Did Human Bipedalism Evolve from a Knuckle-walking Ape Ancestor
Did Human Bipedalism Evolve from a Knuckle-walking Ape Ancestor
Introduction
The narratives of human evolution are one of the oft-told scientific stories whenever the topic of the origin of man crops up. However, this topic is also one of the thorniest and most highly contentious. There are major disagreements in the manner in which man first came to be, and every interpretation of almost every new find on the matter will be sure to find opposition among scholars on the field (Harcourt-Smith & Aiello, 2004). While there are numerous causes of disputes on the matter such as diet and habitat, genetic makeup, extinction, and environmental adaptability, the evolution of human bipedalism is perhaps one of the most disputed subjects on the topic of human evolution (Niemitz, 2010). For instance, during the last century, studies shows that about 30 dissimilar suggestions have been put forward to not only explain the evolution of the human upright posture and locomotion, but also to construct the why, the where, and the how aspect of the development (Niemitz, 2010). Though a good number of these hypotheses are considered outdated, at least in their main ideas and contextual premise of the argument, a considerable number of them still hold significant sway on the matter among the scientific community and are increasingly being held as more accurate (Richmond, Begun, & Strait, 2001). Among these is the acceptance of the ancestral monkey-like arboreal or terrestrial quadrupedalism, the Gibbon (or Orangutan) like forms of climbing and aerial suspension, and the more so often Knuckle-Walking characteristics of the Apes (Kivell & Schmitt, 2009). In this category, there is also the Amphibian Generalist Theory (2000) that asserts that bipedalism began in wooded habitats (Niemitz, 2010). Paleoanthropological evidence, mechanisms of functional anatomy, and findings of evolutionary psychology supports this theory (Niemitz, 2010). Although these approaches provide suitable scenarios on how human evolution could have repressed the functional as well as the anatomical threshold from quadrupedalism to bipedalism, a good number of experts still raise serious questions about some key elements of their assertions (Niemitz, 2010; Putt, 2009). For example, adequate answers still lack on how wading behavior an important factor in the Amphibian Generalist Theory could trigger upright posture or on how first-time bipedal species could maintain the upright position and sustain the bipedal walk over time. There are still hypotheses that, by walking upright especially in grassy ecosystems, man could find food better and escape predators (Carrier, 2011). However, parsimonious arguments claim that the most logical way to look farther is by growing taller which requires fewer adaptations and hence more favorable evolutionarily (Niemitz, 2010). Other suppositions claim that man evolved bipedal locomotion and upright posture to free his hands to carry things (Isbell & Young, 1996). While these arguments are equally logical, some apes (who naturally walk on four) are known to carry around things and climb trees better – a better place to hide from predators (real or perceived). With such arguments and counter- arguments, this paper tries to harmonize some of the hitherto competing points of view by trying to ascertain whether human bipedalism evolved from a knuckle-walking Ape ancestor.
Bipedalism is a story that goes back approximately 3.6 million years ago at Laetoli in modern day Tanzania, Africa where Mary Leakey stumbled upon the most ancient clue of bipedalism (Hominides, 2007). Preserved in volcanic ash were footprints of an adult hominid coupled with those of a child (Hominides, 2007). March 2003, another archeological discovery unearths a 350, 000 year old (Paleolithic) hominid footprint at Roccamonfina (Italy) (Hominides, 2007). According to Hunt (1994), these are just some of the few spottings of bipedalism and that the history of the upright posture dates much farther back. Viewed zoologically, we humans are Homo sapiens, an upright walking, culture-bearing species that likely first evolved in Africa less than 250, 000 years ago and the last of the human tribe (Hominini) on the surface of the earth (Harcourt-Smith & Aiello, 2004). However, abundant fossil records indicate that we are not the first of our kind. Rather we were preceded for millions of years by other hominins such as the Australopithecus (2.9 million to 3.8 million), the more human-like the Neanderthal (Homo neanderthalensis), and other less human-like (but more ape-like) primates such as the long-extinct Dryopithecus of the modern day gorilla family (Harcourt-Smith & Aiello, 2004). In this context, that we and the other hominins (living or extinct) such as the apes are somehow related is increasingly accepted by biologists and anthropologists everywhere like never before. One of our shared traits is bipedalism – locomotion like walking, running, etc. on two legs (Richmond, Begun, & Stait, 2001). Even in our daily routines, it is common to spot animals standing or walking on two legs such as Chimpanzees and gorillas occasionally assuming bipedalism on a temporary basis to carry things on their hands or the wild antelopes and the domestic goats standing on two hind limbs while supporting themselves on forelimbs when reaching for high branches.
