Why cavemen are called Ug

The earliest reference I can find to cavemen being called Og, Ug, Ugg, etc. is from A Child's History of the World by V.M. Hillyer (1924).

"The first people had hair growing, not just on their heads, but all over their bodies, like some shaggy dogs. They simply laid down on the ground when night came. They were bloodthirsty. They liked to drink the warm blood of animals they killed, as you would a glass of milk. They talked to each other with some sort of grunts—umfa umfa—glug glug."

I can't find anything before then on Google Books. Use of Glug or Ug as a name occurs in the 30's and 40's, sometimes in literature for children.