Walking on two legs, or bipedalism, immediately sets us apart form other apes. It frees our arms for using tools and weapons and is a key part of our evolutionary success. Scientists have put forward a few theories to explain how our upright gait evolved, but the ‘savannah theory‘ is by far the most prolific.
It’s nicely illustrated by this misleading image that has become a mainstay of popular culture. It suggests that our ancestors went from four legs to two via the four-legged knuckle-walking gait of gorillas and chimps. Dwindling forests eventually pushed them from knuckle-walking to a full upright posture. This stance is more efficient over long distances and allowed our ancestors to travel across open savannahs.
But this theory fails in the light of new fossils which push back the first appearance of bipedalism to a time before the forests thinned, and even before our ancestors split from those of chimpanzees. Very early hominins, including Lucy (Australopithecus afarensis) and Millennium Man (Orrorin) certainly ambled along on two legs, but they did so through woodland not plains.
Our arms provide a further clue. Even though our ancestors’ back legs quickly picked up adaptations for bipedalism, they steadfastly kept long, grasping arms, an adaptation more suited to moving through branches. To Susannah Thorpe at the University of Birmingham, these are signs that bipedalism evolved while our ancestors were still living in trees. And she has found new support for this theory by studying Sumatra’s orang-utans, who occasionally take to two legs to negotiate tricky canopies.