The panda's thumb is not a thumb but a bone grown inside a tendon
Sesamoid bones sit embedded in tendons, named for their resemblance to sesame seeds. Your kneecap is the largest. In giant and red pandas an enlarged sesamoid in the wrist became a false thumb for gripping bamboo, and the two species evolved it independently, one of them for climbing first.
These bones often form in response to strain. They act as pulleys, giving a tendon a smooth surface to glide over and increasing the force it can transmit. Beyond the kneecap, humans usually have two at the base of the thumb and two beneath the big toe, and the pisiform in the wrist is another, beginning to harden between ages 9 and 12. The fabella, tucked behind the knee in a calf muscle tendon, turns up in 10% to 30% of people.
Being tucked away has drawbacks. Sesamoids generally get a poor blood supply, which makes them vulnerable to avascular necrosis, bone death from lack of blood, a condition that is hard to treat. In the hand it is so rare that a review covering publications from 1990 to 2024 found just two documented cases, both eventually needing the bone removed. Dancers frequently develop sesamoiditis, an inflammation of the tendons around the pair beneath the big toe. Radiologists must also tell harmless extra sesamoids from bone fragments pulled off in injuries; smooth edges and an oval shape point to the former.
The panda story shows evolution reusing parts. Many carnivores have a radial sesamoid, but the red panda probably enlarged its version to help it move through trees, and only later, after switching to bamboo, repurposed it for grasping. The giant panda developed the enlargement at about the same time as its bamboo diet, and it works like one side of a pincer. Researchers have pointed to the genes DYNC2H1 and PCNT as possible drivers. Cotton rats have a strikingly similar bone, perhaps for gripping thin branches.
Elephants take the idea further, with enlarged sesamoids in all four limbs that behave like sixth toes and help spread their weight. Most elephant sesamoids ossify between three and seven years of age, but these extra digits remain unossified even in animals older than 20. Moles have a sickle-shaped version on their digging forepaws, and horses have a pair at the back of each fetlock.
Source: Sesamoid bone
