I learned this once but was not able to look it up again. There is a medical term for the 2 different types of thumbs that humans have. One type is able to be bent back much more than the other, as you can see in the pic on the site below. To such an extent that the tip of the thumb will literally point backwards. Otherwise, "straight" thumbs cannot be bent back like that. http://www.jbhs.k12.nf.ca/biology/THUMBS.HTM Does anyone know the medical/biological term for this?