by 
kuyakev
ANSWERS: 2
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Because we have not yet completed total evolution from apes to humans. By theory of evolution we are descendents of ape species and thus had our spines curved initially. As time progressed we straightened up and started walking (Homo erectus...). Next stage was what is today's modern man Homo sapiens (wise man). However we have yet to give up our vestigial tail (in some cases) and to get our spines fully erect.
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The spine is curved to allow us to maintain a proper centre of gravity and weight transference while walking on two legs, with the least amount of physical effort required (ie the least amount of muscle activity involved to keep us upright). If the spine was not curved, the weight would all be transferred to the posterior of the pelvis, and we would be likely to fall backwards.
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