If you have ever tried to knuckle-walk on all fours like a gorilla, you'd soon realize that it's difficult to do. But some apes do this with ease, partly because they have specialized wrist joints, which we don't have.
Australopithecus afarensis, also known as 'Lucy', is an ape-like creature that some claim is our ancestor. Evolutionary books show pictures of Lucy walking upright, so you would not expect her to have knuckle-walking wrist joints. But recently, scientists were shocked to discover this wrist joint structure in Lucy's kind. According to one of the researchers: 'I walked over to the cabinet, pulled out Lucy, and—shazam!—she had the morphology that was classic for knuckle walkers.'
If 'Lucy' really is our ancestor who walked upright, why does her wrist anatomy suggest she walked on all fours? It just does not add up!