User:Vrabec.4/sandbox

Topic: Evolution of bipedalism in humans

Works Cited

Hasegawa, Masami, Hirohisa Kishino, and Taka-Aki Yano. "Dating of the Human-ape Splitting by a Molecular Clock of Mitochondrial DNA." Journal of Molecular Evolution 22.2 (1985): 160-74. Web. 15 Sept. 2014.

Hunt, Kevin D. "The Evolution of Human Bipedality: Ecology and Functional Morphology." Journal of Human Evolution 26.3 (1994): 183-202. Web. 15 Sept. 2014.

Schmitt, D. "Insights into the Evolution of Human Bipedalism from Experimental Studies of Humans and Other Primates." Journal of Experimental Biology 206.9 (2003): 1437-448. Web. 15 Sept. 2014.

Spoor, Fred, Bernard Wood, and Frans Zonneveld. "Implications of Early Hominid Labyrinthine Morphology Forevolution of Human Bipedal Locomotion." Nature 369.6482 (1994): 645-48. Web. 15 Sept. 2014.

Templeton, Alan R. "Phylogenetic Inference From Restriction Endonuclease Cleavage Site Maps with Particular Reference to the Evolution of Humans and the Apes." Evolution 37.2 (1983): 221. Web. 15 Sept. 2014.

https://en.wikipedia.org/wiki/Human_skeletal_changes_due_to_bipedalism

Limbs[edit] Some more suggestions to help improve the article:

1. I found an article that discussed the changes in bone structure of the femur in infants as they learn to walk. I feel like this may be worth adding to the article.

2. The addition of images may be useful to help illustrate the changes discussed in the article. A simple one such as this might help people to understand how the angle of your femur helps to support your weight.

3. Another article I found discussed how femoral/humeral strength and length changed during a human's development. The researchers also compared these results to those gathered from baboons. Some of this information may be worth adding to the article as well.

Vrabec.4 (talk) 02:13, 2 October 2014 (UTC)

This results in decreased strength in the forelimbs relative to body size for humans compared to apes.[8] source: http://www.sciencedirect.com/science/article/pii/S0047248403001209

Actual formatting in article: (This results in decreased strength in the forelimbs relative to body size for humans compared to apes. )

Edit added to Bipedalism

A similar study conducted by Thorpe et al. looked at how the most arboreal great ape, the orangutan, held onto supporting branches in order to navigate branches that were too flexible or unstable otherwise. They found that in more than 75% of locomotive instances the orangutans used their hands to stabilize themselves while they navigated thinner branches. They hypothesized that increased fragmentation of forests where A. afarensis as well as other ancestors of modern humans and other apes resided could have contributed to this increase of bipedalism in order to navigate the diminishing forests. Their findings also shed light on a couple of discrepancies observed in the anatomy of A. afarensis, such as the ankle joint, which allowed it to “wobble” and long, highly flexible forelimbs. The idea that bipedalism started from walking in trees explains both the increased flexibility in the ankle as well as the long limbs which would be used to grab hold of branches.

