Flexible feet help apes to move around fast providing good grip on the branches. Scientists are now surprised how most humans ended up with rigid feet. Apes spend a lot of their time in trees, so their flexible feet are essential to grip branches and allow them to move around quickly.
Scientists found that when apes lift their heels off the ground their feet are floppy with nothing holding their bones together. This is known as a midtarsal break, and surprisingly was found in some of the participants by the Boston team of scientists.
Many of the participants with flexible feet were not aware of it.
The American Journal of Physical Anthropology has published this research.