ANTH 3A Ch. 10 - Unit 3A of CSN Physical Anthropology 102 Chapter 10 Notes; Dr. Levin - Our Origins PDF

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Unit 3A of CSN Physical Anthropology 102 Chapter 10 Notes; Dr. Levin...

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Ch. 10 Early Hominin Origins and Evolution - The Roots of Humanity “Cradle of humankind,” Olduvai Gorge–a ravine in East Africa’s Great Rift Valley––with many hominin fossils recovered Big Questions: 1) What is a hominin? 2) Why did hominins evolve from an apelike primate? 3) What were the first hominins? 4) What was the evolutionary fate of the first hominins? Mary Leakey and Louis Leakey: found ancient stone tools and ancient animal remains scattered about the landscapes - Important questions they imposed: - Who were the first humans? - They worked hard for little payoff because of these questions - The Leakeys decided to investigate the gorge’s geologic strata, laying the essential groundwork for our current understanding of the first humans and their place in the evolution - Their research added a whole new dimension to their variability and geographic distribution Geologic Strata at Olduvai: one key aspect of excavations at Olduvai is the exposed strata, including volcanic rock (can be radiometrically dated). These fossils can be dated according to the stratum in which they were found. The ages of fossil hominins recovered from Olduvai George help anthropologists reconstruct humans’ family tree. In order of origin and evolution, they are the pre-australopithecines (before the genus Australopithecus), which lived 7–4 mya, and the australopithecines, which lived 4–1 mya. 10.1 What Is a Hominin? What is distinctive about hominins: the morphological characteristics and the behaviors inferred from these characteristics - E.g., living humans speak, use language, depend fully on complex material culture, and have advanced cognition in lieu of complex material culture which apes do not have. - Speech, advanced cognition, and complex material culture evolved in the human line long after the first hominins appeared in Africa, 7-6 mya, so these characteristics do not define a hominin. A hominin is much better understood as having two obligate behaviors: bipedal locomotion and nonhoning chewing, and the suite of associated physical characteristics that manifest these behaviors - The evidence is very clear: bipedal locomotion and nonhoning chewing preceded speech and material culture by several million years. Like large brains, speech and material culture help define humans today but were not attributes of the earliest hominins. Human have and apes lack: bipedalism, nonhoning chewing, dependence on material culture, speech, hunting and cooperation, and domestication of plants and animals Bipedal Locomotion: Getting Around on Two Feet Scientists originally thought that the most evolutionary change was an increase in brain size,

reflecting advanced human intelligence. - They have identified bipedalism to be the foundational behavior of the Hominini (humans and human-like ancestors) - The shift from walking with and running with four limbs (arms and legs) to walking with and running with two limbs (legs) distinguishes hominins from the apes (and other nonhuman primates). Seven distinguishing characteristics in the skeleton are associated with bipedalism: 1) The foramen magnum is positioned on the bottom of the skull 2) The spine is S-shaped 3) The ilium is short from front to back 4) The legs are long relative to the body trunk and arms 5) The knees are angled inward 6) The foot has a longitudinal arch 7) The big toe (hallux) is not opposable Nonhoning Chewing: No Slicing, Mainly Grinding Another difference between the two primates is the way their dentitions process food Apes and humans have evolved different dental characteristics, reflecting how each uses the canine and postcanine teeth. - When apes grab on to food with their front teeth, the upper canines and lower third premolars cut and shred the food. Through evolution, apes’ upper canines have become large, pointed, and projects, with a sharp edge on the back. When the jaws are fully closed, each canine fits snugly in the diastema, the gap located between the canine and the third premolar on the lower jaw and the canine and the second incisor on the upper jaw. The sharp edge on the back of the upper canine hones, or rubs against, a sharp edge on the front of the lower third premolar, or sectorial premolar. This honing action helps maintain a sharp, shearing edge on both the canine and the premolar. - Humans’ nonhoning chewing complex is characterized by canines that are much shorter than those found in nonhuman primates’ honing complex, such as gorillas. Humans lack the diastema that nonhuman primates such as gorillas have. Living and past hominins have small, blunt, and nonprojecting canines and no diastema. Unlike apes, hominins do not hone their canines as they chew. Apes and humans postcanine teeth: similar; the third and fourth premolars, upper and lower, have two cusps each. Apes and hominins upper molars have four cusps, and their lower molars have five cusps. - Apes and humans back teeth crush and slice food: humans crush food more than apes do. - Apes use their molars more for slicing than crushing, reflecting their plant-heavy diet. Science is a powerful way of distinguishing between what is true and what is not true in the natural world. Eoanthropus dawsoni: (meaning “Dawson’s dawn man”), a fossil found of a skull unearthed by Charles Dawson and Sir Arthur Woodward. The Piltdown fossil was purported to be the “missing link” between apes and humans; long after the discovery of the Piltdown fossil, the earliest hominin fossil in South Africa, the Taung child, was discovered. However, this fossil was deemed fake in lieu of modifications of the skull as

