One sort of fiction that skirts the periphery of sword and sorcery fiction is the cave-man tale. Robert E. Howard’s first story in Weird Tales (“Spear and Fang”) was stone age yarn.
P. Schuyler Miller’s “The People of the Arrow” (Amazing Stories, July 1935) is an interesting prehistoric story of genocidal warfare. Miller (1912-1974) was one of the great science fiction writers of the 1930s into the early 40s. He later became Astounding Science Fiction/Analog magazine’s main book reviewer.
His earliest stories have a palpable A. Merritt influence but like Jack Williamson, Miller moved on.
Amazing Stories in the 1930s, the era that T. O’Conor Sloane edited the magazine had a habit of running stories years after acceptance. Miller probably wrote “The People of the Arrow” in the early 1930s.
The story concerns a tribe of prehistoric men, the Arrow-People who migrate north along the Rhone river valley as the glaciers are melting. Their leader is Kor, the Wolf Slayer, son of Kor. Miller describes him as:
“A magnificent bronzed figure, his broad chest streaked with bands of graphite and ochre, the pelt of the white wolf twisted about his muscular waist.”
About two thirds of the story is about the migration. Kor and a hunting band return to find much of their tribe dead by extreme violence. Tracks show splay feet and captured women of the Arrow-People.
The men follow the trail finding the lair of the beast men. They are described as thus:
“They were smaller than a man. Their massive legs were bent and crooked, their backs warped until their great blunt paws hung far below their knees. 
Little red eyes peered under protruding brow; thick, sucking lips slavered and spewed out clucking speech. Man stood facing hairy, grizzled beast, across a hundred feet of rock, as at Kor’s wild scream a score of bowstrings sang.”
What follows is extermination of the beast men. Miller is not specific but the beast men are probably Neanderthals. Miller had an interest in archaeology and has some nice points like hand-axes being used.
There is no dialogue in this story. You see this in the pulps with the big epics compressed into a short story. The stories are more of an outline for a novel. There was an early writer of space opera, J. Schlossel who had stories like this (“Invaders From Outside”) in Weird Tales and Amazing Stories in the 1920s.
Lin Carter had two stories in the pages of Fantastic in the 1970s about the People of the Dragon which read very similar in tone. Carter might have read “The People of the Arrow” as Amazing Stories reprinted the story in the January 1970s issue.
P. Schuyler Miller would return to Neanderthals in the story “Old Man Mulligan” in Astounding Science Fiction (December 1940) which featured an immortal Neanderthal in the future.
You can find P. Schuyler Miller stories in various anthologies of 1930s and 1940s science fiction. There was one collection from Fantasy Press, The Titan in 1952. A shame there was never any paperback collections of his fiction.
Have always thought it a pity that the caveman tale, like the reincarnation story which similarly was once a staple of mainstream as well as pulp fiction, went out of fashion. When each was revived in the 1980s they became the preserve of chick-lit for some peculiar reason: think Jean Auel and Barbara Erskine.
Do you remember the excellent prehistoric story “Through the Dark Past” which Charles Saunders and Gene Day collaborated on for Space & Time? Would appear to harbour echoes of Miller’s tale.