Reversion to type, also known as ATAVISM (7 letters), is a phenomenon in which an organism reverts to ancestral traits that had been lost in its evolutionary history. It is often seen in animals, where a species may suddenly display characteristics of its ancestors, such as a snake growing legs or a whale growing teeth. In humans, it can manifest itself in the form of physical traits, such as a person having a tail, or in behavior, such as a person exhibiting primitive behaviors.