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The Evolution Of Primate Locomotion and Body Configuration
Humans, who have long since abandoned the trees as their principal lodging place, have only partially lost the physical adaptations for tree climbing; their hands, in particular, remain in the arboreal mold. Only the feet have lost their primitive prehensility in adapting to bipedal walking (Primate, 2009). There are many changes that occurred in locomotion and body configuration through primate evolution, many of which are easily seen, but some require some in depth research. This research paper will try to elaborate on the evolutionary modifications and some of the theories that have been proposed for these changes throughout our and other primate evolution.
Grade I The Lemuroids
Some of the greatest diversity of strepsirhines and lemurs are found on Madagascar, where more than 30 species are represented, belonging to five different families (Harrison, 2004). The Lemur ranges in size from as small as a single ounce to as much as twenty two pounds. At one time there we're even larger lemurs that weighed as much as 240k g but are now extinct due to humans settling on Madagascar. Lemurs on the evolution timeline have not changed very much from their ancestors. This helps in the understanding of primate evolution because we can see primates of long ago first hand. One of the best-known members of the lemur family is the Ring-tailed Lemur. Like other lemurs the Ring-tailed lemur move quadrupedally in the trees and on the ground. Although their spine is shaped for walking quadrupedally, Ring-tailed lemurs are able to hop bipedally as well. According to Shapiro, there are a great deal of fossils from the earliest of primates. And although they were not directly ancestral to the lemurs, in terms of their adaptations they were probably most similar to the living lemurs. Shapiro goes on to say that this spine comparison is important because lemur's spine is a very different shape from the spine of a primate that keeps its back upright a lot. Compared to other primates like Chimpanzees, the lemurs spine looks very different. All monkeys or prosimians who walk on all fours most of the time have much longer spines, which are more flexible, and they have a harder time holding their back upright. Shapiro also states that Lemurs do a lot of different kinds of locomotor behaviors. They do walk quadrupedally, but they're also very good at leaping. And in some of the lemurs, they do their leaping from a horizontal position. And their spine looks like that of a primate that walks quadrupedally. Other lemurs leap from a vertical position, and they spend almost all their time either clinging to the trunk of a tree vertically, and then they leap (Shapiro, 2001).
Grade II The Tarsiers
The Tarsier is a one of the smallest primates belonging to a single genus and they can be found on the islands of Southeast Asia. Tarsiers are often called the living fossils because of striking resemblance to fossils of primates from 40 million years ago. These nocturnal primates have some of the largest eyes of the primate family. Their legs are much longer than their forelimbs and they have a long and almost hairless tail. Tarsiers also have pads on the ends of their fingers which are a result of adaption for their specialized lifestyle as vertical clingers and leapers (Harrison, 2004). What makes Tarsiers unique among primates is that they have toilet claws on both their second and third toes, unlike the lemur which only have a toilet claw on their second toe. Strepsirhines and Tarsiers have in the past been grouped together because they still retain many primitive features that become obsolete in higher evolved primates. However, Tarsiers share a number of distinctive specializations with anthropoids (for example, the development of a bony partition that partially separates the eyeball from the chewing muscles behind, lack of a toothcomb, a dry hair-covered nose and undivided upper lip, a short snout with reduced facial whiskers, and the structure of the placenta) that suggest that they are more closely related to each other than either is to the strepsirhines (Harrison, 2004). Tarsier move somewhat like a small, furry frog, and can leap from small branch to small branch. In order to do this efficiently, the tibia and the fibula (the two lower leg bones) are fused about halfway down their length, giving the leg more strength. Tarsiers also have elongated ankle bones, which helps them leap, and which gives them their name, tarsier, a reference to the tarsal, or ankle, region.(Jurmain, 2008) The legs are much longer than their arms. These curious little nocturnal creatures dart around the undergrowth and low trees, keeping out the realm of larger animals until they want to leap across the ground to gather up prey. Tarsiers are carnivorous, eating insects and small lizards (Blashfield, 2009). If the Tarsier had not developed grasping and large eyes through evolution it would have most likely not survived in its present habitat.
Grade III The Monkeys
Monkeys can range in size from 5–6 in long (plus tail) to almost 3.3 ft long. Monkeys can also weigh anywhere from 4–5 ounces to as much as 77 lbs. Some are arboreal (living in trees) while others live on the savanna. Living in such different climates play part in how some Monkeys have different characteristics than others. Some characteristics are shared amongst most Monkeys. New World monkeys have prehensile tails while Old World monkeys have non-prehensile tails or no visible tail at all. Some monkeys have trichromatic color vision like humans do, others are dichromats or monochromats. Although both the New and Old World monkeys, like the apes, have forward facing eyes, the faces of Old World and New World monkeys look very different, though again, each group shares some features such as the types of noses, cheeks and rumps. One monkey that stands out in particular because of its evolutionary traits is the spider monkey. The Spider monkeys use several different types of locomotion. They can use quadrupedal for walking or running, suspensory locomotion when hanging, climbing or moving through the trees and bipedalism when leaping. Quadrupedal locomotion is usually observed if the monkey is on a stable relatively substrate free of obstacles. When they are using suspensory locomotion they may be brachiating (swinging with their arms from one branch to another while often maintaining a tail hold). The most commonly used pattern of body movement while in a feeding pattern is that of quadrupedal, climbing and suspensory locomotion. While traveling they mostly employ quadrupedal walking and running, suspensory locomotion and climbing. The part of the body that helps the spider monkey walk bipedally, almost acting as external spine is the single trait in particular that makes it stand out. Its griping tail. This arboreal monkey has a prehensile tail that is muscular and tactile and is used as an extra hand. The tail is sometimes longer than the body and when stiffly up righted against their back act an external spine so that they can maintain a bipedal walk. Both the underside and tip of the tail are used for climbing and grasping and so the spider monkey uses it like a fifth hand. When swinging by the tail, the hands are free to gather food( Valley, 2009). In the picture to the right you can see how the spider monkeys tail can support the entire weight of the monkey. The Spider monkey may have evolved into this structure so that when eating high in the trees where there is no room for error in grip, it can still have a two handed grip. There is still much speculation as to why primates as they get larger in size start to lose their tails (Kavanagh, 1984).