Unlike this facultative bipedalism, habitual bipedalism (or obligate bipedalism) is very rare, and only a few mammals such as humans and the Kangaroo are known to demonstrate this kind of locomotion on a regular basis (Hunt, 1994). However, even among the land dwelling Kangaroo, several non-archosaurian lizard species, birds, and arboreal primate species (like Gibbons) human bipedalism is incomparable. In the erect-walking penguins, for example, the functional anatomy and biomechanics of locomotion supporting their short rudder-like feet are way too different from that of man (Niemitz, 2010). This shortcoming also applies to the extinct dinosaurs, the ostriches, or any other marsupial moving on their feet. None can show the kind of orthograde spine in locomotion that humans have. Whereas these mammals hop or waddle, humans stride. Because of these differences in bipedal gait, the development of bipedal locomotion is considered one of the most significant adaptations known to occur within the hominin lineage (Carrier, 2011). Consequently, there are substantial pieces of information on the subject, and with that a considerable debate on the same on not only how but also why humans developed bipedal locomotion and an upright posture (Kivell & Schmitt, 2009).
The Evolution of Human Bipedalism
According to the Smithsonian National Museum of Natural History (2016), evolution is a process of natural changes that causes a species to rise, adapt to their surroundings, and to be extinct. Putt further claims that it is through biological evolution that all species came to be while millions vanished in the course of time (particularly during the Miocene epoch) (Isbell & Young, 1996). In sexually reproducing organisms such as humans, species evolution occurs when there is a change in the genetic composition of the offspring through the inherited genes from the parents. Variations in the inherited traits can in turn influence the likelihood of the survival and reproduction of the organism and ultimately the resultant new species. Over time, the genetic changes alter the overall way of life of these species in several ways such as dietary preference, growth patterns, behavior, and choice of habitat (Richmond, Begun, & Strait, 2001). Such are the kind of changes that occurred in the development of the bipedal type of movement and upright posture in humans (Polk, Williams, & Peterson, 2009). However, as pointed out previously there are several theories put forward on how and why humans developed to become bipeds.
Theories on the Origin of Human Bipedalism
One of the most basic hypothesis on the origins of bipedalism is the Postural Feeding Theory formulated by Kevin D. Hunt in 1994 (Hunt, 1994). This theory is based on feeding requirements and suggests that the hominid bipedalism developed at first only as a feeding technique and later evolved into a form of regular locomotion. Like the modern day chimpanzees, the interpretation of australopithecine anatomy of the earliest known fossil remains of hominids suggests that these human ancestors were compromised terrestrial bipeds with substantial adaptations to arboreality (Hunt, 1994). Although several hypothesized selective pressures might have led to the evolution of bipedalism like the need for increased viewing distance (Dart, 1959), and tool use and carriage (Washburn, 1960), meeting food requirements is one the most proposed reason (Carrier, 2011). According to Roman and McHenry (1980), there was an adaptation need for a more efficient long distance travel in the context of foraging, scavenging or hunting for food (Isbell & Young, 1996). Furthermore, Turtle (1995) suggest that in wooded habitats, early hominids developed a significant low-frequency bipedalism similar to the modern day gibbon-like suspensory gait as an important large-branch locomotor mode as well as an arboreal feeding posture (Hunt, 1994). In these proto-hominids, their suspensory forelimbs were hypothetically less adapted for quadrupedalism in a more terrestrial food-gathering strategy, therefore, leading first to them adopting high-frequency and then exclusive bipedalism in terrestrial habitats.
Other theories based on the hominid feeding practices include the hominin hunter-gatherer hypothesis. Here, man the provisioner and the hunter-scavenger and woman the gatherer might have developed bipedalism as an adaptation for terrestrial posturing while looking for food in terrestrial habitats. The idea of the Hunter-Scavenger is based on the premise that at a slower pace (something best achieved with an upright gait), bipedalism best served two purposes. First, it allowed the early hominins greater stamina to track and kill prey more efficiently especially over long distances (Isbell & Young, 1996). Secondly, the seed-eating hypothesis that focused on the demands of collecting small and unevenly distributed food resources close to the ground supports this notion through its corollary locomotor observation that bipedalism evolved as a terrestrial feeding posture to reduce energy costs when traveling between densely packed feeding sites (Richmond, Begun, & Strait, 2001).
