If you rest the back of your wrist on a table and connect your thumb to your pinky, you may see a band of muscle pop up on your wrist. That is a vestigial muscle called the palmaris longus. Amir said this muscle is there because it helped our ancestors climb trees. According to the Encyclopædia Britannica, the muscle likely also helped early humans with their grip. But we started walking on two feet about 3.2 million years ago, making the muscle useless… More here.