The phenomenon of walking upright on two hind limbs, as opposed to using both forelimbs and hind limbs for running, climbing, etc.
Bipedal motion is one of the features of modern humans that most drastically separates us from other primates and great apes. Several different theories as to how we developed bipedal motion relating to diet, terrain, and morphology have developed and changed over the years as physical evidence has increased and refined the ways we think about hominin bipedalism. With the understanding that some theories have been discarded or severely changed since their debuts, presented here is a full analysis of each of the predominant theories of bipedal motion in humans, including those that have not been reconsidered in several years due to compromising evidence. Additionally, there are certain biological and physiological explanations for bipedal locomotion and how that anatomy compares to that of our ancestors and modern nonhuman primates.
One major assumption made by researchers is that adopting a bipedal form of locomotion was adaptive in such a way that it aided survival and carried on into modern human anatomy. The Miocene era, which was a time of dramatic climactic change around 4.4 million years ago, is the approximate time during which bipedalism is thought to have evolved (Hunt 1994). Each of the following theories cites the hypothesized environment during the Miocene era and why bipedal locomotion would have been adaptive for that environment at that time.
Geological as well as fossil evidence has indicated that the climate and environment in Africa were wetter and more lush than they are today. It is supposed that animals could have been living in an area that was relatively well covered by trees, being similar to a forest or woodland (Hunt 1994; Thorpe et al. 2007).
Since the Miocene era was a time of such change, some theorize that hominins would have begun moving from a more forested habitat, to a more open one. As they did so, they would need to develop a bipedal stance to see further, without the assistance of trees (Ravey 1976). A bipedal stance would have also assisted the animal in cooling itself when out in direct sunlight, especially since there would be less skin area coming into view of the sun while standing (Ruxton and Wilkinson 2011). It is also thought that these early hominins would have transported themselves bipedally in the trees before climbing down and continuing to be bipedal (O’Higgins and Elton 2007; Thorpe et al. 2007). Another theme in the theories of bipedalism is that developing a bipedal gait would have freed up the arms for carrying things like tools or food (Brace 1962; Watson et al. 2007). Each of these ideas is examined in greater detail below.
Potential Origins for Bipedal Locomotion
The Miocene period was characterized by a particular group of hominin ancestors, the Australopithecines, and Ardipithecus ramidus. Both of these ancestors to the hominin lineage have left behind important evidence as to how they traveled, and both display lower-limb anatomy that would have likely supported bipedal motion (DeSilva 2009; Lovejoy and McCollum 2010).
As hominins migrated towards open spaces, many may have become bipedal to cope with the wide surroundings and changes in feeding habits, especially for the smaller of these hominoids. Additionally, it has been suggested that as these animals are foraging through tall grasses and may be unable to see over them while in a quadrupedal position, the development of a bipedal stance would have aided in their ability to see threats. While this line of reasoning may not have produced bipedalism quickly, even short moments of bipedal posture may have been the difference between life and death for some of these smaller hominins (Ravey 1976).
Similar to this argument, Brace (1962) points out that Australopiths, defending themselves from large carnivores, would have needed to use handheld weapons and tools, since they lacked the large canines seen on other primates. Without the use of weapons, these early hominins would have been completely defenseless. Since it is clear that they survived, they must have done so by using weapons to defend themselves and developed an upright stance as a response to this (Brace 1962).
While increased viewing distance would have potentially been an immediate difference between life and death, observations of modern nonhuman primates show that they rarely engage in bipedal forms of social display to resolve conflicts or to assist in vigilance over the social group. It has not been truly tested whether or not there would have been a significant increase in viewing distance for the australopithecines, by standing upright, and this would likely not have occurred often enough to substantiate evolving an entirely new method of locomotion (Hunt 1994).
When people walk, they use more energy and produce more heat. This heat can accumulate greatly especially on a fur-covered animal in direct sunlight (Queiroz do Amaral, Quieroz do Amaral 1996). An examination on energy usage by upright-walking hominins can provide evidence for later hair loss in hominins as we became bipedal. Ruxton and Wilkinson (2011) created a model to predict the average amount of energy humans use while walking outside in direct sunlight at different times of day. They compared results of the model between fur-covered and hairless hominins. Results indicated that fur-covered hominins would likely have been unable to walk long distances in direct sunlight, in habitats that would have looked the way Africa did. This assumes that the greatest source of heat comes from internal production from an animal using energy (Ruxton and Wilkinson 2011).
The main conclusion that these authors came to base on their analysis stated that it is unlikely that hominins developed bipedal motion because it would have been more energy efficient. The evidence for this is presented in the fact that walking on 2 ft produces more internal heat than merely standing, and would have made walking long distances incredibly difficult for fur-covered animals. This implies that hominins would have needed to evolve bipedal motion through other circumstances and then lose some of their hair coverage as a result (Ruxton and Wilkinson 2011). It is also hypothesized that there would have likely been a large amount of tree coverage around this time, in Africa, meaning that the thermoregulatory reasons for bipedalism likely would not have been relevant, based on the environment.
