ABSTRACT
Asked to choose the most distinctive feature of thehuman species, many people would cite our massive brain. Others might mention our ability to make and use sophisticated tools. A third feature also sets us apart: our upright mode of locomotion, which is found only in human beings and our immediate ancestors. All other primates are basically quadrupedal, and with good reason: walking on two limbs instead of four has many drawbacks. It deprives us of speed and agility and all but eliminates our capacity to climb trees, which yield many important primate foods, such as fruits and nuts.
