Origin Story: A Big History of Everything

by David Christian

Even the earliest hominin species seem to have walked on two legs, at least some of the time. This is very different from chimps and gorillas, which knuckle-walk. You can tell from bones if a species regularly walks on two legs. In bipedal species, the big toe is no longer used for gripping, so it aligns more closely with the other toes, while the spine enters the skull from below, not from the back (get down on all fours and you’ll understand why). Walking on two legs required rearrangements of the back, the hips, even the braincase. It also favored narrower hips, which made childbearing more difficult and dangerous and probably means that many hominins, like modern humans, gave birth to infants that were not yet capable of surviving on their own. That would have meant that their babies needed more parenting, which may have encouraged sociability and gotten hominin fathers more involved in child-rearing. There were many indirect effects of bipedalism, but we’re not yet sure exactly why hominins became bipedal. Perhaps bipedalism let our ancestors walk or run farther in the grassy savanna lands that had spread around a cooling world in the past thirty million years. It also freed human hands to specialize in manipulative tasks including, eventually, the making of tools.