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AR103 - RN Ch#8 W8.pdf

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Jonathan Haxell

AR103 - Readings pgs. 183-200 - Week 8 Notes - Nov. 2, 2011. Chapter Eight: Primate and Hominin Origin Pages # 183 - 200 - The hominin nature of remains is reveal by more than the morphological structure of teeth and bones; in many cases, we know that these animals are hominins also because of the way they behaved - emphasizing once again the biocultural nature of human evolution. - Hominins is a colloquial term for members of the tribe Hominini, which includes all bipedal hominoids back to the divergence with African great apes. - Biocultural is pertaining to the concept that biology makes culture possible and that culture influences biology. - The earliest fossils identifiable as hominins are all from Africa. They date from perhaps as early as 7 mya, and after 4 mya, varieties of these early hominins became more plentiful and widely distributed in Africa. - Hominins evolved from earlier primates. The earliest members of the human family were confined to Africa. Early Primate Evolution - The roots of the primate order go back to the early stages of the placental mammal radiation at least 65 mya. The earliest primates were diverging from early and still primitive placental mammals. - The earliest primates date to the Paleocene (65-56 mya) and belong to a large and diverse group of primitive mammals called the plesiadapiforms. - From the Eocene epoch more definitive and clearly derived primate features in fossils have been found. - The Eocene fossils found, it’s certain that they were: 1. primates, 2. widely distributed, 3. mostly extinct by the end of the Eocene. - Earliest fossil ever found called Darwinius comes from the Messel site in Germany, dates to 47 mya. - The Oligocene has yielded numerous additional fossil remains of several different early anthropoid species. Most of these are Old World Anthropoids, all discovered at a single locality in Egypt, the Fayum. - By the early Oligocene, continental drift had separated the New World monkey from the Old World monkeys. - The possible roots of anthropoid evolution are illustrated by different forms from the Fayum; one is the genus Apidium. - Apidium is represented by several dozen jaws or partial dentitions as well as many post-cranial remains. - Post-cranial is referring to all or part of the skeleton not including the skull. The term originates from the fact that in quadrupedals, the body is in back of the head; the term literally means “behind the head.” - Aegyptopithecus, another genus, represented by well-preserved crania and abundant jaw and teeth. The largest of the Fayum anthropoids. - Roughly the size of a modern howler monkey. - Bridges the gap between the Eocene fossils and the succeeding Miocene hominoids. - Further suggests that the crucial evolutionary divergence of hominoids from other Old world anthropoids occurred after this time. 1 AR103 - Readings pgs. 183-200 - Week 8 Notes - Nov. 2, 2011. Miocene Fossil Hominoids - During Miocene, in Africa, Asia, and Europe, a diverse and highly successful group of hominoids emerged. - The Miocene could be called “the golden age of hominoids.” - During the Miocene, significant transformations relating to climate and repositioning of landmasses took place. - Migrations of animals from Africa directly into southwest Asia became possible - Among the earliest transcontinental migrants were African hominoids that colonized both Europe and Asia at this time. - Given uncertainties about assemblages of fossils and conclusions about the Miocene, Miocene hominoids have been grouped geographically: African forms (quite generalized from 23-14 mya), European forms (derived and varied from around European countries from 16-11 mya), Asian forms (largest most varied from 15-5 mya). - Four general points are certain concerning Miocene hominoid fossils: - They are widespread geographically - They are numerous - They span essentially the entirety of the Miocene, with known remains dated between 23 and 6 mya - At present, they are poorly understood. - Drawn conclusions of Miocene: - 1. These are hominoids - more closely related to the ape-human lineage than to Old World monkeys - 2. They are mostly large-bodied hominoids, that is more akin to the lineages of orangutans, gorillas, chimpanzees, and humans than to smaller bodied apes (gibbons, siamangs). - 3. Most of the Miocene species thus far discovered are so derived that they are probably not ancestral to any living form. - 4. One lineage that appears well established is Sivapithecus from Turkey and Pakistan, showing highly derived facial features similar to modern orangutans. - 5. Evidence of definite hominins from the Miocene hasn’t been indisputably confirmed. Definition of Hominin - The earliest evidence of hominins dates to the end of the Miocene and mainly includes dental and cranial pieces. - Dental remains alone don’t describe the special features of hominins. - Modern humans & our most immediate hominin ancestors, are distinguished from the great apes by more obvious features than tooth and jaw dimensions. - Over the last several million years of hominin evolution each component (dentition, locomotion, brain size, and toolmaking) have developed at different rates. - This pattern in which physiological and behavioural systems evolve at different rates, is called mosaic evolution. - Mosaic evolution is a pattern of evolution in which the rate of evolution in one functional system varies from that in other systems. - The single most important defining characteristic for the entire course of hominin evolution is bipedal locomotion. - In the earliest stages of hominin emergence, skeletal evidence indicating bipedal locomotion is the only truly reliable indicator that these fossils were indeed hominins. But later stages of hominin evolution, features relating to brain development and behaviour become highly significant. 2 AR103 - Readings pgs. 183-200 - Week 8 Notes - Nov. 2, 2011. What’s in a Name? - Hominoid classification has been significantly revised to show more complete relationships, and two further taxonomic levels (subfamily and tribe) have been added. - It further emphasizes the very close evolutionary relationships of humans with African apes and most especially that with the chimpanzees and bonobos. - The term hominid, which has been used for decades to refer to our specific evolutionary lineage, has a quite different meaning in the revised classification, now it refers to all great apes and humans together. The Bipedal Adaptation - Of all living primates efficient bipedalism as the primary form of locomotion is seen only in hominins. - It’s apparent that while still in trees, our ancestors were adapted to a fair amount of upper-body erectness. - All primates spend considerable time sitting erect while feeding, grooming, or sleeping. Presumably our early ancestors also displayed similar behaviour. - Once the hominoids were on the ground and away from immediate safety offered by trees, bipedal locomotion could become a tremendous advantage. - Bipedal locomotion freed the hands for carrying objects and for making and using tools. - Bipedal stance gives a wider view of the surrounding countryside, and in open terrain, early spotting of predators could be of critical importance. - Bipedal walking is an efficient means of covering long distances, and when large game hunting came into play further refinements increasing the efficiency of bipedalism may have been favored. - During normal walking, both feet are simultaneously on the ground only about 25% of the time, and as speed increases, this percentage becomes less. - Maintaining a stable center of balance calls for many drastic structural / anatomical alterations in the basic primate quadrupedal pattern. - The most dramatic changes are seen in the pelvis. Pelvis is composed of 3 elements: 2 hip bones (ossa coxae), joined at the back to the sacrum. - In hominins, the pelvis is comparatively much shorter and broader and extends around the the side. This configuration helps to stabilize the line of weight transmission in a bipedal posture from the lower back to the hip joint. - The foot must act as a stable support instead of a grasping limb. - Our legs became elongated to increase the length of the stride. - An efficient bipedal adaptation required further remodeling of the lower limb to allow full extension of the knee and to keep the legs close together during walking, in this way maintaining the center of support directly under the body. - Hominin bipedalism is both habitual and obligate. - Habitual bipedalism is bipedal locomotion as the form of locomotion shown by hominins most of the time. - Obligate bipedalism is bipedalism as the only form of hominin terrestrial locomotion. Since major anatomical changes in the spine, pelvis, and lower limb are required for bipedal locomotion, once hominins adapted this mode of locomotion, other forms of locomotion on the ground became so inefficient as to not be sustainable. - Around 4 mya the major anatomical changes required for bipedalism occurred. - Fossil evidence of early hominin foot structure has come from two sites in South Africa; especially important are some fossils from Sterkfontein. 3 AR103 - Readings pgs. 183-200 - Week 8 Notes - Nov. 2, 2011. - The earliest hominins were likely habitual bipeds when they were on the ground, but they were not necessarily obligate bipeds. Only after about 4 mya did further adaptations lead to the fully committed form of bipedalism we see in all later hominins, including us. - During hominin evolution, several major structural features throughout the body have been reorganized (from that seen in other primates) to facilitate efficient bipedal locomotion. Beginning with the head and progressing to the foot: - The foramen magnum, is repositioned farther underneath the skull, so that the head is more or less balanced on the spine (and thus requires less robust neck muscles to hold the head upright). - The spine has 2 distinctive curves - a backward (thoracic) one and a forward (lumbar) one - that keep the trunk (& weight) centered above the pelvis. - The pelvis is shaped more in the form of a basin to support internal organs; the ossa coxae (specifically, iliac blades) are also shorter and broader, thus stabilizing weight transmission. - Lower limbs are elongated, as shown by proportional lengths of various body segments (e.g. in humans the thigh comprises 20% of body heigh, while in gorillas it comprises only 11%) - The femur is angled inward, keeping the legs more directly under the body; modified knee anatomy also permits full extension of this joint. - The big toe is enlarged and brought in line with the other toes; a distinctive longitudinal arch also forms, helping absorb shock and adding propulsive spring. Biocultural Evolution: The Human Capacity for Culture - One of the most distinctive features of humans is our elaboration of and dependence on culture. - Culture is nonbiological adaptations to the environment. This includes learned behaviours that can be communicated to others - especially from one generation to the next. Aspects of this capacity have been identified in our closest ape relatives. - Human culture involves more than toolmaking capacity, it integrates an entire adaptive strategy involving cognitive, political, social, and economic components. - When examining the archaeological record of early hominins, material culture (tools) is mostly the only thing available for study. - The
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