How does the explanation of the origin of the term “alter ego” explain its later use in popular culture and literature?

The meaning of the term is "other I"  in latin an refers to a second part or identity that is contrary to the one that is commonly known.

Explanation:

The phrase "alter ego" comes from latin and means "other I".  It was used in Rome by phiposopher Cicero to describe another part of himself. Later on the term would be used for different psychologists to refer to different personality disorders. Eventually, the term became popular enough to be used by artists creating literature based on that concept , for example,  the tale of Dr. Jeckyll and Mr. Hyde.

in 1897, pershing became a tactical officer at west point, and that's where he was given the nickname "black jack." pershing's first name is "john," for which "jack" is a nickname, and he was a white man who commanded black troops. hence "black jack".