User:Ondrejech.1/sandbox

''Topic (in question form): What Ecological Factors Drove The Evolution of Human Bipedalism?

Andy Ondrejech, Tues: 12:40

Hanna, J.B., and D. Schmitt. Locomotor Energetics in Primates: Gait Mechanics and Their Relationship to the Energetics of Vertical and Horizontal Locomotion. American Journal of Physical Anthropology, 145: 43-54.

Evolutionary Anthropologists of Duke examined the energy cost in vertical and horizontal movements in primates to determine there are selective pressures on the size of primates in arboreal environments. Metabolic rates of primates of various sizes were monitored via oxygen level and compiled when testing their capabilities of moving horizontally and vertically. Large species used almost twice the energy moving vertically compared to horizontally, though these cost were virtually the same for smaller species. This indicates that the cost of locomotion-rate of force relationship varies between vertical and horizontal movement and is further susceptible to species size. These findings back up the idea that small primates were some of the first and most successful animals to inhabit arboreal habitats.

Young, J.W. 2012. Ontogeny of limb force distribution in squirrel monkeys (Saimiri boliviensis): Insights into the mechanical bases of primate hind limb dominance. Journal of Human Evolution, 62:473-485.

Jesse Young, who does research as an assistant professor at Northeast Ohio Medical University, deciphers the biomechanical reasoning as to why primate force distribution remain on the hind legs of primates. In tree-dwelling species, hind leg dominance is viewed as an adaptation to maneuver arboreal territories, though biomechanically it remains controversial. Three methods for the weight proportioning are tested. The Center of Mass (COM) Position model depicts that it is due to the body’s center of mass relative to the hands and feet. The Active Weight Shift model tests if hind limb retractor muscles redistribute the weight off of the forelimbs. The Limb Compliance model argues that primates selectively disperse forelimb forces by maintaining a compliant forelimb and a flat shoulder trajectory, holding on to its tree but also leaving weight on the hind limbs. Over ten months of life, the spider monkeys in testing switched from forelimb dependent forces to hind limb dependent forces, showing a decrease in the peak force in the forelimb and an increase in hind limb weight distribution. All the models were supported though the COM position model and Active weight shift model were much more effective than the limb compliance model, meaning that there are a variety of factors that arboreal and possibly terrestrial primates disperse their weight to their hind legs.

Raichlen, D.A., H. Pontzer, L.J. Shapiro, and M.D. Socko. 2009. Understanding Hind Limb Weight Support in Chimpanzees with Implications for the Evolution of Primate Locomotion. American Journal of Physical Anthropology, 138: 395-402.

Various associate professors from around the country join to analyze what the mechanism used to support most of primates body weight in their hind limbs compared to their forelimbs. This is compared to most quadrupedal mammals that place most of their weight on their fore limbs as compared to their hind limbs. This difference is generally viewed as an adaptation that arose in arboreal primates to facilitate arboreality. Weight support patterns in chimpanzees test the hypothesis that the combination of limb position and whole body center of mass position (COM) explains increased hind limb weight support. A chimpanzee’s center of mass is between their shoulders and hips. When walking, the chimps’ limb kinematics brings their feet closer to the COM than to their hands, generating greater hind limb weight support, and comparative data suggest that this is the same for other terrestrial primates that move upright. It is unclear if these limb characteristics are to increase hind leg support, or if they are a side effect of other gait characteristics.

Pontzer, H., D.A. Raichlen, and  P.S. Rodman. 2014. Bipedal and quadrupedal locomotion in chimpanzees. Journal of Human Evolution 66: 64-82.

Chimpanzees ability to carry themselves bipedally and quadrupedally have been a common comparison to early ape-like hominins in order to study the evolutionary adaption to bipedalism. The kinematics of five captive chimpanzees were recorded via recorded treadmill trials while kinetics were recorded with a forceplate. Oxygen consumption determined metabolic rates during treadmill trials. The spatiotemporal characteristics, joint angles, ground reaction forces, and metabolic cost of bipedal and quadrupedal locomotion are similar to one another in chimpanzees. Notable differences in the two states of movement include hip and trunk angles and mediolateral ground reaction forces, which were more upright and larger during bipedal walking, respectively. Stride frequencies were higher due to step lengths shorter during bipedal trials. The similarity in bipedal and quadrupedal energy costs implies that it would have been an easy transition for early ape-like hominins. It is suggested that this efficient, habitual bipedalism favored hip adaptions such as an increased orthograde posture, and the hind limb abductor mechanisms to efficiently exert mediolateral ground forces, greatly increasing the ability to walk.

