BIPEDALISM Some major changes are associated with bipedalism Human legs are much longer For example human thigh 20 of body height But in Gorilla only 11 Turnbaugh et al 2002 227 Proportional lengths of various body segments One problem with the comparison I just showed you For a long time we thought that the last common ancestor for gorilla chimpanzee bonobo and human was a knucklewalker BUT now we think that ancestor was arboreal AND that knucklewalking among chimp bonobo and gorilla began AFTER humans split away Let s start with the head and move down toward the feet foramen magnum gorilla More centered to balance head on spine Turnbaugh et al 2002 226 230 under head chimp Human Gorilla Smithsonian Institution Whitehead et al 2005 87 The human spine has an S shaped curve which shortened the overall length of the torso and gives rigidity and balance when standing By contrast apes have a relatively straight spine Turnbaugh et al 2002 226 Whitehead et al 2005 184 Pelvis illia shorter broader Pelvis more bowl shaped Red line shows line of weight transmission Box compares length breadth of iliac blade Top Campbell on Talkorigins Bottom Turnbaugh et al 2002 224 Human upper legs angle inward from the hip joints which positioned the knees to better support the body during upright walking The legs of apes on the other hand are positioned almost straight down from the hip so that when an ape walks upright for a short distance its body sways from side to side Campbell on Talkorigins Turnbaugh et al 2002 227 Zihlman 2000 5 15 Knee anatomy permits full extension of joint A afarensis Muscles must also change Turnbaugh et al 2002 224 Zihlman 2000 5 14 Comparison of humerus arm on left to femur thigh on right gorilla human chimpanzee gorilla reversed Human has a relatively longer leg Foot Big toe is no longer divergent less prehensile The shorter less flexible toes provide a rigid lever for pushing off from the ground with each step But doesn t help in climbing trees Human Chimpanzee National Geographic1993 Note outline showing footprint shape Zihlman 2000 5 16 A africanus Campbell on Talkorigins Turnbaugh et al 2002 227 Turnbaugh et al 2002 228 Whitehead et al 2005 192 Laetoli footprint in wet volcanic ash Human long powerful thumb Result great opposability i e you can easily pick up cheerios chimpanzee Ape long powerful fingers weak thumb Result less opposability National Geographic 1993 The first characteristic that sets hominids humans apart from apes is bipedal locomotion Earliest evidence 6 mya thighbones from Tungen Hills Kenya eastern Africa 4 4 mya 45 of skeleton from Ethiopia eastern Africa 3 7 mya footprints in volcanic ash at Laetoli Tanzania eastern Africa 3 2 mya pelvic thigh shin and foot bones from various specimens including LUCY Lucy the nickname for a specimen of Australopithecus afarensis was the first fossil discovered with much of the skeleton intact Why Bipedalism https www toonpool com cartoons bipedalism 20sounds 20good 20fall 20down 99428 Environmental Factors Formation of the Great Rift Valley in East Africa 8 mya East of valley became savannas and woodlands Early hominids isolated from early great apes Models for Evolution of Bipedalism 1 Carrying Model 2 Vigilance Model 3 Heat Dissipation Model 4 Energy Efficiency Model 5 Foraging Bipedal Harvesting Model 6 Display Model 7 Complex Topography Model 1 Carrying Model Bipedalism frees arms from role in walking allowing items to be carried Enables Food to be carried for longer distances and with greater efficiency An infant to be carried Weapons such as stones or sticks to be carried to scare predators 2 Vigilance Model Bipedalism allows for the head to be elevated An elevated head may have allowed early hominids to better locate predators and food sources In a savanna environment bipedalism would allow hominids to see over tall grass 3 Heat Dissipation Model Thermoregulation bipedalism allows the body to cool more efficiently In a hot savanna environment standing upright Exposes less of the body s surface area to the sun Allows heat to escape Allows exposure to cooling breezes Heat Dissipation Model 4 Energy Efficient Model Bipedalism is an energy efficient form of walking Walking long distances to acquire food would require less energy when bipedal Energy Efficient Model However Fossils indicate that early hominids walked differently and less efficiently than modern humans So efficient walking was probably not a major factor in the evolution of bipedalism 5 Foraging Bipedal Harvesting Model Bipedalsim allowed early hominids to reach food in otherwise inaccessible places 6 Display Model Bipedalism allowed males to better compete with one another for dominance Chimpanzees stand upright to display dominance and rise in the dominance hierarchy A higher position on the dominance hierarchy leads to greater access to mates This is the only model that links bipedalism to reproductive success 7 Complex Topography Model This is the most recently proposed model Instead of the lessening of trees in the changing environment it focuses on the development of cliffs and gorges Early humans would have been attracted to rugged landscape that had developed offered shelter opportunity to trap prey But need to be able to scramble around bipedally in rough landscape Later adaptation to good running ability tied to excursions out onto the flat plains for food and to seek new home ranges Which model is the best Criticism and support for each model Most likely all factors played a part in the evolution of bipedalism this Wednesday MOVIE lecture plus movie Lucy in Disguise Print and bring your question sheet to class
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