Grade IV The Apes and Man
Apes and humans differ from all of the other primates in that they lack the external tail. Shapiro believes that as primates move into an upright position that that is when we began to lose our tails. When you lose the tail, the tail musculature gets incorporated into the pelvic floor, which can form a support for your organs. Because of the upright position, your organs will tend to move downward. This is where the support would come in on the pelvic floor. Among all the Primates, Man and Apes are the most intelligent. We are also more dependent for survival. Aside from the lack of the external tail, there are several internal body differences as well, such as the absence of an appendix that monkeys have. Humans and apes are both members of the Hominoidea family. Until recently, humans were separated into our own family. It was believed that we significantly different from the apes. However, in recent genetic studies and discoveries form the fossil record have shed light on that some apes are more similar to humans than previously believed. Apes can be found all over the world. Orangutans and Gibbons reside in Southeast Asia. Gorillas, Bonobos, and Chimpanzees can only be found in Africa. According to fossils it is also believed that humans to originated in Africa. The gibbons are the smallest of the ape family. There are 12-13 species of gibbons. Because of their size however, they are referred to as the "lesser apes." Gibbons only get to be around 3ft in height and range from 12-20 pounds. One of the key characteristics of the gibbons is their long arms. Their long arms, permanently curved fingers, and light bodies make them excellent brachiators. This structure makes moving around trees by swinging from branches ideal. This action is call suspensory climbing. Gibbons also can walk bipedally on top of branches but are better at swinging (O'Neil ,2009). Orangutans are the rarest and largest of the Asian apes. They can grow in weight up to 200 pounds and grow as tall as 4.5 feet. This body structure prevents it from swinging like the gibbons so orangutans must walk on the ground. unlike most primates, orangutans have huge fleshy pads framing the upper part of their faces. Unlike the male orangutans, the female orangutans can move about in the trees thanks to a lighter structure. Not as swift as the gibbons, the female orangutans use a leaning form of brachiation. They carefully shift their body weight to bend a supporting branch and then grab the next one before the original one snaps under pressure. In this part of the evolutionary timeline you begin to notice that primates are coming out from the trees and roaming the flat surface as they grow in body size. Their bodies are becoming less adaptable to trees. The largest of the Ape family is the Gorilla. Gorillas have been known to have been as tall as 6 feet with an arm span of almost 10 feet. This long arm span and introduction of knuckle walking is where quadrupedal is starting to fade and bipedal is beginning for primates. However, gorillas are quadrupedal because the small legs cannot support the massive structure so it is forced to walk on all fours. They walk on the soles of their feet but not on the palms of their hands. Gorillas bend their fingers so that they can support the head end of their bodies. The gorillas head is massive and heavy. With this comes huge muscles on not only the rest of the body but on the jaw. Like humans, gorillas are terrestrial. The more common resemblance to humans does not belong to gorillas however, but chimpanzees(Byrne, 2001). Chimpanzees grow to be around 5.5 feet in height and their average weight is about 100 pounds. Their arms get to span about 6 feet. Because of the evolution of the spine in primates, even though chimpanzees prefer to walk quadrupedally, they can get around bipedally. This is what makes the chimpanzee are closest cousin on the primate table (O'Neil, 2009).
While researching at The Los Angeles Zoo, I got see firsthand the transformation from quadrupedal to bipedal in an afternoon. While at the chimpanzee exhibit I got to see a close family eat and sleep together. When it came time for feeding, one of the males ran bipedally across a flat area. As I walked around viewing the other primates, viewing the Siamangs and their griping abilities, the white sifakas and how it has arching spine when it walks about, and the gorillas and how when they look out at the onlookers, it really does feel like they are looking back at you with some kind of interest. You begin to believe more in Darwin's theory of evolution when you see such striking patterns in structure to yourself.
Although there are significant differences between humans and non human primates, the distance between primates begin to get closer as we humans begin to look deeper into primates evolution. The way we walk, grab, even just stand can all be traced through the bone structure of our non human cousins. As researchers get closer to discovering links between humans and non human primates and filling the gaps of origin information, the gap between ape and man becomes dramatically smaller than once thought.
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Byrne, R. W., Dunbar, R. I.M., McGrew, W. C., Pusey, A. E., Snowdon, C. T., Stanford, C. B., et al. (2001). Tree of Origin: What Primate Behavior Can Tell Us about Human Social Evolution. Massachusetts: Harvard University Press.
Figure 1. Spider Monkey (2009). Source: http://www.primates.com/monkeys/spidermonkey.jpg
Harrison, T. (2004). Primates, Retrieved September 29, 2009, from AccessScience@McGraw- Hill, Website: http://www.accessscience.com
Jurmain, R., Kilgore, L., Trevathan, W., Ciochon, R. L., (2008). Introduction To Physical Anthropology. California: Thomson Higher Education
Kavanagh, M., Morris, D. (1984). A Complete Guide To Monkeys, Apes And Other Primates. New York: The Viking Press
O'Neil, D. (2009) Apes. Retrieved September 30, 2009, from Palomar.edu
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Shapiro, L. (2001) Primate Locomotion. Retrieved September 29, 2009, from PBS.org
Valley Zoo School (2009). Spider Monkey Retrieved September 30, 2009