D'Aout, K., E. Vereecke, K. Schoonaert, D. De Clercq, L. Van Elsacker, and P. Aerts. 2004. Locomotion in bonobos (Pan paniscus): differences and similarities between bipedal and quadrupedal terrestrial walking, and a comparison with other locomotor modes. Journal of Anatomy 204: 353-361.

With humans’ bipedal form of transportation being so distinct from the primitive quadrupedal form of ancient ancestors, it is difficult to find an intermediate underneath any current evolutionary locomotive adaptions. Bonobos are among the closest living relatives to humans, and their locomotion is valuable to compare as an intermediate to both human and other primate locomotion. Kinematically, apart from the trunk being approximately 37degrees more erect during bipedal locomotion, the bonobos bipedal leg movements are rather similar to its quadrapedtic leg movements. Other than the heel, pressure distributions show little difference between bipedal and quadrupedal locomotion. Regardless, variability is high, and various intermediate forms of locomotion exist both in captivity and in the wild. There are many intermediates to these two extreme sets of movements, with more existing based on the environment. There are many varieties of movement which overlap one another reflected in which is reflected in their anatomy. This may prevent the development of bipedalism as a specialized gait, reducing efficiency in other forms of movement. It seems that Bonobos are unique in their locomotion compared to chimps and humans, but is specialized for their environment and lifestyle and not necessarily as a bridge between apes and humans.

Suggestions: 1.) In the biomechanic's section, material can be added to discuss the efficiency of bipedalism compared to quadrapedalism and the kinematics in animals such as chimps that invoke bipedalism. 2.) I agree that the different hypothesis could be discussed more or even outlined and that there is little on here for a terrestrial shift for bipedalism. The inclusion of the Knuckle walking hypothesis would definitely help balance this. 3.) The flexible branch theory could also be added to the end of the Savanna-Based Theory which suggests that bipedalism arose arboreally and was retained as primates descended from trees rather than being innovative at the time.

Change: In an experiment monitoring chimpanzee metabolic rate via oxygen consumption, it was found that the quadrapedal and bipedal energy costs were very similar, implying that this transition in early ape-like ancestors would have not have been very difficult or energetically costing.

FINAL DRAFT STARTS HERE

It goes without saying that bipedalism is one of the most important adaptions in the world. It is what separated Humans from any other primate; it’s the feature that paved the way for the rise of Homo sapiens, which have reached worldwide success and have altered the earth’s habitat and history unlike any other organism before them. Though the impact this adaptation has changed the world, the origins of this adaptation are still debated with many theories due to a wide variety of sparse evidence. It can be commonly agreed that the evolution of bipedalism helped the dispersal of early humans from forested areas and enabled the expansion of the cranial capacity, but what specific advantages led to orthograde posture in the first place? There are many benefits and possibilities that could have led to the adaption of Bipedalism, but none with enough certainty to be the definite reason or mechanism behind that enabled our great ancestors to walk on two legs.

A popular theory is the Knuckle Walking Hypothesis, suggesting that humans evolved from a knuckle walking ancestor to an upright walker. Knuckle Walking is a quadrupedal locomotion technique exhibited by chimps and gorillas that involves placing weight on the middle phalange of their forelimb when moving. This is used to retain digits used in for grasping while preventing them from interfering with terrestrial quadrupedalism (Dean 2000). Although highly debated because there are no fossils of habitually knuckle walking species (Richmond and Strait 2000) make the case that from a phylogenetic point of view, it only makes sense that humans evolved from a knuckle walking ancestor. This is because both the genus Pan, the chimpanzees, and Gorilla are the only extant primates that knuckle walk, with molecular, fossil, and anatomical evidence showing that humans shared a common ancestor with both, and most recently Pan. This means that knuckle walking would have to evolve independently twice for both Pan and Gorilla, rather than having it develop once and then become lost along genus Homo after the advent of a superior form of terrestrial locomotion (Richmond and Strait 2000). Not only is this less parsimonious and illogical, but theories without the knuckle walking hypothesis ignore features shared in terrestrial African great apes and humans such as similar vertebral, limb and intra-limb proportions, carpal, tarsal, and phalangeal morphology. Using the amount of mutations between human and chimp DNA as a time clock, molecular data suggests that humans are believed to have diverged from a common ancestor of Pan approximately 5-6 million years ago, while Fossil evidence demonstrates that hominids displayed adaptations to bipedal walking as long as 4.1 million years ago, if not more. This is an extended period of time in which we do not know much about the human fossil record relative to 1-4 million years ago, in which it is possible for humans to develop bipedalism.