Moreover, studies further indicate that bipedalism developed in the early hominid out of the need to carry the foods collected as well as the tools used to forage and gather the resources. It also allowed the man to be alert and spot potential dangers while foraging. Another bipedal theory based on the hominid early feeding requirement is the environmental food distribution theory (Richmond, Begun, & Strait, 2001). This theory suggests that as food became scarcer and more dispersed due to the changing environment particularly because of the receding forests resources which were also the primary habitat for the early man, the need to travel further in search of adequate food became necessary (Isbell & Young, 1996). Ecologically, the thinning and drying out of the forests paved the way for the development of open-field grasslands that significantly changed not only the distribution of food for most species at the time but the availability of it as well. For the early hominids, of all forms of travel, upright gait was the easiest to develop for two reasons: firstly, the posture provided an ability to carry food over distances and secondly it allowed for quicker transport between the ecosystems patches between the receding forests and the expanding grasslands (Richmond, Begun, & Strait, 2001). According to this model, a change in the environment would mean that the Australopithecine needed to straighten up in the Savannah to have a wider visual angle, find prey easily, as well as anticipate potential attacks (Richmond, Begun, & Strait, 2001). As this upright stance freed their hands, the forelimbs developed to other necessary functions such as tool carrying as pointed out before.
The theory of Thermoregulation first suggested by Wheeler in 1984 is yet another hypothesis mostly put forward for the evolution of bipedalism. The thermoregulation theory suggests that due to the changing environment the Australopithecine had to forage in the newly created open grasslands for the better part of the day when the sun was overhead and insolation at its peak (Hunt, 1994). Because the changing environment brought with itself an increase in temperature in the man’s early habitats, the need to reduce the effects of the heat, heat gain, and accelerate the rate of dispelling heat became paramount (Isbell & Young, 1996). Because of this, man started to adopt the upright posture. The science behind this theory was that the bipedal walking position reduced the surface area of the early man under the direct impact of the sun while raising the body above the ground higher than the nearby grasses enabled it to benefit from the cooling effect of the fast blowing cooler winds above. The upright stance also meant that unlike other quadrupeds who have to stop looking for food to cool under shades, bipeds would not have to do that. According to Harcourt-Smith and Aiello (2004), this is because bipeds expose lesser percentage of their total body surface area directly to the sun than quadrupeds do (Harcourt-Smith & Aiello, 2004). This mechanism, therefore, meant that the upright walking early man had to deal with less thermal stress and the likelihood of overheating was significantly reduced. The stance promoted rapid loss and dissipation of heat to the surrounding because of the reduced skin surface under direct sunlight. Increased heat loss mechanism further meant that bipedals required less amount of water that could necessitate frequent bending than quadrapedals. The vertical posture reduced the rate of body water loss by decreasing the evaporative cooling temperatures. This mechanism helped the australopithecine retain much of its body water.
Closely related to the food requirement hypothesis, is the bipedal threat-display model (Williams, 2010). According to Carrier (2011) to this line of thought, the development of bipedalism in early hominin species not only enabled the medieval man to hold and wield tools and weapons, but it also allowed them to appear threatening to any danger. This threatening façade was mainly employed during competition for food and mates against fellow Australopithecines to appear dominant in the group or to scare potential predators by appearing prominent and dangerous. Out in the open savannahs, the development of the upright stance accorded the Australopithecines greater vigilance against attacks because of the possible increased height and visual range bipedalism would have provided them (Thorpe, McClymont, & Crompton, 2014).
So which theory is correct?
In the analysis of all the competing theories provided above, it is problematic to determine with absolute certainty which theory best explains the origins of bipedalism in humans. While all of the hypotheses seem feasible and equally provide plausible scientific evidence in support of their claims, neither of them has no flaw (Schmitt, 2003). Nevertheless, we cannot ascertain whether such flaws are due to lack of supporting evidence or mere scientific misrepresentation and analysis of archeological findings. Still, all the hypotheses show that the early man went through significant pressures that could have led to his bipedalism state. Of these pressures are provisioning requirements and access to food, the need for protection, and survivorship all of which make much profound scientific sense. While there is still much controversy on which school of thought best explains this evolution, the most widely accepted fact among the scientific school of thoughts is that man developed his human bipedalism from his knuckle-walking ape ancestor for two main reasons. First as a way of coping with the changes in his early environment for reasons such as holding tools and weapons while in motion, long distance travel to find food, and observance when foraging. Secondly, as a locomotive adaptation to suit the changing state of the environment, that is, ground walking in grasslands from arboreal suspensory locomotion in woodlands.
