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The family of primates, ranging from 13 to 16 separate families, includes over 200 individual species. Within these grouped species is our own species, Homo sapiens. In terms of primate locomotion, expressed forms of locomotion include various forms of quadrupedalism, tripedalism, vertical climbing, leaping, tail swinging, suspensory, and bipedalism, as discussed in John Fleagle’s 1998 book Primate Adaptation and Evolution (see also Hunt et al., 1996). Bipedalism is a remarkable form of locomotion. Though many nonhuman primates occasionally use a form of bipedalism, humans are the only primate species that uses a distinct and obligate form of bipedalism as a primary form of locomotion.
Similarities among nonhuman primates, particularly the great apes and the human primate in terms of morphological, physiological, and social characteristics has confirmed many early naturalists’ views of humankind’s origin and relationships with extant primates. Such shared characteristics within an evolutionary framework affirmed Charles Darwin’s view on humankind’s “lowly origin” with his summation that humans differ only in degree but not in kind from the great apes. With the discovery of fossil hominins and hominids, the classically “human” distinguishing features become less pronounced. As more fossil evidence is recovered, the scientific community is faced with questions regarding the emergence of human bipedality among various forms of primate locomotion within the fossil record. Though the fossil record is incomplete and recovered fossils have blended ape and human characteristics, bipedalism (at least in some form) is suggested to have existed about 6 million years ago. If these dates and the interpretation of the fossil evidence are correct, a nonhuman form of primate bipedalism and quadrupedalism coexisted farther back than previously estimated. Primate locomotion, similar to morphology, becomes an interesting aspect of our evolutionary descent.
Types of Primate Locomotion
The geographic distribution of primates is limited to the tropical, temperate, subalpine, and alpine biomes of South America, Central America, Africa (including Madagascar), Asia, and the Oceania areas. Nonhuman primates can be found in rainforests, seasonal forests, woodlands, savannas, semideserts, evergreen forests, elfinwoods, and meadows. With the addition of the human species, primates can be found throughout the globe and in every habitat. Considering the totality of the nonhuman primate species, a particular mode of locomotion is not species specific. Rather, activity (e.g., being diurnal or nocturnal) and arboreal stratification and terrestrial territoriality foster evolutionary competition. The habitual modes of locomotion for arboreal primates primarily consist of branch quadrupedalism and/or bipedalism and brachiation. Vertical climbing and leaping are the second and third most common mode of locomotion, respectively. Suspensory behavior, including arms, feet, and tail, are least common.
Primate locomotion, whether arboreal or terrestrial, is determined on the physical terrain and by the placement of the upper and lower limbs in relation to the adjacent limbs. While in motion, hand and foot configurations can be palmigrade, plantigrade, or digitgrade, and manifest as knuckle-walking, fist-walking, grasp-walking, serpentine grasp-walking, schizodactyl grasp-walking, or clawed quadrupedalism (Hunt et al., 1996; Vilensky & Larson, 1989). Consequently, locomotive gaits among primates could be either symmetrical or asymmetrical, for example, an evenly timed footfall pattern as opposed to an unevenly timed footfall pattern. In addition, gait sequence can be either a diagonal sequence (DS) and diagonal couplet (DSDC) or a lateral sequence (LS) and lateral couplet (LSLC). In a diagonal sequence, the placement of the forefoot is followed by the diagonal hindfoot. The lateral sequence has the forefoot following the ipsilateral hindfoot. It is interesting to note that the human primate develops along similar patterns. During the emerging human locomotive development, both types of gait sequence and variations in locomotive behavior are normally experienced (Higurashi, Hirasaki, & Kumakura, 2009; Shapiro & Raichlen, 2005; Vilensky & Larson, 1989). Although primates use both DS and LS gaits, recent research suggests that DS gaits are preferred over LS gaits. DS gaits are almost exclusively used (Cartmill, Lemelin, & Schmitt, 2007; Stevens, 2008).
The latest accepted theory as to why primates prefer DS gaits is related to an increase in neurological pathways and an increase in neurological motor control in order to have balance control while on branches or terminal supports; however, there is disagreement on the advantage of gait sequence or change of gait on stability (Davis, DeLuca, & Ounpuu, 2003; Higurashi et al., 2009; Stevens, 2008). Although there are disagreements regarding the advantages of gait sequence, branch diameter appears to have an effect on the speed of the gait cycle (Franz, Demes, & Carlson, 2005).
