Ardipithecus Ramidus and Its Relationship to Humans and Chimpanzees - Term Paper Example

?Running head: ANTHROPOLOGY Ardipithecus Ramidus and Its Relationship to Humans and Chimpanzees Insert Insert Grade Insert 09 March 2012 Ardipithecus Ramidus and Its Relationship to Humans and Chimpanzees Introduction There have been two major gaps in the fossil record in tracing human evolution. The hominoid gap is the first and is said to date back to approximately 22-32 million years ago. This is the period that the human and ape ancestors were diverging from the monkeys’ evolution path. The other gap is the hominid gap and is estimated to date back to about 4.5 to 14 million years ago. It is believed that during this period, the human ancestors and extant ape ancestors separated from a common evolution path. Hominids refer to the human ancestors after the divergence from the last common ancestor with extant apes. Recently, fossil evidence bridging the hominid gap was discovered in 1992 in Aramis, Northern Ethiopia by a group of researchers. The discovery of Ardipithecus ramidus changes the initial belief that the common ancestor between man and extant apes looked more like a chimpanzee. The characteristics of the Ardipithecus ramidus are more similar to those of humans and therefore, the extant apes must have developed their traits after their evolution path and that of humans separated. This discussion explores the Ardipithecus ramidus’ relationship to Chimpanzees and humans. Ardipithecus Ramidus Ardipithecus ramidus is believed to be 4.4 million years old and its discovery has changed the belief that human beings evolved from chimpanzees. Ardipithecus ramidus walked upright like humans and had a big toe for holding trees like apes. Before the discovery of Ardipithecus ramidus, the oldest early human skeleton was that of Australopithecus afarensis, also known as Lucy. Lucy is believed to be 3.2 million years old. Australopithecus afarensis was also discovered in Ethiopia and had many common characteristics with human beings. Therefore, the earlier ancestry between humans and apes was missing. The first specimen of Ardipithecus ramidus was discovered in 1992. The fossils were found in sediment layers which are approximately 4.4 million years old. The skull, arms feet, pelvis, and teeth were later discovered. The features of Ardipithecus ramidus such as lack of ape-like canines show that she was not an ancestor of modern chimpanzees (White et al., 2009). Studies of evolution have been trying to determine the reason for the distinctiveness of human beings in terms of being bipedal, having high intellectual and technological capabilities, and other traits that distinguish them from extant apes. Before the discovery of Ardipithecus ramidus fossils, the evolution of humans was based on the characteristics of African apes. It was believed that these unique abilities that humans possess evolved from the modification of the traits of chimpanzees and bonobos, which are the closest human relatives. Human behavior was viewed as a modification of the chimpanzees’ characteristics such as the dominance of males, hunting, culture, upright feeding posture, among others (Harrison, 2010). The discovery of Ardipithecus ramidus sheds light into the period of hominid development after splitting from the common ancestor with apes. Before Ardipithecus ramidus was discovered, scientists believed that the last common ancestor looked more like a chimpanzee than humans. They believed that the modern apes got many of their traits from this ancestor, such as walking on knuckles and hanging and swinging on trees. Characteristics of apes such as short backs, limbs, and pelvis which are appropriate for walking on knuckles and arms for swinging and hanging on trees were believed to have originated from the last common ancestor. It was believed that after the split, the human ancestors dropped the characteristics of the common ancestor such as sharp canines and walking on knuckles while the ancestors of the chimpanzees retained them up to the present. However, these beliefs are refuted by Ardipithecus ramidus. Ardipithecus lived in woodlands and climbed on trees with four legs. However, they walked upright on the ground. The form of their limbs reveals that they did not walk on their knuckles. They also did not hang and swing from trees as is common with modern chimpanzees. Therefore, chimpanzees are not representative of the last common ancestor with humans as each took a different path of evolution after the split (Dalton, 2009). Before Ardipithecus ramidus was discovered, the oldest known human ancestor was Australopithecus