Jeremy DeSilva writes:

Long before our ancestors evolved large brains and language, even before they tamed fire or made stone tools, they started doing something no mammal had done before: walking on two legs. Skeletal adaptations for traveling upright are evident in fossils of the very oldest hominins—members of the human family—which date to between seven million and five million years ago. Moving on two legs rather than four set the stage for subsequent evolutionary changes in our lineage. It allowed our predecessors to expand their home ranges and diversify their diets, and it transformed the way we give birth and parent our children. This peculiar mode of locomotion was foundational to virtually all the other characteristics that make humans unique.

In the iconic representation of human evolution, a procession of ancestors starting with a chimplike creature ambling on all fours gives way to a series of ever more erect forebears, culminating in a fully upright Homo sapiens striding triumphantly on two legs. First popularized in the 1960s, the March of Progress, as this image and its variants are known, has decorated countless books, T-shirts, bumper stickers and coffee mugs.

But paleoanthropological discoveries made over the past two decades are forcing scientists to redraw this traditional, linear imagery. We now know that various hominin species living in different environments throughout Africa, sometimes contemporaneously, evolved different ways to walk on two legs. The emergence of bipedalism kicked off a long phase of rampant evolutionary riffing on this form of locomotion. Our modern stride was not predetermined, with each successive ancestor marching closer to a particular end goal (evolution has no plans, after all). Rather it’s one of many forms of upright walking that early hominins tried out—and the version that ultimately prevailed. [Continue reading…]