The fabella is a small sesamoid bone found in some mammals embedded in the tendon of the lateral head of the gastrocnemius muscle behind the lateral condyle of the femur.

It is an accessory bone, an anatomical variation present in 39% of humans.

[4] Although the fabella seems to have disappeared with the evolution of Hominidae, it reappeared in humans sometime after they diverged from chimpanzees.

It is unknown whether it reappeared soon after this divergence, 5–7 million years ago, or more recently in human evolution.

[5] The re-emergence of the lateral fabella may be correlated with "straight-legged, bipedal locomotion" in humans.