Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Why Bipedalism?Environmental FactorsModels for Evolution of Bipedalism1. Carrying ModelSlide 242. Vigilance Model3. Heat Dissipation ModelHeat Dissipation Model4. Energy Efficient ModelEnergy Efficient Model5. Foraging/Bipedal Harvesting Model6. Display ModelSlide 32Slide 33Which model is the best?Slide 35BIPEDALISMSome major changes are associated with bipedalismProportional lengths of various body segmentsHuman legs aremuch longerFor example,human thigh = 20%of body heightBut in Gorilla only11%(Turnbaugh et al. 2002:227)One problem with the comparison I justshowed you:For a long time we thought that the lastcommon ancestor for gorilla, chimpanzee/bonobo,and human was a knucklewalkerBUT now we think that ancestor was arborealAND that knucklewalking among chimp/bonoboand gorilla began AFTER humans split awayLet’s start with the head and move down toward the feet(Turnbaugh et al. 2002:226,230)gorillachimpforamenmagnumMore centered to balance head on spineunder head(Whitehead et al. 2005:87)(Smithsonian Institution)GorillaHuman(Turnbaugh et al. 2002:226; Whitehead et al. 2005:184))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.Pelvis: illia shorter & broader. Pelvis more bowl-shapedRed 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 thehip joints, which positioned the knees tobetter support the body during uprightwalking. The legs of apes, on the other hand, are positioned almoststraight 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)A. afarensisKnee anatomy permits full extension of joint(Turnbaugh et al. 2002:224)Muscles must also change(Zihlman 2000:5-14)Comparison of humerus (arm) on left to femur (thigh) on right gorilla human chimpanzeeHuman has a relatively longer leggorilla reversedFoot:Big toe is nolonger divergent;less prehensileThe shorter &less flexibletoes provide arigid lever forpushing offfrom theground witheach stepBut doesn’t helpin climbing treesChimpanzeeHuman(National Geographic1993)(Zihlman 2000:5-16)Note outlineshowingfootprint shapeA. africanus(Campbell on Talkorigins; Turnbaugh et al. 2002:227)(Turnbaugh et al. 2002:228)(Whitehead et al. 2005:192)Laetolifootprintin wetvolcanic ashHuman: long, powerful thumbResult: great opposabilityi.e., you can easily pick up cheerios!!!Ape: long, powerful fingers, weak thumbResult: less opposabilitychimpanzee(National Geographic 1993)The first characteristic that sets hominids (humans)apart from apes is bipedal locomotionEarliest 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 LUCYLucy, the nickname for a specimen of Australopithecus afarensis, was the first fossil discovered with much of the skeleton intactWhy Bipedalism?https://www.toonpool.com/cartoons/bipedalism%20sounds%20good%20fall%20down_99428Environmental FactorsFormation of the Great Rift Valley in East Africa- 8 mya–East of valley became savannas and woodlands–Early hominids isolated from early great apesModels for Evolution of Bipedalism1. Carrying Model2. Vigilance Model3. Heat Dissipation Model4. Energy Efficiency Model5. Foraging/Bipedal Harvesting Model6. Display Model7. Complex Topography Model1. 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 carriedWeapons, 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 ModelThermoregulation: 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 breezesHeat Dissipation Model4. 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 ModelHowever:•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 bipedalism5. Foraging/Bipedal Harvesting ModelBipedalsim allowed early hominids to reach food in otherwise inaccessible places.6. Display ModelBipedalism 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 ModelThis is the most recently proposed model:Instead of the lessening of trees in thechanging environment, it focuses on thedevelopment of cliffs and gorgesEarly humans would have been attracted torugged landscape that had developed:-offered shelter- opportunity to trap preyBut need to be able to scramble aroundbipedally in rough landscapeLater adaptation to good running abilitytied to excursions out onto the flat plainsfor food and to seek new home rangesWhich model is the best?•Criticism and support for each model•Most likely all factors played a part in the evolution of bipedalismthis Wednesday: MOVIE lecture plus movie: Lucy in DisguisePrint and bring your question sheet to
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