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Lecture 16

BIOLOGY 1M03 Lecture 16: 16- Hominin Diversification, Locomotion and Bipedalism

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McMaster University
Ben Evans

Bio 1M03 February 10, 2016 Hominin Diversification, Locomotion and Bipedalism How Are Humans Different from Apes? - Fully bipedal - Parabolic dental arcade, thick molar enamel, reduced canine teeth, large molar relative to other teeth - Long juvenile period - Large brains relative to body size - Spoken language - Symbolic culture - Ancestral hominins shared some of the features of humans and some features of contemporary chimpanzees Late Miocene/Pliocene Primates - Global cooling was associate with decreased rainfall and seasonality, dry seasons - Tropical rainforests shrank in size - Dry woodland and grassland habitat expanded - Ancestors of humans (hominins) moved into grassland habitat Chimp-Human Split - 5 to 7 million years ago - The shift from four to two legs; anatomical changes were required for the shift, especially in the structure of the skull, spine, legs and feet - The foramen magnum, the hole in the skull through which the spinal cord passes is repositioned so that the human skull balances directly on top of the vertebral column - The human spine is S-shaped so weight is directly over the pelvis - The pelvis is extensively reconfigured for an upright posture, with internal organs over it - Legs are no longer to enable long stride length for efficient locomotion and their anatomy altered so the legs are directly under the body - The foot is narrower and has more developed heel and larger big toe, which contributes to a springier foot - Genetic data suggests that the most recent common ancestor (MRCA) of chimps and humans lived about 5-7 million years ago - 3 fossils have illuminated what this ancestor was like o Ardipithecus ramidus o Orrorin tigenesis o Sahelanthropus tchadensis - These fossils had a mixture of primitive features (thick enamel, ape sized brains) and derived features (shorter arms, upright posture, less prognathism, long femoral neck, foramen magnum under skull) Hominins - 4 to 6 million years ago - Ancestral features o Small molars o Thin enamel o Canines larger than humans o Large brow ridge o Small braincase relative to modern humans - Derived features o Forward location of the foramen magnum; this is a signature of bipedalism o Relatively small canine teeth compared to chimps that are not sharpened by first lower premolar o Changes in the femur and pelvis, knee and ankle o Flattening of the face - Hominin lineage diversified ~4mya and there were 4-7 lineages in Africa for the next 2 million years - Australopithecus: small bipeds with small teeth - Paranthropus: small bipeds with bug teeth, probably ate plants - Kenyanthropus: small teeth and a flat face Pelvic Variation - Morphology of pelvis is influenced by bipedalism - Alignment of femur and tibia is influenced by bipedalism - Laetoli footprints o 3.5 million years old o Illustrates that Australopithecus afarensis was bipedal Theories of Bipedalism - Not clear that bipedal walking is any more efficient locomotion on the ground than Quadra pedal walking - Walking on two legs keeps a hominin cooler - Walking on two legs leaves the arms free to carry objects and forage - Could be a response to increased dependent of offspring - Adaptation for suspensory motion o Short trunk and legs
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