well as being purposely placed in the gravel pit at Piltdown. Au. Africanus was the human ancestor; Dart was correct In apes and humans, grinding and slicing are facilitated by powerful chewing, or masticatory, muscles, especially the temporalis, masseter, and pterygoid muscles Hominins place more emphasis on the front portion of these muscles, to provide greater vertical force in crushing food. Apes place more emphasis on the back portion of the masticatory muscles because slicing requires more horizontally oriented forces. Homininae is located in the anatomical complexes associated with acquiring and transporting food (locomotion) and chewing food (mastication). 10.2 Why Did Hominins Emerge? The central point to most arguments on why hominins have evolved is bipedalism, the focal point in the study of human origins. Charles Darwin’s Hunting Hypothesis Darwin: offered the first serious hypothesis about the first appearance of hominins - Conclusion: because of the remarkable anatomical similarity between humans and African apes, Africa was hominins’ likely place of origin - One key evolutionary event in their common ancestor: the shift from living in the trees to life on the ground. - Four characteristics that set living humans and living apes apart: 1) Humans are bipedal, while apes are quadrupedal 2) Humans have tiny canines, while apes have large canines 3) Humans rely on tools in their adaptation, while apes do not 4) Humans have big brains, while apes have small brains Thomas Huxley: studied the anatomical features of primates. He studied apes’ and humans’ anatomies. The major differences between apes’and humans’ skeletons: limb proportions, pelvic shape, and spine curvature Darwin believed that humans’ large brain resulted mainly from the presence of language in humans - Scientists are aware that tool use and the increase in brain size began well after the appearance of bipedalism and the reduction in canine size. - The earliest tools date about 3.3 mya and evidence of brain expansion dates to 2 mya - It is doubtful that canine reduction began with tool use Hypotheses have emerged to answer the question of why there are hominins - Starting point: after 17 mya, a massive adaptive diversification of apes occurred in Africa, resulting in many different taxa; diversity declined in lieu of competition between apes and to the rising number of monkey species that were also evolving in the late Miocene epoch; changes in climate were also a factor in the declination of ape taxa - Darwin proposed that hunting was at the basis of the divergence. Hunting did not begin, however, until human evolution, 2 mya, during the same time the brain began to expand Peter Rodman and Henry McHenry’s Patchy Forest Hypothesis These two gentlemen have proposed that human origins and bipedalism, in particular, may be related to the greater efficiency, in certain habits, of walking on two feet rather than four feet. - Suggested: bipedalism arose in areas where the forest was becoming fragmented, a

process that began toward the end of the Miocene - It all depended on the resources Owen Lovejoy’s Provisioning Hypothesis Lovejoy provided an alternative to Darwin’s ideas about the arboreal-to-terrestrial shift and the origins of bipedalism. - Hypothesis: freeing the early hominins’ hands was important in initiating bipedal locomotion but not for the reasons Darwin cited. Lovejoy observes that in many species of monkeys and apes, males compete for sexual access to females. - The young are cared for by the mother without involvement of the father. The mother is not able to care for more than one infant at a time; also, she is unreceptive to mating until the child can find food on its own. - Lovejoy hypothesizes that if infants and mothers were provided with more food, they would not have to move around as much for resources. - Lovejoy favored the idea that they were monogamous, that the father enhanced the survival of the mother and offspring by providing both food and protection from predators; this habitual provisioning required the male to have free hands for carrying food, so bipedalism arose. Sexual Dimorphism and Human Behavior With the study of body size, many living primate species are highly dimorphic sexually: males are considerably larger than females. - The larger the male, the more equipped they are to outcompete other males for sexual access to females. - Reduced sexual dimorphism in body size suggests that males were cooperative, not competitive, meaning they were led to be monogamous which supports Lovejoy’s hypothesis. Bipedalism Had Its Benefits and Costs: An Evolutionary Trade-Off All the hypotheses about human origins have suggested that an apelike primate evolved into an early hominin through complete positive adaptation. - Bipedalism dominance: ability to see greater distances because of upright posture, transportation of food and children, running, and free hands for skill as well as activities for tool use. - Costs of bipedalism: upright standing causes greater exposure to predators, back injuries from standing too much, a burden on the circulatory system as it moves blood from the legs to the heart (varicose veins; bulged veins), if one of the feet are injured, the ability to walk is reduced causing limited chances of surviving and reproducing. 10.3 What Were the First Hominins? The oldest hominin fossil dated to less than 4 mya. The earliest hominins were known from one genus, Australopithecus, found in two areas of Africa: limestone caves in South Africa and in sedimentary basis and associated river drainages in the Eastern Rift Valley in Ethiopia, Kenya, and Tanzania. The first group is identified as about 8 mya. - The crucial time period during which hominins and the last common ancestor with apes (chimpanzees) split into separate lineages had been an unknown because of the 4-

million-year gap in the fossil record (8–4 mya). The Pre-Australopithecines - These fossils are few in number and quite fragmentary but provide information on the evolution of the Hominini. Sahelanthropus tchadensis (7-6 Mya) - central Africa, a close relation to apes. They bridge the gap between other hominin ancestors and apes. Cranial capacity is one important quantitative characteristic with which anthropologists determine the degree of humanness in individual fossil hominins. - Increase in brain size of about 350 cc - Massive brow ridge, larger than modern gorillas. - The primate was most likely bipedal in lieu of: the position of the foramen magnum at the base of the skull and the canine-premolar chewing complex was nonhoning. - All found in the same site: fish, crocodiles, amphibious mammals associated with aquatic settings, hoofed mammals, horses, elephants, primates, and rodents associated with forests and grasslands. Orrorin Tugenesis (6 mya) - “original man” Bipedal beings; found partial femurs; spent time in trees in lieu of hand phalanx which was curved like a living ape’s; the canines had worn on the tips and were nonhoning; they lived in the forest - Their most important skeletal remain is a proximal, or upper, portion of the femur, which has a long femoral neck and a groove for the obturator externus muscle. Ardipithecus kadabba and Ardipithecus ramidus (5.8-4.4 mya)...