Final Draft

The evolution of human bipedalism has been researched extensively and, while researchers have a good idea as to why we walk upright, these is no concrete answer that has been found. Instead it seems that a variety of factors came into play and each one helped to exert a selection pressure in the favor of bipedalism. There are many hypotheses that support the evolution of bipedalism in humans and while they share a lot in common there are also subtle differences between them. One of those ideas is that “hominid bipedalism may have evolved in conjunction with arm-hanging as a specialized feeding adaptation that allowed for efficient harvesting of fruits among open-forest or woodland trees (Hunt, 1994).” Instead of walking more upright to allow for easier feeding, which has been argued by some scientists, Kevin Hunt proposed that it may be the other way around. Finding the answer to why humans walk upright is important to the evolution of humans as well as our closely related ancestors because no other species of our size has developed bipedalism. The two leading ideas common to almost all the hypotheses however are that it was energetically favorable to walk more up right as opposed to walking with bent knees and hips and that a more upright posture allowed for increased balance and feeding ability. In a paper published by J.T. Stern Jr. and R.L. Susman back in 1983 they analyzed the locomotive anatomy of Australopithecus afarensis in an attempt to determine to what extent A. afarensis was terrestrial compared to arboreal. A. afarensis is thought to be the most closely related to the genus Homo than another known primate at that time. In a comparison between specimens they found that the smaller ones, assumed to be female, probably spent more time in the trees while the larger males presumably spent more time on the ground based on the anatomy of the specimens. This supports the idea of sexual dimorphism and provides another potential selective pressure for or against bipedalism (Stern Jr. and Susman, 1983). If the females had a preference for males that could walk upright that would have been a contributing factor to the evolution of bipedalism. Unfortunately there is no way to directly test this and it is very hard to make assumptions about sexual selection because of it's complexity. A. afarensis is believed to be the most common ancestor between humans and apes. A. afarensis is also thought to be one of the first ancestors able to walk bipedally but also frequently walked on their knuckles in a bent hip-bent knee gait. To this day there is still debate about how much time A. afarensis spent living and moving in trees compared to walking as well as how much they walked upright. In an attempt to help determine the main method of locomotion for A. afarensis researchers Tanya Carey and Robin Crompton tried to estimate the metabolic costs for A. afarensis for walking on land versus living and moving through trees. They looked at if it was more beneficial to walk upright or to walk with bent hips and bent knees for these activities. Their results showed that by moving with bent hips and knees there was a 50% increase in basal metabolic rate as well as an increased heat load. The increased heat load would have required an increased resting time following physical activity thus making it more inefficient than walking more upright. A. afarensis are also hypothesized to have been scavengers, thus they would have wanted to be able to forage in a large area, which would have been unfeasible given these results (Carey and Crompton, 2004). This study supports the idea that if bent hip-bent knee walking was energetically unfavorable there would have had to have been some sort of selection pressure that made the more energetically favorable mode of bipedalism more unfit than bent hip-bent knee. Another study performed by Wang et al. also returns the same conclusions. In this study eight adults were asked to walk across a 25 meter plywood walkway. They crossed it by walking either at a slow, comfortable or fast upright pace as well as using bent hip-bent knee walking. The researchers then determined the amount of “energy recover” between the four gaits (Wang et al., 2003). Essentially they calculated how much energy was wasted in the transition from potential to kinetic energy when walking. They found that the bent hip-bent knee gait was the most ineffective by twice as much as the other three. Increased recovery time was also seen when using the bent hip-bent knee gait when compared to the other gaits of walking (Wang et al. 2003). This result is also supported in the study by Carey and Crompton. They found that on average twice as much oxygen is consumed when walking crouched than when walking comfortably upright (Carey and Crompton, 2004). Wang et al. additionally demonstrated that neither reduced load on the sacroiliac joint nor advantages in speed or changing direction were significant enough to provide enough selective pressure on bipedalism for it to not seem more beneficial than bent hip-bent knee (Wang et al., 2003). These reasons are the most common ones used when trying to explain why bent hip-bent knee would have been more beneficial than terrestrial bipedalism. They finished by concluding that a potential selective pressure that makes bent hip-bent knee better than bipedalism remains unidentified (Wang et al., 2003). Between these two studies and what is currently known about the anatomy and hypothesized behavior of A. afarensis, it seems that bipedalism has a clear fitness and selection advantage over bent hip-bent knee. The second leading hypothesis of how bipedalism might have evolved comes from the idea that flexibility and the ability to walk in an upright position provided increased balance and ability to grab food when compared with those using a bent hip-bent knee posture. In a study by Thorpe et al. they show that the most arboreal great ape, the orangutan, is able to navigate branches that are too