However, it should be noted that the observed type of locomotion and gait sequence can be problematic and can lead to erroneous statistical significance. The problem is method. Although the observations of primates in their natural habitats can be more realistic, controlled experiments that involve modified habitats—the use of re-created “trees” and obstacles, for example—may affect their natural forms of locomotion and gait preference. To this author, variations seen in those “branched trees” that are re-created for experimental purposes may alter natural primate behavior. This alteration may not be in purely psychological terms but rather a biomechanical response to an “unnatural” terrain.
Beyond gait sequence and mode of gait, primate locomotion as defined by this author is best described as that behavior consistent with the biomechanical limitations of the primate skeleton and neurological control within an ecological niche. The diagnostic skeletal features are critical in both comparing and contrasting primates with the possibility of inferring a mode of locomotion. For example, consider the morphological differences that can be seen between arboreal and terrestrial quadrupeds. As detailed in Fleagle’s 1998 book Primate Adaptation and Evolution, arboreal quadrupeds have distinct diagnostic features: similar lengths (usually short) of both forelimbs and hindlimbs; elliptically shaped glenoid fossa; a broad humeral head with a moderately robust shaft; medial epicondyle, which are oriented medially; a long olecranon process of the ulna; a broad hamate; a relatively high angled femoral shaft; an asymmetrical size of femoral condyles and articulating surfaces on the tibia; an asymmetrical tibiotalar joint; and a large hallux. Fleagle further reported that differing from their arboreal counterparts, terrestrial quadrupeds have limited anterior-posterior motion of the shoulder (scapula and humeral articulation), an olecranon process extending dorsally, a deep olecranon fossa, a short and posteriorly orientated medial epicondyle, a robust tarsal and robust metatarsals, and short and broad carpal bones.
Differences can also be seen among brachiating and leaping primates, as discussed in Fleagle’s 1998 book Primate Adaptation and Evolution: Leaping primates have diagnostic features that include a deep femoral condyle, a narrow tibia, a short femoral neck, a slender fibula, a long calcaneus and navicular, and a long ischium. Brachiating and suspensory primates have narrow and dorsally orientated scapulas, a small and round glenoid fossa with a large humeral head, medially orientated medial epicondyle, a short olecranon process, curved phalanges, broad and a shallow femoral condyle, and a shallow patellar groove. Also, the ulna does not articulate with the carpals and the distal and proximal row of carpals via a ball and socket joint contribute to the range of movement.
Given the commonalities and diagnostic features among quadrupeds, brachiators, and leapers, as Esteban E. Sarmiento noted in his 1998 book Generalized Quadrupeds, Committed Bipeds, and the Shift to Open Habitats: An Evolutionary Model of Hominid Divergence, there is very little disagreement that the commonalities show a deep common evolutionary ancestry while the diagnostic features show evolutionary adaptation to various ecological niches. Species with overlapping modes of locomotion and morphological similarities have homologous and homoplasy issues regarding morphological traits. This issue can be problematic in determining the origin of human bipedality. Among the various traits attributed to our species (e.g., expansion of the human brain, tool use, and material culture), the emergence of obligate bipedalism preceded all other traits that are considered uniquely human.
Human Bipedal Locomotion
Human bipedalism can be defined as the constant utilization of alternating hindlimbs as a means for movement between two points, according to Fleagle in his 1998 book Primate Adaptation and Evolution. The gait is a cycle (stride) that consists of a stance phase, midstance phase, and swing phase. The moment the heel strikes, the hip becomes flexed, and the knee is extended while the leg is laterally rotated. The adductor muscles then shift the body’s weight over the supporting limb (midstance phase), and the opposite hip and knee are extended. During the increase in forward momentum, the ankle dorsiflexes, and the hip and knee pass the supporting leg. After toe-off, the weight passes the toe and hyperextends the hip joint. The swing phase completes the cycle, ending with the leg laterally rotated in preparation for another cycle, as noted by Leslie C. Aiello and Christopher Dean in their 2002 book An Introduction to Human Evolutionary Anatomy. This motion makes human bipedality different from the bipedalism of any nonhuman primate. For example, when a chimpanzee uses bipedalism, the cycle differs in its phases due to a lack of full hip and knee extension. In addition, the knee and ankle joints do not pass the hip joint, and the femur does not have a bicondylar angle. Aiello and Dean also noted the absence of abductor muscles and a reverse of the pelvic tilt during the stance phase as additional differences. Altogether, the differences between human bipedality and nonhuman primate bipedality are fourfold in (1) the degree of spinal curvature, (2) pelvic configuration, (3) foot morphology, and (4) biomechanical modifications in related muscles, tendons, and ligaments.