afarensis. Ardipithecus ramidus is approximately one million years older than Australopithecus afarensis. Australopithecus afarensis’ fossils were discovered in 1970s in eastern Africa and they had close similarities with the chimpanzees. She was believed to be only 3.2 million years old. She looked a lot like humans and it was therefore difficult to use her to deduce the traits of earlier human ancestors. With the discovery of Ardipithecus ramidus, scientists know how early human ancestors looked like. Fossils of other hominids have been discovered in Chad and Kenya, but Ardipithecus ramidus is the most complete. Sahelanthropus tchadensis was discovered in Chad and is the oldest discovered hominid estimated to be approximately 7 million years old. It had human similarities but had a thick brow ridge the brain size of Chimpanzees. Only the skull of Sahelanthropus tchadensis was found and therefore, it was hard to determine its other characteristics. The second oldest discovered hominid, Orrorin tugenensis was found in Kenya on the Tugen Hills and is believed to be approximately 5.6 to 6.2 million years old. The Orrorin is believed to have been bipedal but there is no sufficient evidence to support this because only a few bones were discovered. The fossils found near the bones revealed that Orrorin lived in the woodlands. Ardipithecus ramidus comes third among the oldest discovered hominids. Several fossils of Ardipithecus were found including jaw bones, teeth, foot, leg, ankle, palm bones, pelvis, cranium and arm and hand bones. Ardipithecus ramidus had a height of about 120cm and 50Kg of weight (Harrison, 2010). The discovery of Ardipithecus ramidus has revealed that the early paths of evolution of hominids and apes differed since the beginning. Ardipithecus ramidus was bipedal. She was also omnivorous unlike the Gorillas and Pans. She also lacked the sharp and long canines of the ancestors of modern apes. The Ardipithecus ramidus was also believed to have lived in the woodlands judging from the contents of the sediments that were found covering the fossils. This refutes the earlier theory that human beings learned to be bipedal due to climate changes that led to dryness and made the environment more open. It was initially believed that hominids learned to walk upright when the vegetation dried up and the environment was left grassy. The Ardipithecus ramidus confirmed the early clues that there was a rift between the evolution paths of hominids and apes. For instance, the anatomy and reproductive behavior of apes differs from humans, the structure of their gut is also different and apes are clumsy when walking upright, among other disparities (White et al., 2009). Ardipithecus ramidus is believed to be close to the common ancestor between humans and chimpanzees. The last common ancestor between apes and humans is believed to have lived around six million years ago and Ardipithecus ramidus has taken the evolutionists closer to finding it. Ardipithecus ramidus lacked most of the characteristics of present-day chimpanzees such as the structures of the feet, arms, pelvis and spine that enable them to knuckle-walk and climb vertically. The chimpanzees must have inherited these traits from ancestors from their evolution path after the split from the common ancestor. Ardipithecus ramidus had a big toe used to climb trees but her pelvis differed from that of an ape in that it was broader and shorter. Therefore, she could walk upright. Ardipithecus ramidus was omnivorous judging from the thickness of its teeth enamel. The enamel was neither too thick nor too thin and this implies that she fed on meat, plants and fruits (Gibbons, 2009). Relationship of Ardipithecus ramidus to humans and Chimpanzees Ardipithecus ramidus has brought a new dimension to the theories of evolution. Being closer to the last common ancestor between man and apes, Ardipithecus ramidus had both the characteristics of chimpanzees and humans. However, her traits resembled the humans more than chimpanzees. Initial evolution theories suggested that the last common ancestor was more similar to chimpanzees. Since the Ardipithecus ramidus has different traits from the modern apes, it is believed that after the split in the line of ancestry, the apes continued evolving into modern apes such as chimpanzees while humans underwent a little change. The characteristics of the Ardipithecus ramidus’ skull, pelvis, limbs and dental structure are explored in relation to humans and chimpanzees. The Ardipithecus ramidus had a pelvis and femur adapted for both walking upright and climbing trees on four limbs. Though the Ardipithecus ramidus walked