There are ideas that suggest that Bipedalism was mostly developed in the trees, leading to the shift to a terrestrial environment to be less of an innovation and more of a trait vestigial to its ancestral lifestyle. The biggest question behind this would be to wonder why bipedalism would an arboreal adaptation as opposed to a terrestrial adaption. It is a riskier form of movement as it consists of less contact with the branch supporting the primate as well as an unbalanced center of gravity, and is not completely clear as to why it would have developed in an environment much more dangerous to it. There is a hypothesis suggesting that bipedalism is an adaptation to move on flexible branches (Thorpe and Crompton 2007), in that they have increased hip and knee extension in relation to an increase in to branch flexibility. The fact that only orangutans and humans react in this way is thought provoking, and entices the idea that maybe bipedalism was originally arboreal, helping to explain why terrestrial species have yet to wean off of quadrupedalism. This statement was enforced with the idea that arboreal feeding could be advantageous in allowing a greater range of food by allowing further foraging on small branches, serving as an appropriate niche. This was compared to how chimps are restricted to bipedal feeding on low, thick branches, and that the Orangutan adaptations would be more expand the feeding range throughout the tree, as well as being more energy efficient to traveling between the thin branches associated with the upper canopy in forests.

However, those who published their idea of the knuckle walking thesis commented on this hypothesis, saying that “their model is problematic and they neglected many important facts that point to a knuckle-walking ancestry.” (Strait et al. 2007) Such important facts include the knuckle-walking features found in early hominids, Austripithicus amanensis and Austripithicus afarensis that are found in extant Chimpanzees and Gorillas today. This connects the human lineage back to knuckle walking origins still dominant in extant great apes today, and can even show similarities to the Orangutan’s terrestrial locomotion of fist walking, where they put their weight on the base of the first digit connected to their palm instead of placing their weight on their second digit as in knuckle walking. This quadrupedal movement found in Orangutans when they are faced with ground movement indicates that even if bipedalism developed arboreally, it may have been assisted by a form of unknown hybrid terrestrial quadrupedalism. However, if arboreal bipedalism is ruled out completely, it seems likely that this shared reaction to flexible branches in arboreal settings is nothing more than a case of convergent evolution found in developed hind limb travel regardless of environment.

Another variation on the theme of locomotion leading up to Bipedalism draws from the idea that human ancestors passed through a variety of stages to retain the variety of elaborate features contained in Australopithicus fossils. It describes early apes as arboreal quadrupeds, before developing an orthograde, brachiating forelimb suspensory based locomotion, which was then adapted to a mix of arboreal and terrestrial quadrupedalism before developing bipedalism. (Gebo 1996) This is backed up by both behavioral analyses that follow and mirror the progression of ape locomotion over time, as well as by morphological evidence that point out similarities in anatomy between branchiating Atelines and terrestrially adapted hands and feet of terrestrial primates that occurred prior to bipedalism. I think it is possible that this branchiating phase prepared ape anatomy for the tradition to bipedalism, as it developed limb proportions to help facilitate knuckle walking, as well as hind limb dependency when necessary. Most primates already support a greater portion of their weight on their hind legs, though this trend is reversed for almost all other mammals. In a study on chimpanzees, it is found that the center of mass is located between the shoulders, yet is positioned closer to their feet during movement (Raichlen et al. 2009). It is possible that is a spandrel, or a trait created as a byproduct for an adaption for this gait, but it is also possible that this is an adaption to take weight off the forelimbs and free them up for short periods at a time, which is important to all primates regardless of dependency on their hands. This latter hypothesis could draw conclusions to the idea that complete hind limb support was developed in terrestrial species to free up their hands, which ties into many other positive adaptations that come with bipedalism.

There are numerous benefits that make the transition from four to two limb locomotion a very advantageous adaptation, though the central factor behind it is that Bipedalism frees hands that had become well adapted to doing complex tasks such as climbing or nurturing young all of the time. (Dean 2000) This allowed for potential gathering of food, not only for collection purposes but also to provide for females that may be nurturing. The ability to have two free hands also drastically altered the nurturing of children, and may have led to increased infant dependency on mothers instead of clinging to them, leading to the increased nurturing period allowing for the brain to develop longer outside of the body. This freedom of hands could have also led to the increased use of tools, as well as increased hunting with said tools. It is also possible that creating a better portrayal of sexual features and signals helped lead to higher reproductive rates. The prospect of an orthograde position is a significant feature, as early humans traveled through the flat, treeless savannah, giving bipedalism further momentum that could face off against many selective pressures. One of which would be to assist in staying cool in hot environments by creating less area to be in direct impact of sunlight, maintain cool body temperatures without nearby shade. It can also be helpful to look over tall patches of grass in order to survey a target, which is useful for both hunting and preventing from being hunted. Bipedalism is also much more efficient then quadrupedalism, (Pontzer et al. 2014) and is necessary for long distance traveling in flat environments, which was crucial to the dispersal and success of early hominids. The benefit of bipedalism on these selective pressures such as food gathering, nurturing, sexual selection, dispersal, and protection made its natural selection extremely practical, despite how profound.