Evolutionary History of Human Primate Bipedality
As many previous primates throughout North America, Europe, and Asia went extinct during the Eocene to the middle Oligocene, the emergence of apes in Africa presented a unique evolutionary adaptation that would eventually be seen within our species in the form of bipedal locomotion. From its evolutionary ancestors, the African Miocene apes Orrorin tugenesis and Sahelanthropus tchadsis are considered the earliest bipeds. These specimens represent the earliest date for primate bipedalism— far earlier than the later genera of Australopithecus and early Homo.
Orrorin tugenesis, considered one of the oldest bipedal hominins, dated from 6 to 8.5 million years ago. The diagnostic features, which indicate bipedalism, include an anteriorly convex curvature of the femoral shaft, the blending of the tubercle into the greater trochanter, the presence of an intertrochanteric line (absent in other Miocene apes), a medially salient and well-developed lesser trochanter, elongation of the femoral neck (closer to the Australopithecines and humans), a proximo-distally elongated gluteal tuberosity with a distal leading crest, a marked pectineal line with a spiral line from below and medially orientated to the lesser trochanter and a linea aspera with mild cresting located below the trochanter, a right angled intertrochanteric crest, an obturator groove originating from the fossa to the inferior margin of the femoral neck (indicating femoral hyperextension), and a developed but shallow hypotrochanteric fossa. Orrorin’s femoral neck is antero-posteriorly compressed, along with the presence of a obturator externus groove, which differs from both the Miocene and modern apes (Pickford, 2006; Pickford & Senut, 2001). Similarly, the asymmetrical distribution of the cortex, inferiorly thick and superiorly thin, suggests loading patterns or weight distribution that is indicative of having a degree of bipedalism and being orthograde (Galik et al., 2004).
Another Miocene hominin, Sahelanthropus tchadensis, dates back 6 to 7 million years ago. The diagnostic feature indicating bipedality, though controversial, revolves around a cranium (TM 266), which is partial and distorted. Through virtual reconstruction, the cranium depicts an orthognathic face, short premaxilla, a large foramen magnum (greater in length than breadth) that is positioned more anteriorly, and a short basiocipital. The orientation of the foramen magnum suggests bipedality. This is due to the relative angle of the foramen magnum to the orbital plane—103.2 degrees +/−6.9 degrees for humans and 95 degrees for Sahelanthropus, for example, and the orientation of the flat nuchal plane being 36 degrees to the Frankfurt horizontal (Zollikofer et al., 2005). When these morphological features are compared among fossil apes, hominids, modern apes, and humans, Sahelanthropus is closer to both Australopithecus and modern Homo. Displaying morphological synapomorphines with other bipeds, bipedalism becomes the assumed form of locomotion (Guy et al., 2005).
The emergence of hominins in Africa provides the best evidence to date for the appearance of bipedalism. The subfamily Australopithecine consists of three genera: Ardipithecus, Australopithecus, and Paranthropus (previously robust australopithecines). For the Aridpithecus genera, A. ramidus and A. kadabba are suggested to be possibly bipedal and the beginning of the hominin line. This determination is based on the combined features of a short cranial base, an anteriorly positioned foramen magnum, and a strong plantar curvature of the proximal foot phalanx (Harcourt-Smith, 2007). Although Ardipithecus, as well as Orrin and Sahelanthropus, is controversial regarding its bipedality, that the genus Australopithecus was bipedal is accepted with few reservations (e.g., the degree of bipedality Harcourt-Smith, 2007).
There are five species of Australopithecus located at sites in the southern, western, and central regions of Africa: A. anamensis, A. afarensis, A. africanus, A. aethiopicus, and A. garhi. Australopithecus anamensis (4.2 million years ago to 3.9 million years ago), though having blended human and ape characteristics, represents a more positive shift in hominin locomotion toward bipedalism. The locomotive indicators are limited. This hominin has a tiba shaft, which is oriented orthogonally to the talar joint surface; the metaphyses are flared both proximally and distally, which is an indication of bipedal locomotion, as Glenn C. Conroy reported in his 2005 book titled Reconstructing Human Origins (see also Ward, 2007).