on two feet, its bipedalism was not as advanced as that of Australopithecus afarensis. The Ardipithecus ramidus had not fully developed obligate bipedalism like humans but had facultative bipedalism since she could walk upright on the ground and also on four limbs on tree branches. The formation of the pelvis of hominids enabled them to be balanced on one limb like humans while walking, unlike the chimpanzees. The shape of the ilium of the Ardipithecus ramidus and other hominids splayed out towards the body. This shape of the ilium enabled the hominids to support their weight on one limb while walking upright because the gluteal muscles were moved towards the outside of the body. The pelvis of Ardipithecus ramidus resembled that of chimpanzees in that the surface of intersection between the pelvis and hamstring muscle is not vertically pointed, but inclined to an angle to enable it to climb trees. In humans and Australopithecus afarensis, this surface is vertically-pointed to enhance bipedalism. Ardipithecus’ bipedalism was not as advanced as that of the Australopithecus or humans. On the pelvis, the ilium of Ardipithecus resembled that of humans and Australopithecus afarensis. In addition, the Ardipithecus ramidus resembled humans and Australopithecus afarensis in the anterior inferior iliac spine and the sciatic notch on the pelvis to enhance bipedalism. These characteristics differ in chimpanzees (Lovejoy, et al., 2009a). The limbs of chimpanzees and those of Ardipithecus ramidus are very different. Ardipithecus ramidus had very flexible joints on the arms. The metacarpal and wrist joints were very flexible. The hands of chimpanzees are more rigid. This shows that though both the chimpanzees and Ardipithecus ramidus climbed trees, they used completely different styles. The feet of Ardipithecus ramidus were suited for both walking on the ground and climbing trees. Ardipithecus and humans, unlike chimpanzees have a bone to help them pull out the big toe to hold things and stiffen the feet. The chimpanzees’ feet are flexible and suited climbing trees quickly. For the Ardipithecus ramidus, only the big toe is flexible to enhance grasping while the other four toes are rigid (Lovejoy, et al., 2009b). The region between the wrist and fingers was shorter in Ardipithecus ramidus than chimpanzees. They also had no characteristics such as grooves on the part connecting the wrist to the hand bones that knuckle-walking apes have. Apes such as chimpanzees which walk on their knuckles have metacarpal heads expansion which is not found in Ardipithecus ramidus. Knuckle-walking refers to the unique walking by chimpanzees without touching the ground with their palms. Instead, they fold their fingers into a fist and touch the ground with the proximal phalanges of their fingers’ dorsal side. The finger bones on the hands of Ardipithecus ramidus are shorter than chimpanzees’ but longer than those found on other hominids. The Ardipithecus ramidus’ forelimbs were not adapted to hanging and swinging on trees and walking on knuckles. The same applies to humans. Chimpanzees have stiff wrists which enable them to walk on their knuckles while those of Ardipithecus ramidus are flexible. The hamstring muscles of the Ardipithecus ramidus were well developed judging from the ischium size and shape. This condition is present in chimpanzees and it enables them to climb trees and walk on four limbs on the branches. The upper arm bone of Ardipithecus ramidus and other hominids differ from that of chimpanzees. The part of the upper arm bone connecting to the shoulder blade of Ardipithecus ramidus is shaped elliptically unlike that of chimpanzees. The bones on the inside and outside of Ardipithecus ramidus’ lower arms differ from those of chimpanzees. The part connecting the upper arm bone and the inside bone of the lower arm of Ardipithecus ramidus faces forward while that of chimpanzees and other extant apes faces upward (Lovejoy, et al., 2009b). The brain of the Ardipithecus ramidus was small judging from the size of the cranium vault on the skull. It was approximately 320 to 380 cubic centimeters. It was almost the same size as that of the chimpanzees. The human brain is approximately 1150 to 1500 cubic centimeters. The brain of Ardipithecus ramidus was smaller than that of Australopithecus which is approximately 400 to 550 cubic centimeters. Australopithecus afarensis had larger head and strong chewing muscles as it lived on a rough diet. On the other hand, Ardipithecus ramidus had a small round face since its chewing muscles were not modified for hard chewing. The structure of its enamel revealed that Ardipithecus ramidus