved. This signaled a longer lifespan for the offspring, a greater ability to survive, and a superior rate of reproduction compared to the parents. It was therefore necessary for all males to use two limbs for locomotion in order to secure mates (Kinzey, 1987).

However, a shortcoming of this theory is its argument that hominids had a monogamous mating arrangement, which is inconsistent with the sexual dimorphism in body size observed among hominids. Among non-human primates, sexual dimorphism in body size is most convincingly explained by sexual selection resulting from differences in reproductive potential between sexes. It is difficult to accept that greater male body size in hominids did not accompany greater variance in male versus female reproductive success, and some degree of polygyny. In light of this, Lovejoy's arguments regarding size differences reducing food competition are not entirely persuasive. It is only in orangutans and chimpanzees, among primates, that a notable degree of sexual segregation in food use is observed — and in both of these non-human primates, sexual dimorphism in body size aligns with male reproductive competition and polygamous mating arrangements (Kinzey, 1987).

Conclusion

Bipedalism is a unique feature of human beings. The evolution of this attribute is elucidated by different theories that seek to explain its origin and the way in which it has given rise to several adaptations and developments in the origin story of human beings. This paper discussed three particular theories: the behavioral theory, the postural feeding theory, and the thermoregulatory structures theory. Despite the fact that all three theories advance arguments that may seem plausible, each has its shortcomings and points of incongruence with the fossil record. From the author's perspective, the postural feeding theory offers the most convincing explanation for the origin of bipedalism and its subsequent developments in hominids. Lovejoy's behavioral theory links bipedalism to the social, sexual, and reproductive characteristics of early hominids. Hunt's ecological postural feeding theory outlines the need for hominids to employ their arms to reach food and maintain an upright posture.

In essence, the postural feeding theory most convincingly accounts for the origin of bipedalism because fossil archives have shown that early humans used multiple postures for feeding — both terrestrial and arboreal — which plausibly explains where hominid bipedalism originated. This is tied to the effort of hominids to reach up into trees while simultaneously remaining erect on the ground, or to climb into trees and maintain balance using branches so as to grasp and eat fruits and pods. Moreover, this theory is grounded in the most fundamental necessity of any organism — finding food in order to survive. Throughout the story of evolution, animals have repeatedly adapted their body features to become more effective feeders, thereby surviving to successive generations. Human bipedalism, as explained by the postural feeding theory, represents exactly this kind of adaptive transformation.

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