Humans walk far more easily than our nearest living relatives. Our upright gait is much better at covering ground than that of chimpanzees for example. It's always been assumed the other apes don't walk as well as us because there is a trade-off between walking and climbing capability, but reconstructions of some of our early ancestors have complicated that theory. There are signs, at least when it comes to hip design, they didn't immediately lose their climbing skills as they got better at walking. The finding throws a question mark over why we, and other apes, haven't retained the best of both worlds.
Hunter College's Dr Herman Pontzer studied the way the force generated at the knee varies for different hip constructions. He found the other great apes are able to produce much more force than humans, which is very useful for scrambling up trees for food or safety. However, the apes' pelvises do not allow the hips to extend beyond 160, when 180 is required to walk upright as humans do.
This appears to back the trade-off theory, but in Proceedings of the National Academy of Sciences, Pontzer and co-authors look at the shape of the hips of the early hominins Ardipithecus ramidus, Australopithecus afarensis, and Australopithecus africanus.
All three extinct species, which lived between 4.4 and 2.1 million years ago and in some cases may have been our direct ancestors, had a human-like capacity to extend their hips, indicating the ability to walk upright as we do had emerged by this point. Intriguingly, however, Ardipithecus was also able to produce similar force to the non-human great apes, suggesting it would have had considerable climbing capacity, as well.
It seems likely then that at this point in our evolution we were both roaming widely on foot, and spending time in the trees. This is consistent with the recent discovery the famous skeleton Australopithecus afarensis, or Lucy as she's more widely known, died from falling out of a tree.
The paper notes we changed in other ways that undermine climbing capacity, such as the loss of a grasping foot and elongated arms. The fossil record is too incomplete to determine the timing of many of these alterations, and the new work raises questions about the simple story that these were all necessary and related.
In particular, there is a question of why we lost our climbing capacity. Is it because some subtler trade-off meant legs needed to change to maximize movement on the ground, or did we simply spend so little time in the trees we lost our ability to go back there without really needing to? The authors hope further digging into the shapes of species living and extinct will answer these questions.