This would then cause selection for reduced hair coverage as a way to promote heat loss. Evidence from human body parasites from around 3mya suggests that this may have been around the period when hominins were losing body hair. This is likely already one million years after hominins came definitively bipedal; however, this is a loose hypothesis (Ruxton and Wilkinson 2011). However, Queiroz do Amaral asks whether or not being hairless would have been more adaptive for regulating heat loss as hominins became bipedal, not after (1996). She suggests that loss of body hair would have occurred before or in conjunction with the evolution of bipedalism, not after bipedalism had already evolved. This is because the reduction in body hair could have compensated for the higher energy required to move bipedally, even in a potentially forested environment (Queiroz do Amaral, Quieroz do Amaral 1996).
While it may seem convincing that increased sunlight would have been an important motivator for bipedalism to occur, early Miocene habitats would have been much more covered than the environment in Africa is today. This means that while walking upright would have decreased the amount of sunlight and heat reaching an animal, there would not have been an intense need to do so because the amount of sunlight reaching the ground would already have been compromised by tree coverage. Essentially, the amount of heat reduced by standing upright in these conditions would have likely been so insignificant that adapting an entirely new form of transportation would have been unlikely and inefficient (Hunt 1994; Ruxton and Wilkinson 2011).
Observations on the locomotor patterns of nonhuman apes indicate that the posture and gait of orangutans when walking across tree branches is different than that of other apes. Interestingly enough, orangutans appear to move through the trees in ways similar to that of modern humans moving across the ground (O’Higgins and Elton 2007; Thorpe et al. 2007). Orangutans could have developed this form of walking through branches since it appears to be safer, as they have the ability to grasp upper branches for balance. Thorpe et al. (2007) use this to later assert that one particular early hominin, Australopithecus afarensis, could have been a biped at short distances.
Again, as the arboreal climate in Africa became less dense, primates had few options. It is hypothesized that arboreal bipedalism would have transferred effectively into terrestrial bipedalism, while other great apes, who were built for vertical motion in trees, would have more likely moved towards the knuckle-walking we see today in Gorillas and Chimps. This can be substantiated by certain physiological evidence found in the skeleton of modern humans. Evidence for cranial morphology that allow for the skull to sit right at the top of the spinal column in a vertical fashion suggests an upright posture, since humans have the ability to balance themselves above a central point. Lumbar and spine anatomy in modern nonhuman apes, they are unable to naturally balance themselves this way and therefore would struggle to walk bipedally. To adjust for this, they are forced to bend their knees and hips to achieve this balance. These features were likely not part of the anatomy of the last common ancestor that humans shared with these apes (Lovejoy and McCollum 2010). Therefore, the authors assert that the knuckle-walking we see in modern nonhuman primates was never a precursor to modern human bipedalism because of this necessary skeletal realignment (Thorpe et al. 2007). Other evidence provided by the skeletal remains of Ardipithecus ramidus, discussed in detail later, also helped to remove the presumption that humans walked with a bent-hip-bent-knee locomotor pattern (Lovejoy and McCollum 2010).
Lovejoy and McCollum (2010) hypothesize what the locomotor patterns of the last common ancestor between humans and chimps may have been like. They suggest that the LCA would have likely moved around the lower arboreal setting, as opposed to leaping across canopies at the top. This may have also put them closer to the ground, where they may have moved in small bursts as well. It is possible that this form of terrestrial transport may have occurred on four limbs instead of two, based on metacarpal evidence found in Ardipithecus ramidus (Lovejoy and McCollum 2010).
However, the evidence provided by Ardipithecus ramidus potentially provides an important shift in these origins by making it unlikely that hominins were arboreal bipeds after a certain point in our evolution. The upright hanging anatomy seen in chimps and other apes were constructed for suspension, and modern humans have no similar anatomy that would suggest that we would have spent large amounts of time putting our arms up or putting weight on our shoulders (Lovejoy and McCollum 2010). Additionally, the anatomy of the lower limbs from remains from early hominins does not suggest they would have had the flexible ankle joint necessary for climbing or moving around in an arboreal setting (DeSilva 2009).
Because there has been substantial evidence that upright climbing in trees tends to lead to the development of a stiff lumbar spine, which would be unideal for bipedal motion, early hominins were likely not bipedal in an arboreal setting before being bipedal in a terrestrial setting. The authors mention that it would have been the differences in lumbar anatomy that fueled bipedal motion more so than pelvic anatomy, as was previously assumed (Lovejoy and McCollum 2010). Adjustments to the pelvis likely would have come after these adaptations to the spine.
