The Relationship Between Human and Hominid : FICTION : NEANDERTHAL,<i> By John Darnton (Random House: $24; 256 pp.)</i> : ALMOST ADAM,<i> By Petru Popescu (William Morrow: $24; 544 pp.)</i>

Here’s a novelist’s nightmare: You concoct an original plot gimmick, labor long to turn it into a thriller, then, just as your magnum opus goes to press, discover that another writer is using exactly the same gimmick in a book that will hit the stands exactly the same time as your own.

Such is the fate of John Darnton and Petru Popescu, both of whose novels pose the question: What would happen if a dwindling band of cavemen survived to the present age, were discovered in a little-explored quarter of the globe and had to at long last confront their old enemy--you and me?

Pulitzer Prize-winning journalist Darnton starts his yarn, “Neanderthal,” in a fevered style not seen since H. Rider Haggard published “King Solomon’s Mines.” The tale begins, as such tales must, with a mysterious archeological find. Shortly thereafter a venerable researcher disappears in remote Central Asia. Enter the protagonists, a sexy pair of thirtysomething paleontologists. Enter the villains, a brace of improbably weird CIA types. The plucky paleontologists dash off to rescue the vanished researcher. Consistent with the best traditions of the genre, a cryptic diary is soon discovered. What comes next are rapid-fire Saturday matinee episodes of derring-do designed less to be read than chug-a-lugged. Darnton gives us life-and-death chases, hairbreadth escapes, avalanches, cave-ins, cave bears, cannibals, cliffhangers, brain-eating savages, half-human witch doctors, gratuitous sex and. . . .

. . . And that’s it. The book just stops. Dead. No action, no plot. It’s the literary equivalent of coitus interruptus.

Midway through, the author inexplicably veers into a mean-spirited sermon against modern science, a subject about which he is not well-informed. Darnton’s diatribe is classic pseudoscience--a dreary litany of the tired arguments of kooks, cranks and quacksalvers from time immemorial. That he uses politically correct Neanderthals possessed of Shirley MacLaine-style psychic powers to illustrate his own crackpot ideas adds little to their credibility.

Ah well, Steven Spielberg has bought the movie rights. Perhaps he’ll be able to do a better job with the material than its creator.

Written with more grace and fewer dangling participles, Petru Popescu’s “Almost Adam” is the richer yarn. Popescu tracks a more intriguing quarry: Australopithecus, that upright ape who, 3 million years ago, was humankind’s earliest sapient ancestor.

Popescu sets his novel in East Africa, a landscape that he vividly brings to life. A struggling young American paleontologist named Ken Lauder stumbles across ambiguous evidence that, deep in an isolated savanna, a handful of australopithecines may survive. Lauder sets off to investigate but soon enough he loses his gear--and with it any hope of making it back to civilization. No truck, no radio, no water, no food, no gun. He is alone on the veld with only two bad-attitude lions to keep him company. The lions are hungry. And Lauder is limping.

Those lions are the best villains of the piece. They’re pug-ugly bullies on the prowl for mates, munchies and murder, not necessarily in that order. Popescu’s loving description of leonine table manners is not for the squeamish, but, like much else in the book, it is exceptionally well-observed and tellingly written.

Lauder’s salvation presents itself in the form of an australopithecine boy. Nameless and incapable of speech, the more-than-monkey, less-than-human child is nothing if not a survivor. The lad takes Lauder in tow, guiding him to safety, and teaching him those cunning strategies that our earliest ancestors improvised to survive.

The result is both thrilling and charming. Lauder’s new-found friend is a sort of Pliocene Artful Dodger. Intelligent in ways that humans are not, the boy’s quick wits and exuberant bravado not only bail his hopelessly civilized companion out of countless perils, but also teach Lauder some important lessons about himself--and about humankind in general.

There’s more to Popescu’s plot than an encounter between two distant relatives. “Almost Adam” has enough bad guys for a book twice its length, and therein lies Popescu’s problem: His story line is too busy. The author who has many well-intentioned messages to send frequently strays from the emotionally powerful heart of his tale: the relationship between human and hominid.

Nonetheless, there is much to praise in “Almost Adam.” Throughout his tale, Popescu paints striking portraits of the African landscape. His often lyrical, occasionally cluttered prose brings sights and scents of the veld to life.

Any number of novelists have tried to get into the heads of early man, but few have done so believably. Only three come to mind: William Golding, Elizabeth Marshall Thomas and novelist-paleontologist Bjorn Kurten.

And now, Petru Popescu--whose portrayal of australopithecine life is as compelling as it is revelatory.

“Neanderthal” is available on two audiocassettes, read by Jay O. Sanders, abridged, from Random House; $18.

“Almost Adam” is available on four audiocassettes, abridged, from Audio Renaissance; $24.