About a third of the human population, including Adele, Jessica Alba, and Pedro Pascal, have what is known as a “hitchhiker’s thumb”. By itself, it has no complications, and doesn’t cause any problems. It is thought to be inherited, but its exact cause is not known. Distal extensibility is a spectrum, and there is debate over what should and should not count as hitchhiker’s thumb. Also, it doesn’t just affect thumbs - any distal finger bone can be hyper-extensible. My most backward-bending are my right thumb and my left pinky. I often use it to pretend I’ve been injured.
Almost every time I say something like “I’ve got five fingers on my hand,” a smart butt inevitably pipes in and says, “Uh, don’t you mean four fingers and a thumb?” Guess what, Kyle? The word “finger” most likely comes from the Proto-Indo-European root word “pinkway,” which means “five”. So, call it what you will, but your central nervous system treats your thumb like a fifth finger.