lecture 23 notes.docx

Introduction to Human Evolution: Friday November 3rd Today: Origins of Bipedalism Origins of bipedalism “Bipedalism is a highly specialized and unusual form of primate locomotion that is found today only in modern humans. The majority of extinct taxa within the Hominini were bipedal, but the d...

Introduction to Human Evolution: Friday November 3rd Today: Origins of Bipedalism Origins of bipedalism “Bipedalism is a highly specialized and unusual form of primate locomotion that is found today only in modern humans. The majority of extinct taxa within the Hominini were bipedal, but the degree to which they were bipedal remains the subject of considerable debate…. evidence point to at least two distinct adaptive shifts. First, there was a shift to habitual bipedalism, as typified by certain members of australopithicines. Such taxa were bipedal, but also retained a number of significant adaptations to arboreal climbing. The second shift was to fully obligate bipedalism, and coincides with the emergence of the genus homo…” Hominin Bipedalism Both habitual and obligate Habitual bipedalism means that hominins, unlike any other primates, move bipedally as their standard and most efficient mode of locomotion Obligate bipedalism means that hominins are committed to bipedalism and cannot locomote efficiently in any other way for the long term Habitual and Obligate Bipedalism Habitual: this form of locomotion is the standard and most efficient in certain contexts Obligate: only form of [terrestrial] locomotion – restricted to this form; adaptations allow for this Adaptation an anatomical, physiological, or behavioral response of organisms or populations to the environment results from evolutionary changes (specifically, as a result of natural selection) Hominins Members of the tribe Hominini, which includes all bipedal hominoids back to the divergence with African great apes Hominoids= members of the primate superfamily (Hominidae) that includes apes and humans Hominins are joined by similar behaviors, emphasizing the biocultural nature of human evolution (biology makes culture possible and that culture influences biology) Primate Bipedalism Primates who adaptations for erect body posture and some species are occasionally bipedal Among living primates, efficient bipedalism as the primary form of locomotion is seen only in hominins Functionally, the human mode of locomotion is most clearly shown in our striding gait, where our weight is alternately placed on a single fully extended hind limb This specialized form of locomotion has developed to a point where energy levels are used to near peak efficiency This isn’t true of nonhuman primates, who move bipedally with hips and knees bent in a much less efficient manner Why Become Bipedeal Perhaps natural selection favored some Miocene hominoids coming occasionally to the ground to forage for food on the forest floor When on the ground and away from the safety of trees, bipedal locomotion had tremendous advantages Freed the hands for carrying objects and for making and using tools In the bipedal stance, animals have a wider view of the surrounding countryside Bipedal walking is an efficient means of covering long distances How? Anatomical Changes Associated with Bipedalism Cranial Foramen magnum Postcranial Spine Pelvis Lower limbs Evidence for Bipedalism Anatomical evidence for bipedalism position of the foramen magnum centered beneath the skull Basin-shaped pelvis Femurs angled inward (Knock-kneed) Big toe in line with the other toes Direct evidence for bipedalism Trail of fossilized footprints in Laetoli, Tanzania, dating to 3.6 mya Evidence of Bipedalism the earliest evidence of hominin bipedalism - 3.6 mya fossil footprints preserved in volcanic ash at Laetoli, Tanzania Apes (Left) are adapted anatomically for quadrupedal locomotion but often stand upright and occasionally walk on their hind limbs in short distances Humans (right) show anatomy, which is adapted to being bipedal Foramen Magnum= ‘‘big opening’’ Shift in position more centered under the skull Spine S-shaped spine in bipeds— helps keep trunk and weight centered above the pelvis Pelvis Bowl shaped pelvis iliac blades - shorter and broader & extend around the side **note the length and breadth (width) of the iliac blade (boxed) and the line of weight transmission Structural and Anatomical Alterations Foot as stable support instead of grasping limbs Elongated legs to increase the length of stride Full extension of knee to maintain center of support directly under the body Lower Limbs Femur angled inward keeping legs more directly under body Lower limbs elongated Modification in the knee allowing full extension enlarged big toe & inline with other toes distinctive arch have to look at the whole picture of these markers to tell if an organism is an obligate biped