flexible and too unstable to be navigated by other apes with reduced flexibility. The researchers observed orangutans for one year at Gunung Leuser National Park and found that in more than 75% of locomotive instances the orangutans used their hands to hold branches to stabilize themselves while they navigated thinner branches. They hypothesized that increased fragmentation of forests where A. afarensis as well as other ancestors of modern humans and other apes resided could have contributed to this increase of bipedalism in order to navigate the diminishing forests (Thrope et al., 2007). Their findings also shed light on a couple of discrepancies observed in the anatomy of A. afarensis, such as the ankle joint, which allowed it to “wobble” and long, highly flexible forelimbs. The idea that bipedalism started from walking in trees explains both the increased flexibility in the ankle as well as the long limbs which would be used to grab hold of branches. They conclude the paper by stating that it was the knuckle walking ancestors that innovated and evolved the ability to move on land as a quadrupedal while the ancestors that gave rise to bipedalism simply conserved their existing adaptations and perfected them to be able to walk on land (Thrope et al., 2007). Contrary to this study was one conducted by Brian Richmond and David Strait in which they hypothesized that due to the unique morphology of the distal radius found in A. afarensis and the fact that it differs from that found in later hominids and non-knuckle walking primates that it was the knuckle walking ancestor that gave rise to bipedalism. By comparing fossils of those found from A. afarensis and other ancestors with those found in modern day humans and primates as well as other recent ancestors they found that the morphology of knuckle-walking primates wrists were less flexible. This is because they need to be able to bear much more weight than the wrists of non-knuckle walkers. In the conclusion of the paper they acknowledge that locomotion by A. afarensis was likely a combination of both terrestrial knuckle-walking and arboreal climbing even with the wrist morphology less suited for climbing. They also note that there would be fairly low selective pressure for terrestrial bipedalism since they already had a suitable adaptation for it (Richmond and Strait, 2000). It seems that scientists have found many different explanations for how bipedalism could have evolved. They all seem to over lap with one another at the same time though. The general consensus seems to be that through a combination of walking upright in trees, having reduced energy expenditure, sexual selection, and an already viable method of terrestrial locomotion that bipedalism was selected for by the environment and led to where we are today. While scientists will probably never know for sure if one of these methods is more correct than the others they seem to be very close to determining all the factors that led to the evolution of bipedalism. The evolution of human bipedalism has been researched extensively and, while researchers have a good idea as to why we walk upright, these is no concrete answer that has been found. Instead it seems that a variety of factors came into play and each one helped to exert a selection pressure in the favor of bipedalism. There are many hypotheses that support the evolution of bipedalism in humans and while they share a lot in common there are also subtle differences between them. One of those ideas is that “hominid bipedalism may have evolved in conjunction with arm-hanging as a specialized feeding adaptation that allowed for efficient harvesting of fruits among open-forest or woodland trees (Hunt, 1994).” Instead of walking more upright to allow for easier feeding, which has been argued by some scientists, Kevin Hunt proposed that it may be the other way around. Finding the answer to why humans walk upright is important to the evolution of humans as well as our closely related ancestors because no other species of our size has developed bipedalism. The two leading ideas common to almost all the hypotheses however are that it was energetically favorable to walk more up right as opposed to walking with bent knees and hips and that a more upright posture allowed for increased balance and feeding ability. In a paper published by J.T. Stern Jr. and R.L. Susman back in 1983 they analyzed the locomotive anatomy of Australopithecus afarensis in an attempt to determine to what extent A. afarensis was terrestrial compared to arboreal. A. afarensis is thought to be the most closely related to the genus Homo than another known primate at that time. In a comparison between specimens they found that the smaller ones, assumed to be female, probably spent more time in the trees while the larger males presumably spent more time on the ground based on the anatomy of the specimens. This supports the idea of sexual dimorphism and provides another potential selective pressure for or against bipedalism (Stern Jr. and Susman, 1983). If the females had a preference for males that could walk upright that would have been a contributing factor to the evolution of bipedalism. Unfortunately there is no way to directly test this and it is very hard to make assumptions about sexual selection because of it's complexity. A. afarensis is believed to be the most common ancestor between humans and apes. A. afarensis is also thought to be one of the first ancestors able to walk bipedally but also frequently walked on their knuckles in a bent hip-bent knee gait. To this day there is still debate about how much time A. afarensis spent living and moving in trees compared to walking as well as how much they walked upright. In an attempt to help determine the main method of locomotion for A. afarensis researchers Tanya Carey and Robin Crompton tried to estimate the metabolic costs for A. afarensis for walking on land versus living and moving through trees. They looked at if it was more beneficial to