William Golding, The Inheritors (1955, re-reading). In Northern Eurasia during the Upper Paleolithic, a band of eight Neandertals (two young men, two young women, and old man, an old woman, a little girl and an infant) think that they are the only humans (using the word to mean all animals of the genus Homo) in the world. They are omnivorous scavengers (modern paleoanthropologists think that the historic Neandertals were actually skilled hunters, though I don't know whether this was known in 1955), do not possess specialized weapons or wear clothing (both things are untrue of the historic Neandertals), live in peace and worship Mother Earth. Instead of complicated language and thought they have telepathy. The band encounters an expedition of about a half dozen behaviorally modern humans (a few young men, an old man, a young woman and a little girl), who travel in dugout canoes with sails, hunt with bows and arrows, make pottery, wear clothing, create art, drink mead, and have a religious ceremony that was apparently inspired by the deer-man painting at the Trois Frères cave in southwestern France. Unlike the Neandertals, modern humans are capable of murder and genocide, intrigue, and passionate sex. All this fascinates but confuses the Neandertals who observe it. Modern humans kill most of the "forest devils" and kidnap the infant on the request of one of them, a nursing mother whose own infant has died. Presumably, after the events of the novel the infant grew up among modern humans, and became as much an ancestor of the later humanity as they.
It is my understanding that Golding's novel was not meant to be paleoanthropologically accurate; he wanted to illustrate the concept of the Fall of Man from Christian theology: it is the capacity for thought of modern humans that makes them capable of doing good and evil. The problem is that the Neandertals are a poor model of the prelapsarian humanity. They were a sister species to modern humans, like coyotes are to wolves and polar bears are to grizzly bears, adapted to life in a different environment but fundamentally the same kind of creature with the same nature. At the Shanidar Cave in northern Iraq there is a buried Neandertal man; according to the Smithsonian Institution's paleoanthropology site, he "most likely died from a stab wound to his chest"; another Neandertal man buried in the same cave "experienced a crushing blow to his head" and a fractured bone in his foot, but both injuries healed and he lived for many years after them; the Smithsonian site says that "he would probably not have been able to survive without the care of his social group". It looks like there was no Fall of Man because early humans were just as capable of doing good and evil as modern humans.