Australopithecus afarensis (3.6 million years ago to 2.9 million years ago), having similar qualities, presents the greatest blending of primitively derived traits from previous hominids with added humanlike characteristics. The lower limbs are typified by having major features, which include long and curved proximal phalanges with a circumferential trochlea, a navicular with a low dorsoplantar height and a large right-angled cuboid facet, a robust and triangular diaphysis of the first metatarsal, a calcaneus with a horizontal sustenacular shelf, a convergent hallux, a lateral cuneiform with plantar tuberosity, a proximal femur with a short neck relative to femoral length, a high bicondylar angle, an elliptical lateral condyle, a posterior angle of the distal tibia, and a distal fibula with a deep peroneal groove. In addition, the iliac blade faces posteriorly, including robust anterior and posterior superior iliac spines incorporating a sigmoid curve of the iliac crest, a thickened pubic symphysis (dorso-ventrally), and a short ischial shank (McHenry, 1991).
A. afarensis presents an interesting problem in its morphological interpretation. Bipedalism is cited as transpiring due to a lumbar lordosis, a high bicondylar angle, short and wide iliac blades, a prominent anterior inferior iliac spine, mediolateral orientation of the talar surface (distal tibia), a trochlear surface of the talus, and a lateral plantar process of the calcaneus. The dorsal orientation of the proximal articulating facets on the proximal pedal phalanges suggests dorsiflexion (needed during bipedal walking), but it falls outside the human range. Contrary to existing bipedal indicators, certain morphology indicating an arboreal nature and locomotion includes the curved and long proximal phalanges with flexor ridges, a medial cuneiform that is similar to apes (suggesting halluical opposability), a well-developed lateral trochlear crest of the distal humerus (prevents dislocation of the elbow joint during either climbing or suspension), and a cranially oriented glenoid, as discussed in Aiello and Dean’s 2002 book An Introduction to Human Evolutionary Anatomy (see also Harcourt-Smith, 2007).
Australopithecus africanus (3 million years ago to 2.4 million years ago) shares many affinities with A. afarensis. Postcranial elements are fragmentary: capitate, scapula, proximal humerus, distal femora, pelvic blade, adolescent ischium, a fragmented piece of humeral shaft, vertebrae, left and right os coax, fragmentary sacrum, and a left proximal femur without a head. Generally, A. africanus has wide and lateral flaring iliac blades and a small acetabulum and iliosacral joint. The proximal femur has a long neck with a small head. Similar to A. afarensis, the bicondylar angle is high (McHenry, 1986; Wood & Lonergan, 2008).
Paranthropus boisei (2.3 million years ago to 1.4 million years ago) and Paranthropus robustus (2 million years ago to 1 million years ago) are suggested to be bipedal. The locomotion of Paranthropus boisei is uncertain due to the fragmentary nature of the recovered postcranial elements. Postcranial fragmentary elements attributed to this species include a clavicle, a distal humeral shaft, a proximal radius (left and right fragments), an ulna (proximal and shaft fragments), a distal femur, a tibia (left and fragments of the right), a distal fibula, and the right proximal third metatarsal (Wood & Constantino, 2007). However, the morphological similarities between recovered elements of Paranthropus and Australopithecus, especially the existence of an obturator externus groove, suggest a degree of bipedalism. Paranthropus robustus (2 million years ago to 1 million years ago) has femoral features, though fragmentary, that include the lack in the lateral expansion of the great trochanter, an absence of a trochanteric line and a femoral tubercle, and a deep trochanter fossa with an obturator externus groove (Wood & Richmond, 2000).
Although a degree of bipedality is assigned to Australopithecus and Paranthropus, the emergence of Homo is unequivocally bipedal. Included in the genus Homo (starting at 1.9 million years ago) are three early species that exhibit diagnostic features indicative of bipedality. They include H. habilis, H. rudolfensis (or H. habilis), and H. erectus. The major indicators include a large tibial tuberosity, an obturator externus groove located on the femoral neck, a bicondylar angle, the position of the foramen magnum, and pelvic and foot morphology. These features are discussed in Aiello and Dean’s 2002 book An Introduction to Human Evolutionary Anatomy and in Fleagle’s 1998 Primate Adaptation and Evolution (see also Wood & Richmond, 2000). However, it must be stressed that there remains a degree of morphological variation associated with bipedalism. Today, as in the past, this variation can be found within and among bipedal species.
