Are you scared of the dark? It's okay. That is quite a natural atavistic fear — that is, a fear related to an ancient way of thinking or behaving.
Something atavistic doesn't have to be a feeling; it can be anything that's a throwback to an earlier form of life or way of looking at things. For example, a new building can be atavistic if it contains strong elements of ancient architectural styles. A physical genealogical trait handed down from many generations is also atavistic. For example, the pointed canine teeth in humans, originally used to tear meat apart when hunting, is an atavistic trait in mankind.