There is no definitive answer, but its structure suggests a key role in the strong grip for hanging and power grasping.
The middle finger’s length is one aspect of hand evolution that people have in common with apes. For many primates, the middle finger is the functional axis of the hand and the longest finger.
All the muscles of the hand are arranged around an axis through the middle finger, so it is in position to exert the greatest amount of force, and the middle finger bones are the longest and most robust of any finger.
All this suggests that this part of the hand blueprint came from apelike ancestors 5 million to 6 million years ago. Apes all have a powerful grasp, with well-developed fingers and a tiny thumb, unlike the long and very maneuverable human thumb.
The middle finger’s bony anatomy suggests it is important in power grasping, while the thumb and index finger are for precision grasping, like holding a needle or stone tool.
The third finger would be more devoted to the hook grip, used in hanging from a branch or, for modern humans, holding a heavy suitcase.