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Humans are unique among primates because our chief mode of locomotion is walking fully upright . This way of moving frees our hands up for using tools. Unfortunately, the changes made in our pelvis to help us move on two legs, in combination with babies with large brains, makes human childbirth unusually dangerous compared with the rest of the animal kingdom. Unlike other primates, humans have a lumbar curve in the lower back, which helps us maintain our balance as we stand and walk, but it also leaves us vulnerable to lower back pain and strain.