Archeological findings collaborate this stance that human bipedalism evolved from the knuckle-walking apes. Today, for instance, there are remarkable similarities between man and other primates. Both can stand and sit upright, and in occasions especially among primates, a few can walk upright. What these similarities show is that man shares an evolutionary pattern with primates of an upright body position. The upright position is whether in suspension, swinging through branches like the chimpanzee or the ape, leaping like a lemur, or walking on the ground like gorillas – the transition to bipedalism might have built on this evolutionary pattern (Richmond, Begun, & Strait, 2001). That is the reason for the existence of bipedal specializations in the remains of Australopithecus (4.2 - 3.9 million years ago) and similarities between the modern human sexual dimorphism in the lumbar spine and the Australopithecus africanus pre-modern primate (Carrier, 2011).
Pieces of available evidence also suggest that the earliest humans climbed trees and walked on the ground, the observation prevalent in modern day apes. This observation further help fortify that human bipedalism evolved from the knuckle-walking apes. The earliest of such evidence is found on the fossil bones of Sahelanthropus (7-6 million years ago) that combined ape-like and human-like ways of moving round (The Smithsonian National Museum of Natural History, 2016) (Fig 1). Secondly, there is the outstanding similarity between the upper part of the thigh of the fossil remains of both the early human ancestors like the Orrorin tugenensis (6 million years) and that of modern humans to that of other larger apes. The image (b) in Fig. 1 shows that the upper part of the thighbone of both the apes and early humans are identical. This similarity suggests a common line of ancestry (Smithsonian National Museum of Natural History, 2016). While there is a significant dissimilarity between the phalangeal shaft of modern humans and other primates, the mere presence of curvature in their forelimbs indicates that at some point along the evolutionary line, man and apes spend a lot of time grasping and suspending from curved branches. Only the differences in the frequency of the arboreal behavior in the recent past due to selective evolution has made the difference. Nevertheless, the presence of this curvature in the Australopith phalanges - a common ancestor of man and apes – indicates that man’s bipedal activity evolved from the same fossil as the subjective bipedalism in apes and chimpanzees (Williams, 2010).
Besides, anatomical similarities between primates such as the shoulder anatomy in South East Asian Gibbons and the brachiating shoulder mechanism in apes show similar arm behavior like in humans (Richmond, Begun, & Strait, 2001). Together, with other archeological and behavioral evidence, close anatomical similarities between the early human ancestors indicates that man developed his bipedalism from the knuckle-walking ape ancestors. These similarities are accentuated by the resemblance of the anatomical design of the shoulder and arm, elbow, and wrist and hands of the African apes and which goes back to their fossil relatives (Schmitt, 2003). This anatomical connection therefore strongly supports the knuckle-walking suggestion that the hominid ancestor of this trait was being adapted to arboreal climbing and knuckle-walking (Schmitt, 2003; Richmond, Begun, & Strait, 2001).
Conclusion
Whereas several hypotheses try to explain the origin of human bipedalism as discussed in the paper, postural feeding hypothesis (part of the Hominid food requirement models) provides the better assessment to the transformation of human bipedalism from quadupedalism (knuckle-walking). The reason for this is varied. However, the discussion in this paper has shown that through environmental and practical morphology, the ancestor of man and apes exhibited two different feeding postures depending on the availability and reachability of food: terrestrial and arboreal postures. The exploitation of these gaits by the common ancestor served as the origin of human bipedalism. Anatomical similarities in vertebrae and femur, as well as fossil records, between man, the apes, and earlier hominids suggest that the origin of human bipedalism arose from a shared trait with the apes. This paper in support of this conclusion has discussed pertinent issues aimed at achieving the same end. First, the article has reviewed the concept of bipedalism and the two of its kind. In shedding more light on the matter, the paper has also highlighted some of the fundamental theories that explain the origin of the evolution of human bipedal posture and the various factors that might have driven the development in each. Lastly, the paper has presented key reasons in support of the overriding thesis that human bipedalism evolved from the knuckle-walking ape ancestors.
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Appendix
Figure 1: The Evolution of Human Bipedalism
Image as retrieved from The Smithsonian National Museum of Natural History