Gathering and Carrying Food
Likely the most popular theory behind bipedal motion in humans is the Food-Carrying theory, which dictates essentially that hominins would have started to collect small amounts of food from trees and then would have needed to walk short distances to deposit it, or to collect more food from different trees. Studies and observational research have been done to examine how nonhuman primates cope with situations that may have been similar to those in the EEA. Research on Japanese Macaques helps provide support for bipedal locomotion as an adaptation for changes in food sources or collection (Hewes 1964).
It has already been suggested that as the environment in Africa moved from being forested to more open, early hominins would have needed to walk longer distances between food sources or may have even begun scavenging (Hewes 1964). Observations on macaques demonstrated that if one finds a food source that is too large to eat quickly, the macaque will bipedally carry it back to a safer place. Other macaques who observed this process repeated it themselves (Hewes 1964). It appears as though the longest distance for this sprint of bipedal running or walking is usually around 30 m.
Studies on modern macaques have provided all sorts of evidence that help support this theory. A change in diet changed the behavior of the Macaques as they began to walk short distances bipedally, to access or collect more food. It is possible that they learned this in part from seeing the human observers walking on 2 ft (O’Higgins and Elton 2007).
An observational study on chimpanzees was done to test and observe the kinds of bipedal carrying behaviors they exhibited. The researchers changed the availability of certain kinds of nuts to see whether or not behaviors differed between moments of low-competition and high-competition for resources (Carvalho et al. 2012). During periods of high competition, instances of bipedal carrying quadrupled, and the number of resources carried at a time while walking bipedally was double that when the chimps were walking on three limbs. In additional studies, chimpanzees carried significantly larger amounts off food while walking bipedally in other scenarios. They were also able to transport all of their resources further, to private areas where they could be consumed without the risk of sharing. Environmental uncertainty and change may have created situations that favored those who could carry as many resources as possible (Carvalho et al. 2012).
These findings are in tangent with evidence presented by Hunt (1994). Chimpanzees prefer to gather food from smaller trees when possible and would usually be positioned on the ground while doing so. This meant that they did not have to do as much vertical climbing to get to the food they needed. If the chimpanzees of today are similar to our large-bodied primate ancestors, it is possible that these ancestors participated in similar terrestrial bipedal food gathering at smaller trees. He also noted that chimpanzees were able to collect more food while moving bipedally (Hunt 1994).
Feeding and gathering habits that combined terrestrial motion and arboreal climbing could be a potential explanation for this odd combination in Australopithecine anatomy. The evidence suggests that short bursts of bipedal walking combined with small amounts of arboreal climbing could have been the main protocol for getting food. The evidence suggests that australopithecines would have both gathered fruits by suspending themselves from branches, as potentially transported other foods across short distances on the ground (Hunt 1994).
Hypotheses concerning the carrying of tools have also been proposed; however, modern chimpanzees have been observed using and carrying tools quadrupedally, or holding a stick or rock with one limb and walking on the other three. This indicates that bipedalism is certainly not necessary to use tools, especially because there is no reason that tools need to be used while standing up. Other nonhuman primates show signs of using tools while sitting or kneeling (Hunt 1994). This implies that there would have to be other selective pressures causing bipedalism in tangent with tool use, for this to be a viable hypothesis. Infant carrying may have also influenced the evolution of bipedal motion, as the early australopithecines no longer had the flexible, grasping foot seen in other primates. Since they would have been unable to cling to their parents for long periods of time, parents would have needed to carry them (Watson et al. 2007).
Biology of Bipedalism
There is still a certain amount of disagreement between scholars on whether or not early hominins were tree-climbers, before adapting for terrestrial bipedalism. Fossil evidence indicates that bipedal locomotion likely emerged around four million years ago, again, during the Miocene period. There may have been an important split in lineage between what are now modern humans and chimpanzees, as it is hypothesized that our last common ancestor existed between four and eight million years ago, (DeSilva 2009). Many believe this ancestor would have been more closely related to modern chimpanzees than modern humans. A comparison between the knee and ankle anatomy of modern chimpanzees and early hominins suggests that while these early humans may have been adept climbers, they were nowhere near as anatomically suited for it as modern chimpanzees are (DeSilva 2009). The joint angle of the tibia in chimpanzees is naturally bent, while that of humans is much more vertical, it would have likely been unsafe for early hominins to attempt to climb the way other primates did.
Bipedal motion in humans came with significant changes to skeletal structure. One specific change would be the fact that humans are upright, meaning the hands and arms can be used for carrying or gathering food (Lloyd du Brul 1962). Here (Lloyd du Brul 1962) insinuates that the origin of a bipedal posture came from this need to carry food, as was mentioned earlier. Additional skeletal changes include the reconstruction of the foot to be arched and elongated, the pelvis is flared, and knees are bent. Knees have also been reconstructed to maintain the weight of the human body while standing, without too much fatigue (Lloyd du Brul 1962). Additionally it is noted that flexibility in the hip and shoulder for humans is part of what defined human ability to walk on 2 ft without the added need of a tail for balance (Marks 1987).