Today, many researchers conclude that bipedalism could have evolved multiple times in primate history. Given the incompleteness of the fossil record and no definitive relationships among hominin and hominid genera, there is the possibility that multiple lines of bipedal apes could have evolved separately from the direct ancestral descent of the Homo line. The committed time to bipedality, even among Australopithecus morphology and the associated Laetoli footprints, is unknown and very controversial. This is primarily due to the blended characteristics associated with an arboreal setting. Given that Plesiadapiforms, Adapoidea, and Omomyoidea were arboreal and assumed quadrupeds, the question remains: What eventually prompted the emergence of bipedality as early as the Miocene? Assuming bipedality was established during the Miocene, what major influences would cause the greater diversity of hominins to develop this bipedal characteristic? Finally, what evolutionary pressures between the Miocene and Pliocene could account for the morphological differences seen among bipedal primates? The theoretical answers are far from being conclusive. In terms of evolution, it is very probable that natural selection is responsible for the divergence from quadrupedalism to a form of bipedality.
Theories on the Origin of Human Bipedalism
The environment, serving as a selective force, may explain the diversity of primates within various biomes and consequently the various modes of locomotion exhibited by primate species, as Alison F. Richard suggested in 1985 in Primates in Nature. The role of the environment acting as a selective pressure is widely recognized as a major factor (Kingston, 2007). Based on multiple analyses of marine isotopes, ice cores, sediments, loess sequences, pollen sequences, and stable isotopes, environmental conditions have varied throughout the world, including in Africa. Locations at hominin and hominid sites in Africa indicate a mixture of open woodlands, wooded savannas, grassy plains, and closed vegetation. Fluctuations in aridity and environmental changes during the Miocene to the Pleistocene are caused by various factors. Milankovitch cycling, plate tectonics, glacial and interglacial periods, the Messianian salinity crisis, and Walker circulation patterns reshaped the African coastline and the African Rift Valley. However, re-creation of the hominin and hominid environment is far from complete. Yet interestingly enough, extinct bipedal primates are found in various terrestrial settings and are not limited to one type of environment—the savanna, for example. The importance of the environment on the interpretation of hominin and hominid evolution poses critical questions. If hominids and hominins evolved a form of bipedalism independently from either wooded savannas or closed vegetation, then what was the driving force for the development of bipedalism? And what implications does this have on this “unique” trait that is considered a hallmark of being human? Various hypotheses are proposed to answer such questions.
Scientific consensus on the origin of human bipedalism is not conclusive. However, some theories are more acceptable than others. Some of the more accepted hypotheses include the knuckle-walking hypothesis, the brachiation hypothesis, the climbing hypothesis, the energetic hypothesis, and the thermal regulation hypothesis. The foci of these hypotheses are dependent on models established on extant primate behavior. Two major models include the hylobatian model and troglodytian model. In the hylobatian model, it is suggested that a small brachiating and tailless gibbonlike primate made the transition to become a terrestrial biped. It is suggested that the bipedal precursor would have entailed a combination of arm hanging and arboreal branch bipedalism. The characteristics included long forelimbs, mobile shoulder and wrist joints, broad and coronally orientated iliac blades, a laterally facing scapula, long and curved fingers, and highly developed thumbs and first toes. Although the arboreal setting deep within human evolution is widely accepted, the hylobatian model and the brachication hypothesis have very little evidence for their support (Crompton, Vereecke, & Thorpe, 2008).
