Evolution of Human Walking

�ked to choose the most distinc­ tive feature of the human spe­ cies, many people would cite our massive brain. Others might men­ tion our ability to make and use so­ phisticated tools. A third feature also sets us apart: our upright mode of locomotion, which is found only in human beings and our immediate an­ cestors. All other primates are basical­ ly 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 elim­ inates our capacity to climb trees, which yield many important primate foods, such as fruits and nuts. For most of this century evolution­ ary theorists have held that human ancestors evolved this strange mode of locomotion because it freed their hands to carry the tools their larger brains enabled them to make. Over the past two decades, however, knowledge of the human fossil record has ex­ panded. Neither a unique brain nor stone tools are in evidence among our earliest known ancestors, the austra­ lopithecines of three million years ago and more. Yet these same ancestors do clearly show many of the hallmarks of bipedal walking. How long had human ancestors been walking upright? Was bipedal­ ity fully developed in the hominids of three million years ago, or did they