The australopithecines appeared to have certain anatomical features that were almost transitional between earlier apes and modern humans, similar to what is seen in the later discovered Ardipithecus ramidus (Shipman 2010). Pelvic shape, for example, is much wider than that of modern chimpanzees, but is not nearly as bowl-shaped and circular as modern humans (Hunt 1994).
In summary of the pattern that we find in Australopithecus afarensis, the upper body features anatomy that appears to be ideal for arm-hanging, whereas the lower body appears to be more adept for bipedal walking. The curved finger bones and short lumbar indicate that this animal would have been a capable climber, likely hanging from tree branches to gather food (Hunt 1994; Tuttle 1981). However, the anatomy of the lower body of Australopithecus afarensis suggests that it would be sufficient for bipedal motion, although this likely would have been less efficient and more stressful to the skeletal system, compared to bipedalism in modern humans. They likely wouldn’t have been as habitually bipedal as modern humans are today (Hunt 1994).
Overall the comparison of morphology between modern humans, modern chimpanzees and Australopithecus afarensis suggests flaws in previous assumptions for the origins of bipedality. The presence of both arboreal upper anatomy and bipedal lower anatomy in A. afarensis indicates that their method for gathering food would have likely not been purely due to either terrestrial feeding or arboreal feeding (Hunt 1994).
Homo erectus demonstrates the first real indication of changes in post-cranial anatomy, compared to Australopithecus afarensis. Arboreal adaptations appear to stay all throughout the anatomy of the Australopithecines until the next species in the hominin lineage evolved. This is a sign that the arboreal upper body of the australopithecines must have been adaptive for one reason or another, of course, until it wasn’t (Hunt 1994).
In 2009, analysis was published on recent anthropological evidence that had been discovered in Ethiopia. The findings were that of 36 individuals from around 4.4 million years ago. The most complete skeleton was from a female, nicknamed “Ardi.” Hers is currently the oldest hominin skeleton to date, and combined with the remains of the 35 others, provides compelling evidence that they likely walked bipedally. The full name for this group has been termed Ardipithecus ramidus (Shipman 2010).
It was originally believed that our large human brains evolved long before other human features such as bipedalism, or our dental anatomy, however these remains indicated that this was not the case. Ardipithecus Ramidus showed both indications of being bipedal, as well as having dentition that had human-like features. Much of her anatomy appears to be transitional between humans and chimps. She shares many features with both modern nonhuman apes, and modern hominins, however she appears to be evolving further away from our last common ancestor (Shipman 2010).
For many years, the remains of several different kinds of australopiths have been unearthed, all of whom are very different, but the one trait that they all shared, was that they were bipedal. While Ardipithecus was likely bipedal, they likely had the ability to maneuver around trees like most apes. Comparatively, their arboreal motion was likely similar to that of another early ape, the Proconsul, which was likely not as easy as earlier ancestors.
While Ardipithecus was likely not the last common ancestor between humans and chimpanzees, analyzing “Ardi’s” remains can provide insight as to what that ancestor may have looked like or how it may have moved. Contrary to what may have been previously thought, Ardipithecus ramidus is not a transitional biped, she is a fully-established biped (Shipman 2010). As an additional hominin, she presents movement from potentially awkward bipedalism to more efficient bipedalism.
The evidence above provides several different and enticing arguments for the origins of bipedal locomotion. From changes in evolutionary environment suggesting the need for increased visibility, to adaptations that came from being bipedal in the previous arboreal setting. However, it appears that as more evidence on the Miocene hominins is discovered, the more complicated it becomes to explain their anatomy and potential behavior.
Modern non-human primates have demonstrated that there are ways of coping with certain environmental issues without needing to walk upright, indicating that the selective pressures that caused modern humans to evolve bipedal locomotion would have had to be intense and prolonged to justify a complete transformation in skeletal structure. To date it appears that the best evidence we have for studying hominin bipedalism comes from the recently discovered remains of Ardipithecus ramidus. Her anatomy still features arboreal adaptations in the upper body, but this could have been carryover from an earlier ancestor.
It is important to remember the amount of time it takes certain adaptations like this to evolve. When comparing the skeletons of modern humans with modern nonhuman apes it is clear that there was a complete structural reorganization. Even if this process started around 4.4 million years ago, it would have likely been hundreds of thousands of years before there were indications of truly bipedal skeletons, devoid of any arboreal upper body. Patience is required when sifting through the evidence, to deal with the fact that we, as modern humans, exist much further down the evolutionary line, and have much to thank our ancestors for.
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