The troglodytian model suggests that knuckle-walking was the precursor to human bipedalism. Proponents for the knuckle-walking theory have a focus on the morphological features of the shoulder, arm (forelimbs), and the wrist and hand in terms of functionality. Functional morphology becomes the critical factor in discriminating between apes in Africa and apes in Asia and hence the locomotive difference that is seen between knuckle-walkers and brachiators (Begun, 2004). The wrist and hand have many shared features between African apes and humans that are indicative of a knuckle-walking origin. The main morphological features include an early fusion of the os centrale, or central portion, to the scaphoid; the size and facet orientation of the scaphoid; a dorsally orientated scaphoid notch; a broad capitate and hamate with dorsal ridges; an enlarged trapezoid; a small triquetrum; and a palmer and proximal pisiform. The articulation of the scaphoid with the lunate, trapezium, trapezoid, and capitates, along with the articulation of the trapezoid with the capitate (which articulates with the second and third metacarpals and the hamate, which articulates with both the fourth and fifth metacarpals) and the second metacarpal, have biomechanical implications for knuckle-walking. During knuckle-walking, compression and shear stress are placed on the hands and wrists. Shear stress is placed on the carpals when the weight is transferred in a rolling manner from the fourth digit to the second digit, although sometimes to the fifth digit during locomotion (Richmond, Begun, & Strait, 2001).
Among these models, the climbing hypothesis for the origin of human bipedalism is based on both the morphological features and limb patterns involving muscle movements. The humeral shaft profile, the scapula orientation, and the position of the vertebral column, together with the perspective of the center of gravity, have many similarities with humans. Although human hand and wrist morphology have suggested an affinity with knuckle-walkers, evidence from hominid morphology has implied an “intermediate” form of an arboreal existence with adaptive bipedal traits. The climbing behavior of great apes, as with other primates, closely approximated human bipedalism more than any other mode of locomotion, including nonhuman primate bipedalism. This is based on both kinematic and electromyographic studies of monkeys and apes that showed that limb patterns and muscle movements (latissmus dorsi, caudal serratus anterior, deltoid, pectoralis major, and biceps brachii) during climbing were similar to human locomotion, as reported by Fleagle in Primate Adaptation and Evolution (1998). The climbing hypothesis is recognized as a possibility for the origin of bipedality.
Energy efficiency, or energetics, is another theory considered as the origin for human bipedality. When looking at extant primates as possible models, energetics among primates vary according to species and gait velocity. For example, the amount of energy expenditures of chimpanzees and gorillas are not significant when either walking bipedal or quadrupedal (Okada, 2006). Yet Japanese macaques spend more energy walking bipedal than quadrupedal, and spider monkeys and lorises have lower energy costs for suspended walking than for either brachiation or quadrupedalism (Nakatsukasa, Hirasaki, & Ogihara, 2006). For humans, energy expenditure is greater during running than walking, but walking has slightly greater efficiency than quadrupedalism. Tied in with environmental conditions, it is suggested that this new form of locomotion was necessary for hominins to travel between wooded patches among the savannas, as Conroy explained in 2005 in Reconstructing Human Origins.
Associated with energy efficiency, the thermal regulation hypothesis states that bipedality is a regulatory process between the environment and the body’s thermal regulation. The combination of an upright stance with the reduction of body hair, water retention rate, and energy efficiency, or metabolism, would reduce the amount of heat stress experienced. An increase in diurnal water consumption between a naked biped and a fully haired biped at 30 degrees Celsius with the same metabolic expenditure (2.0 BMR) increases from 0.62 kg/12hr to 0.82 kg/12hr. This water consumption is lower than both a naked and fully haired quadruped at the same temperature and metabolic rate, which is 1.16 kg/12hr and 1.11 kg/12hr, respectively (Amarl, 1996; Wheller, 1991). Even though the evidence on environmental re-creation and the emergence of bipedality can be contradictory, the energetics and thermal regulation remain strong possibilities.
There are several other hypotheses that are known but not widely accepted among members of the scientific community. These hypotheses include migration, food transport, posture, reproduction, birthing, and the aquatic ape. Although these hypotheses do not rely on extensive morphological or primate behavior data, many of the conclusions are based on assumptions and possible scenarios that fit a preconceived conclusion. Nevertheless, each of these hypotheses remains as possible but improbable explanations for the origin of bipedalism.
The migration hypothesis suggests that the development of bipedalism occurred during long distance scavenging. Any mutation(s) that furthered the development of bipedality would increase the amount of food found over greater distances. This adaptation would favor bipedalism over quadrupedalism. Furthermore, the freeing of the hands would allow for tool usage designed for the quicker butchering of carcasses (Sinclair, Leakey, & NortonGriffiths, 1986).