walk upright or to walk with bent hips and bent knees for these activities. Their results showed that by moving with bent hips and knees there was a 50% increase in basal metabolic rate as well as an increased heat load. The increased heat load would have required an increased resting time following physical activity thus making it more inefficient than walking more upright. A. afarensis are also hypothesized to have been scavengers, thus they would have wanted to be able to forage in a large area, which would have been unfeasible given these results (Carey and Crompton, 2004). This study supports the idea that if bent hip-bent knee walking was energetically unfavorable there would have had to have been some sort of selection pressure that made the more energetically favorable mode of bipedalism more unfit than bent hip-bent knee. Another study performed by Wang et al. also returns the same conclusions. In this study eight adults were asked to walk across a 25 meter plywood walkway. They crossed it by walking either at a slow, comfortable or fast upright pace as well as using bent hip-bent knee walking. The researchers then determined the amount of “energy recover” between the four gaits (Wang et al., 2003). Essentially they calculated how much energy was wasted in the transition from potential to kinetic energy when walking. They found that the bent hip-bent knee gait was the most ineffective by twice as much as the other three. Increased recovery time was also seen when using the bent hip-bent knee gait when compared to the other gaits of walking (Wang et al. 2003). This result is also supported in the study by Carey and Crompton. They found that on average twice as much oxygen is consumed when walking crouched than when walking comfortably upright (Carey and Crompton, 2004). Wang et al. additionally demonstrated that neither reduced load on the sacroiliac joint nor advantages in speed or changing direction were significant enough to provide enough selective pressure on bipedalism for it to not seem more beneficial than bent hip-bent knee (Wang et al., 2003). These reasons are the most common ones used when trying to explain why bent hip-bent knee would have been more beneficial than terrestrial bipedalism. They finished by concluding that a potential selective pressure that makes bent hip-bent knee better than bipedalism remains unidentified (Wang et al., 2003). Between these two studies and what is currently known about the anatomy and hypothesized behavior of A. afarensis, it seems that bipedalism has a clear fitness and selection advantage over bent hip-bent knee. The second leading hypothesis of how bipedalism might have evolved comes from the idea that flexibility and the ability to walk in an upright position provided increased balance and ability to grab food when compared with those using a bent hip-bent knee posture. In a study by Thorpe et al. they show that the most arboreal great ape, the orangutan, is able to navigate branches that are too flexible and too unstable to be navigated by other apes with reduced flexibility. The researchers observed orangutans for one year at Gunung Leuser National Park and found that in more than 75% of locomotive instances the orangutans used their hands to hold branches to stabilize themselves while they navigated thinner branches. They hypothesized that increased fragmentation of forests where A. afarensis as well as other ancestors of modern humans and other apes resided could have contributed to this increase of bipedalism in order to navigate the diminishing forests (Thrope et al., 2007). Their findings also shed light on a couple of discrepancies observed in the anatomy of A. afarensis, such as the ankle joint, which allowed it to “wobble” and long, highly flexible forelimbs. The idea that bipedalism started from walking in trees explains both the increased flexibility in the ankle as well as the long limbs which would be used to grab hold of branches. They conclude the paper by stating that it was the knuckle walking ancestors that innovated and evolved the ability to move on land as a quadrupedal while the ancestors that gave rise to bipedalism simply conserved their existing adaptations and perfected them to be able to walk on land (Thrope et al., 2007). Contrary to this study was one conducted by Brian Richmond and David Strait in which they hypothesized that due to the unique morphology of the distal radius found in A. afarensis and the fact that it differs from that found in later hominids and non-knuckle walking primates that it was the knuckle walking ancestor that gave rise to bipedalism. By comparing fossils of those found from A. afarensis and other ancestors with those found in modern day humans and primates as well as other recent ancestors they found that the morphology of knuckle-walking primates wrists were less flexible. This is because they need to be able to bear much more weight than the wrists of non-knuckle walkers. In the conclusion of the paper they acknowledge that locomotion by A. afarensis was likely a combination of both terrestrial knuckle-walking and arboreal climbing even with the wrist morphology less suited for climbing. They also note that there would be fairly low selective pressure for terrestrial bipedalism since they already had a suitable adaptation for it (Richmond and Strait, 2000). It seems that scientists have found many different explanations for how bipedalism could have evolved. They all seem to over lap with one another at the same time though. The general consensus seems to be that through a combination of walking upright in trees, having reduced energy expenditure, sexual selection, and an already viable method of terrestrial locomotion that bipedalism was selected for by the environment and led to where we are today. While scientists will probably never know for sure if one of these methods is more correct than the others they seem to be very close to determining all the factors that led to the evolution of bipedalism.