was omnivorous. The cranium of the Ardipithecus ramidus was different from that of chimpanzees and other present-day apes. The Ardipithecus ramidus’ posterior base of the skull where the brain rests was shorter than that of the Pan species. The Ardipithecus ramidus had started inclining towards the development of superior brain features. The spine bone of the Ardipithecus ramidus connecting to the brain had a structure that could allow the posterior parietal cortex to expand. Spatial coordination is enabled by this expansion. Such features are not present in chimpanzees’ ancestors. The position of the skull of Ardipithecus ramidus is not in front of the vertebral column but on top of it, revealing that it was adapted to bipedalism. The Ardipithecus closely resembles the Sahelanthropus but is smaller. Compared to a chimpanzee, the Ardipithecus ramidus’ forward facial projection is weaker and the face shorter. In Ardipithecus ramidus, the forward projection of the face is absent below the nasal aperture unlike in chimpanzees. Their cranial base towards the back is also shorter than that of chimpanzees (Suwa, et al., 2009a). The dental structure of Ardipithecus ramidus differed from that of humans and chimpanzees. The teeth of Ardipithecus ramidus were different from those of chimpanzees in that the chimpanzees’ canines are long and very sharp while those of Ardipithecus were shorter and less sharp. However, the thickness of the canines was almost the same. Human canines are small and less sharp. Sexual dimorphism is evident in male chimpanzees in their competitions for females. Sexual dimorphism is related to the males’ huge upper canines and body size. The males compete in mating the females but play no role in bringing up the offspring. The difference in dental structure and the lack of sexual dimorphism in Ardipithecus ramidus suggest that their sexual selection was different from that of chimpanzees. The sexual dimorphism of Ardipithecus ramidus is a bit higher than that of humans since human canines are smaller. Male chimpanzees have larger canines than the females. Both male and female Ardipithecus ramidus have moderately small upper canines. The height of the upper canines of a male Ardipithecus ramidus is almost the same as that of Pan Paniscus but much smaller than a male Pan Paniscus’. Sexual dimorphism in chimpanzees is also manifested in body size where the males weigh more than the females (Suwa, et al., 2009b). The lower premolars of the Ardipithecus are also smaller than those of chimpanzees. The upper and lower incisors of Ardipithecus ramidus were smaller than chimpanzees’ but bigger than those of Australopithecus afarensis. Both chimpanzees and the Ardipithecus ramidus have a gap between the canines and incisors, even though the one found on the Ardipithecus is smaller in size. The lower molars are wider than the chimpanzees’. The enamel of Ardipithecus is thicker than that of Pan Paniscus but thinner than Australopithecus’. The enamel of the Pan Species has the same patterns of distribution and crown structure. Their enamel also has a thin coating which is absent in Ardipithecus ramidus and Australopithecus afarensis. The thin enamel of the Pan species is suited for a fruit diet. The enamel of Ardipithecus ramidus is a bit thicker and is suited for an omnivorous diet. The thick enamel of Australopithecus was adapted to tough foods (Haile-Selassie et al., 2004). Conclusion The discovery of the Ardipithecus ramidus fossils has enabled researchers to gain more insight into the characteristics of the last ancestor that humans and chimpanzees shared. The characteristics of Ardipithecus ramidus such as reduced sexual dimorphism reveal that the last common ancestor looked more like humans and therefore, chimpanzees and other extant apes developed characteristics such as knuckle-walking and swinging on trees after the paths of evolution diverged. There are several similarities and the differences in the structure of Ardipithecus ramidus, chimpanzees and humans. Researchers may not have found the last common ancestor between man and chimpanzees yet, but Ardipithecus ramidus has given them several clues about his traits and changed the initial understanding of our evolution path. Reference List Dalton, R. (2009). Fossil Rewrites Early Human Evolution. Nature, Vol. 461, p.705. Gibbons, A. (2009). A New Kind Of Ancestor: Ardipithecus Unveiled. Science. Vol. 326, pp. 36-40. Haile-Selassie, Y. et al. (2004). Late Miocene Teeth from Middle Awash, Ethiopia, and Early Hominid Dental Evolution, Science, Vol. 303 no. 5663 pp. 1503-1505. 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