Similar to the migration hypothesis, the food transport hypothesis states that erect posture and habitual bipedalism allowed for the freeing of hands in order to carry scavenged food. The combination of body size, distance between food sources, predation, and competition could have favored a greater degree of bipedality (Hewes, 1961). This view is similar to the tripedal hypotheses where objects are carried either close to the abdomen or slightly behind the back. The key to the transport model is based on the efficiency of the transportation of objects. However, relatively recent research in body proportions and biomechanics, which would include gait, shows an increase in efficiency in carrying goods (while walking) in modern humans as opposed to nonhuman primates with an inference to our hominid ancestors (Wang & Crompton, 2004). Though efficiency in carrying loads among hominids may differ in degree, the morphological changes that result in a greater degree of bipedalism would have a greater biomechanical advantage over less efficient hominids.
Stemming from the migration and food carrying hypotheses, hominid posture is considered as another possible explanation for the origin of bipedalism. The upright posture is suggested to allow the ability to evade predators and for displays of aggression. Aggressive displays, along with the ability to exploit new food sources, would have conveyed a greater advantage to a hominid that had both a larger body size and a more erect posture. Research has shown that the stance of Pan troglodytes during defensive displays were either primary or secondary (passive and aggressive, respectively). Depending on the circumstance and the presence and distance of predators or competition, the display or postural stance would have changed with every situation. An upright posture would have allowed not only for a greater amount of time for the flight versus fight response and/or cost benefit analysis but also the possibility of intimidation with or without physical contact. In any terms, the speed of a quadrupedal gait versus the benefits of an upright posture was unquestionable during defensive displays. An erect posture would have allowed for a greater line of sight and a greater awareness of the surrounding environment (Walter, 2004).
The reproductive hypothesis suggests that human upright posture and bipedal gait are conducive for reproductive efficiency. Unlike the nonhuman primate counterpart, human sexual adaptations for efficiency were seen in a copulation position (ventral-ventral) that would have allowed the female reproductive tract (vaginal angle) to be parallel with gravity, resulting in a greater sperm retention around the cervix. Additional adaptations to increase sperm retention included the sedative effect of an orgasm, females capable of multiple orgasms, the dominance of nocturnal copulation, and a pair-bond sleeping arrangement (Gallup & Suarez, 1983).
As for the final product of copulation, the birthing hypothesis suggests that bipedalism transformed the birthing process from an individual experience to a social experience. For example, in the position of the female reproductive tract in a nonhuman primate, an infant emerges from the birth canal facing the female without the necessity of rotating the infant. This differs from the human primate. Due to the human infant head and shoulder dimensions, as compared to the dimensional opening of the human female birth canal, human infants typically need to be rotated to ensure that they emerge facing away from the female. The vulnerability of females during the birthing process has made assistance a necessity. In terms of hominid evolution, the emergence of complex social systems aided not only in the birthing process, but also provided vigilance against predation during this process (Trevathan, 1996).
The aquatic ape hypothesis suggests an aquatic phase during human evolution with implications for bipedalism. This explanation points to humankind’s untraditional features, such as the great ability to swim, a reduction of body hair, hair tracking toward the midline of the body, subcutaneous fat, and unspecialized hands. Additional features such as body odor, a voluntary control of respiration, a pronounced bradycardia, salt tears, round female breasts, ventroventral mating, and dilute urine are suggested by some researchers as evidence for an aquatic phase in human evolution. It is claimed that our early bipedal ancestors existed in a semiaquatic state within the tropical rainforests. These early ancestors are depicted as “advancing” in a linear progression. At first, these ancestral descendants lived an idyllic life feasting on plants, and then later, they consumed shellfish, and finally, they became terrestrial hunters of game with a spoken language, according to Elaine Morgan in her 1997 book titled The Aquatic Ape Hypothesis: Most Credible Theory of Human Evolution (see also Verhaegen, 1985). It may be concluded, based on this theory, that bipedality would have an advantage in the water. However, the efficiency of humans compared to other aquatic mammals within water may suggest otherwise.
Future Areas of Research
While comparing and contrasting the morphology between humans and nonhuman primates can give clues to the relationship among primate taxa, other areas of research are shedding light on the nature of locomotion. The area of biomechanics can aid in the development of new models of origin by understanding the interaction and relationship between bones and muscles. Even the influence of stress and shear on bone morphology can aid in the understanding of locomotive behavior. In addition, contributions made in the area of robotics can also further the understanding of the mind’s (computer’s) control over multiple mechanical systems. The future development and advancement in understanding locomotion could be within these areas of research and development.
