It's in the Irish gene pool, and appears in some illustrations like from the Book of Kells. Some people have a sixth digit, often placed lower and slightly offset, like a smaller thumb. It is extremely useful in not dropping things like glasses.

Also, some people use a prosthetic extra thumb called a shucking peg. They come in many styles and can be worn in different directions, sometimes close to the thumb or on the palm, but other people like them pointed them other way.