Another commonly discussed but often disregarded idea concerning the development of bipedalism is the Wading model, often known as the aquatic ape theory. (Verhaegen et al. 2002) In this theory, early apes exploited aquatic environments for food either along banks, at the bottom of shallow water, or separated by a small, shallow body of water. The wading associated is thought to help develop bipedalism for its transition to mainland, and may have influenced apes in a many ways. It provided a much larger source of food which is helpful for omnivorous apes that need to scrounge for whatever they can obtain, and was much more constant. It also helped facilitate travel along shores and rivers. They may have even lost climbing abilities while exploiting these resources before becoming well enough adapted to bipedalism to travel efficiently on land. However outlandish this may seem, it is still a slight possibility that should not be disregarded, as it could be an contributing factor in many other theories contributing to the adaption of bipedalism.

Despite over a century of work, there is still no hard evidence on the exact origin of our locomotion. It is revolutionary, no doubt, as it would eventually result in our great cranial capacity and intelligence through nutrition and success, but the roots of it consist only of a variety of conclusions can be drawn from theories, morphological evidence, behavior, and fossils, without the being fully compatible with other theories. A combination of these driving evolutionary factors may have resulted in the evolution of bipedalism, but until we find more definite fossil data in our direct ancestors, it may never be more than a culmination of theories on what makes us so special.

References

Begun, D. R., B. G. Richmond, D. S. Strait. 2007. Comment on "origin of human bipedalism as an adaptation for locomotion on flexible branches". Science: 1066.

Dean, F. 2000. Primate diversity. W.W. Norton & Company, Inc: New York. Print.

Gebo D. L. 1996. Climbing, brachiation, and terrestrial quadrupedalism: historical precursors of hominid bipedalism. American Journal of Physical Anthropology: 55-92.

Pontzer, H., D.A. Raichlen, P.S. Rodman. 2014. Bipedal and quadrupedal locomotion in chimpanzees. Journal of Human Evolution 66: 64-82.

Raichlen, D.A., H. Pontzer, L.J. Shapiro, and M.D. Socko. 2009. Understanding hind limb weight support in chimpanzees with implications for the evolution of primate locomotion. American Journal of Physical Anthropology, 138: 395-402.

Richmond, B. G., and D. S. Strait. 2000. Evidence that humans evolved from a knuckle-walking ancestor. Nature: 382.

Thorpe S. K., R.L Holder, R. H. Crompton. 2007. Origin of human bipedalism as an adaptation for locomotion on flexible branches. Science: 1328-31.

Verhaegena, M., P. F. Puechb, S. Munro. 2002. Aquaboreal ancestors? Trends in Evolution and Ecology: 212 – 217.

FINAL ADDITIONS TO WIKIPEDIA

"This stone tools theory is very unlikely, as though ancient humans were known to hunt, the discovery of tools was not discovered for thousands of years after the origin of bipedalism, temporally preventing it from being a driving force of evolution." - Carrying Models

" It has also been thought that consistent water-based food sources had developed early hominid dependency and facilitated dispersal along seas and rivers." -Wading Models

"...as well as increased sexual signaling in upright female posture.[54]" -Other Behavioral Models

"This theory is closely related to the knuckle-walking hypothesis, which states that human ancestors used quadrupedal locomotion on the savanna, as evidenced by morphological characteristics found in Austripithicus amanensis and Austripithicus afarensis forelimbs, and that it is less parsimonious to assume that knuckle walking evolved developed twice in Genus' Pan and Gorilla instead of evolving it once as synapomorphy for Pan and Gorilla before losing it in Australopithecus.[35] The evolution of a orthograde posture would have been very helpful on a savanna as it would allow the ability to look over tall grasses in order to watch out for predators, or terrestrially hunt and sneak up on prey. [36]." -Savanna Hypothesis

" It is possible that Bipedalism evolved in the trees, and was later applied to the Savannah as a vestigial trait. Humans and orangutans are both unique to a bipedal reactive adaptation when climbing on thin branches, in which they have increased hip and knee extension in relation to the diameter of the branch, which can increase an arboreal feeding range and can be attributed to a convergent evolution of bipedalism evolving in arboreal environments.[39]" -Savanna Hypothesis" - Savanna Hypothesis

"In an experiment monitoring chimpanzee metabolic rate via oxygen consumption, it was found that the quadrapedal and bipedal energy costs were very similar, implying that this transition in early ape-like ancestors would have not have been very difficult or energetically costing.[43] This increased travel efficiency is likely to have been selected for as it assisted the wide dispersal of early hominids across the Savannah to create start populations." -Effiecient-Traveling Hypothesis