One of the best areas for future research is genetics. Since the developmental basis of vertebral morphology and sequences are the same for all human and nonhuman primates, the differences reflected in both posture and mode of locomotion are suggested to reside in the genetic controls, particularly homeobox genes. Homeobox genes (ranging from 100 to 1,000 in number) are a set of DNA sequences involved in regulating embryonic development (Gehring, 1994). As for humans, there are four Hox gene (39 genes) sequences that control vertebral segmentation: Hox A, Hox B, Hox C, and Hox D. These gene sequences are located in various chromosomes and are expressed in the cells of both the mesoderm and ectoderm (body axis). Research has suggested that changes in the Hox gene code depend on the timed introduction of retinoic acid by means of binding to the transcription regulatory sites and the naturally timed introduction of a teratogen that is controlled by retinoic acid receptor genes. Thus, a change in the timing of the introduction of retinoic acid will change vertebral morphology. For example, inactivation of the RAR-γ changes the anterior formation of the vertebrae and Pax genes (a binding protein) that control sclerotome differentiation (Dietrich & Kessel, 1997).
While humans and pongids differ in their number of chromosomes, 46 and 48 chromosomes respectively, the changes in Hox genes could contribute to the “sudden” emergence and continuing development of the erect posture and bipedal locomotion of hominids. Research has suggested that changes in the distal sequence of the Hox D gene (located on the second chromosome) are responsible for the junction between the lumbar and sacral vertebral region. These changes are suggested to be a product of a mutation, which resulted in the chromosomal fusion of panine chromosomes 12 and 13 into hominine chromosome two. This structural alteration would have changed the rate of protein “evolution” and would have allowed for a high rate of protein change. This change in the rate of protein evolution is suggested to account for the sudden emergence of bipedalism via vertebral morphology. The control of Hox genes on axial morphology and comparative chromosomes among primates are supported by other research (Bowers, 2006; Chimpanzee Sequencing and Analysis Consortium, 2005; Navarro & Barton, 2003). In addition to modifications of the vertebral region, other research determined that Hox b9, Hox c9, and Hox d9 were responsible for limb structure and placement in relation to the axial skeleton during application of fibroblast growth factors (chick embryos). Interestingly, any changes in Hox genes resulted in appendicular skeletal placement with no changes in the axial morphology (Cohn et al., 1997; Muragaki, Mundlos, Upton, & Olsen, 1996).
When evaluating primate locomotion, opinions vary across the scientific community. Nonhuman primates use many forms of locomotion. From quadrupedalism to bipedalism, nonhuman primates exhibit these forms of locomotion depending on the environment (e.g., arboreal or terrestrial) and their biomechanical limitations. The expansion of the neural complexity within primate evolution can explain gait sequence (LS or DS) and limb preference. This is not to say that the neural complexity is near that of humans; rather, the complexity of the human brain far exceeds the complexity of any nonhuman primate. In terms of locomotion, neural complexity may contribute to limb movement and propulsion, but complexity may not have any influence on posture or preference for a mode of locomotion.
Humans provide an interesting inquiry on the evolution of bipedality. As depicted within the fossil record, forms of bipedalism have evolved multiple times, stretching back to 6 million years ago. Since these hominins and hominids probably did not have the complexity of the modern human brain, what prompted these individuals to stand erect or semierect and walk bipedal is still unknown. However, bipedalism was well established within the Australopithecines. The reason for the emergence of bipedalism from the Miocene apes is still unknown.
Today, hypotheses on the origin of bipedalism and the relationship to the locomotion of extant primates express various opinions. No researcher has complete answers to the many questions posed by human evolution. Morphologically, humans have many shared characteristics with nonhuman primates. However, do these shared evolutionary characteristics show the origin of bipedalism? The presented hypothetical descriptions have attempted to illustrate the deep philosophical problem when attempting to tie shared morphological characteristics with a particular behavior. Experience and probability define the validity in any of these explanations. Through future research, greater light will be shed on the origin of bipedalism, echoing Darwin’s words that our species differs in degree and not in kind from the great apes.
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