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Human Evolution and PREHISTORY Chapter Six: THE FIRST BIPEDS Link to the Canadian Association for Physical Anthropology COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Chapter Preview What is the Anatomy of Bipedalism and How is it Preserved in the Fossil Record? Who Were the Australopithecines and What Were They Like? Why Had Australopithecus Become A Bipedal Walker? COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. THE ANATOMY OF BIPEDALISM Shared derived characteristics distinguishing hominins from the other African apes Position of foramen magnum is more forward S-shaped spinal column Wide and foreshortened pelvis “kneeing-in” of thigh bones (femora) Stable arched foot and absent opposable big toe Shorter toes COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. THE ANATOMY OF BIPEDALISM COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. THE ANATOMY OF BIPEDALISM COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Australopithecus a well-known hominin that lived between 4.2 and 1 mya in East and South Africa Bipedal when on the ground Apelike brain COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Australopithecus COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Early hominins COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Gracile Australopithecines 3.9 to 2 mya in East Africa 3.5 to 2.3 mya in South Africa Erect bipeds, about 1-1.5 m. in stature Apelike skull morphology Teeth for chewing food in a hominin fashion COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Gracile Australopithecines Earlier fossils show dental features similar to some late Miocene apes; later South African fossils do not Sexually dimorphic, in body size and canine tooth size COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Gracile Australopithecines Foramen magnum is forward and downwardlooking, as in genus Homo Brain is about a third of a modern human brain in size, and three times larger than Miocene apes Endocasts suggest that the human cerebral reorganization has not yet occurred Blood drainage system for the brain is significantly different from genus Homo COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Australopithecines Two indisputable facts: 1. Retention of some form of adaptation to arboreal life 2. Evolution of erect bipedal position long before acquiring highly enlarged brain COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Robust Australopithecines South Africa 1.8 to 1 million years ago Thick bones for their size, with prominent muscle markings Sagittal crest, for huge temporal muscles (more evident in males) – an example of convergent evolution in gorillas and hominins COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Robust Australopithecines East Africa 2.5 to 1.3 million years ago More massive skull and larger body size than South African relatives Enormous molars, premolars, mandible and palate COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Gracile-Robust Relationship East African robust forms likely evolved from earlier gracile forms in east Africa In South Africa the robusts were either an offshoot of the East African lineage, or convergent evolution from a South African ancestor COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Robust-Homo Relationship Robust australopithecines had evolved into highly efficient and specialized consumers of plant food Many anthropologists believe this allowed early Homo and robust australopithecines to co-exist for 1.5 million years because they avoided competition for the same ecological niche ** law of competitive exclusion COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. AUSTRALOPITHECINE PREDECESSORS Sahelanthropus tchadensis (Chad) 7-6 mya Small canines and humanlike face Probably bipedal Recent 3D reconstruction confirms it is more closely related to hominins Likely close to common ancestor COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. AUSTRALOPITHECINE PREDECESSORS Orrorin tugenensis (east Africa) 6 mya 13 fragments of lower jaw, teeth, thigh bones Molars are thickly enameled like Australopithecines, but smaller Suggestion of bipedalism Uncertain evolutionary relationship COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. AUSTRALOPITHECINE PREDECESSORS Ardipithecus (east Africa) 5.8 to 4.4 mya Sp. kadabba and ramidus Likely bipedal when on the ground Mixture of ape and hominin dental features Forested environment COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. AUSTRALOPITHECINE PREDECESSORS Kenyanthropus platyops (east Africa) Contemporary with early east African australopithecines Maeve Leakey sees her fossil as ancestral to the genus Homo COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. AUSTRALOPITHECINE PREDECESSORS Relationship to Hominins Current evidence indicates that hominins evolved from late Miocene apes, becoming distinct about 5 mya More than one bipedal model emerged from this new primate niche; one of them was Australopithecus Are any of these predecessors ancestral to the australopithecines or to chimpanzees, or did they become extinct? Pattern in early hominin evolution has been short periods of change, marked by prolonged periods of stasis COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. ENVIRONMENT, DIET, AND AUSTRALOPITHECINE ORIGINS Major climatic changes in the late Miocene: Drying up of the Mediterranean Sea Breaking up of forests Creation of a mosaic environment with more open areas, interspersed with forest patches COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Effect of Climatic Change 1. Change in diet - less “tree” food - more open ground foraging, e.g. seeds, grasses, roots 2. Change in dentition - smaller canine teeth - male canines became as small as those of females COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Australopithecines and Tools No evidence of toolmaking clearly associated with Australopithecines, although hands of later Australopithecines were suitable Could Australopithecus have been a tool-user and maker of simple tools such as bonobos and chimpanzees today? They could have used wooden tools, convenient stones, animal bones COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. HUMAN BIPEDALISM Disadvantages More visible to predators Exposes soft underbelly Interferes with ability to change direction instantly while running Not a fast locomotion method Frequent lower back problems and circulatory problems Serious impediment if one leg is injured ** all of these disadvantages placed our early hominin ancestors at risk COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. HUMAN BIPEDALISM How did bipedalism become a viable adaptation? Possible selective pressures: 1. Males gather and transport food to females, who were restricted by dependency of offspring 2. Nonterritorial, far-ranging scavenging, because a biped is able to travel long distances without tiring 3. To cope with heat stress out in the open COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. ORIGINAL STUDY The Naked and the Bipedal Upright stance greatly reduces the amount of the body’s surface area that is directly exposed to the sun and increases the amount exposed to the cooler breezes a few feet above the ground Hair loss helps to lose more body heat The “naked biped” becomes an adaptation to the heat of the savanna by keeping the brain cool and allowing for its expansion COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. HUMAN BIPEDALISM 4. Bipedalism is far more economical than quadrupedal locomotion at walking speed The causes of bipedalism are likely to be multiple e.g. food transport AND carrying infants AND reaching for food AND seeing predators AND using hands as protection COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. NEXT TIME: Homo habilis and Cultural Origins COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED.