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Disclaimer: The following is an 8th grade, second trimester essay on a hypothesis concerning the origins of bipedalism that was submitted to the Dupont Challenge 2000. In the process of reading, please be aware of the following errors contained in the text: (1) “Lynne Isabell” should be Lynne Isbell, (2) “Thomas Young” should be Truman Young, (3) the mentioned temperature shift occurred in the mid-late Miocene, not the “mid-late Pliocene”, (4) the earliest putative hominid ancestor is not Ardipithecus ramidus, but the seven million year old Sahelanthropus tchadensis. Proceed here for a revised version of this essay. The Ascent of Man (original) Have you ever wondered why humans have evolved bipedalism and the vast majority of the Earth’s terrestrial organisms have not? No, it’s not because we’re special. It’s because, in almost every instance, quadrupedalism [four-legged land walking] has proven a more efficient means of locomotion [traveling about] than bipedalism [two-legged walking]. Then why did we evolve this queer habit anyway? The early paleoanthropologists believed the large brain gave rise to tool use, so that it was necessary to stand up in order to free the hands to manufacture and implement stone tools. History proved them wrong. Washburn proposed that bipedalism arose to accommodate hunting behaviors. History proved him wrong as well. (Washburn, 1960). Then, in 1981, Owen Lovejoy associated bipedalism with a shift in reproductive strategies (Lovejoy, 1981). To this day, the Lovejoy Hypothesis remains the most popular. However, it predicts very little sexual dimorphism [difference in height and weight between the two genders] in the early hominids when in fact we observe a lot. As a result, several new hypotheses have been suggested, among them, the Alternative Responses Hypothesis, developed by Lynne Isabell and Thomas Young in 1996. Their paper, published in the Journal of Human Evolution, argued that bipedalism was one of the responses to diminishing food sources caused by the shift in the habitat. (Isabell, Young, 1996). As temperatures plummeted in the mid-late Pliocene, the rainforests thinned and food became scarcer and more widely scattered. Based on the composition of stable carbon at a site in Kenya, scientists have concluded that a mixture of C3 and C4 plants [adapted to cold, wet environments] existed between 12 and 5 million years ago. (Kingston et al., 1994). Also, by 5 million years ago, there was a marked transition to C3 plants adapted to warm, dry climates. (Cerling et al., 1993). Interestingly, this period coincides with the ape-hominid divergence period, calculated by biochemists Vincent Sarich and Allen Wilson to be 5 to 7 million years ago. (Sarich, Wilson).  
Copyright ©2001-2003, Allegra H., all rights reserved. Please contact me via e-mail if you wish to reproduce this material.
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