Occasionally there appear works of hard science fiction that seem more about science than about fiction. Greg Bear has penned many such novels over the years, including a whole book about unraveling the geology of a far-off planet. His latest novel, Darwin's Radio, which in April 2001 won the Science Fiction Writers of America's prestigious Nebula Award for best novel, is the latest in this genre.

In it, Bear posits a molecular mechanism behind punctuated equilibrium, the theory that holds that evolution goes through long periods of relative stasis, followed by bursts of speciation and then more long silences. The theory explains otherwise puzzling gaps in the fossil record.

This book opens with a particularly surprising addition to human paleontology. High in the Italian Alps, discredited anthropologist (and maverick researcher) Mitch Rafelson is led by a couple of treasure-seeking acquaintances to a frozen cave. There he finds the well-preserved remains of a pair of Neanderthals, an adult male and an adult female. The real surprise, however, lies nearby in a bundle of furs: It is an infant - an anatomically modern infant.

In the next chapter, we are whisked off to the Republic of Georgia, where microbiologist and virologist Kaye Lang is sampling sewage treatment facilities to find new examples of bacteriophages, the viruses that infect bacteria. During her stay, a United Nations peacekeeping force asks her to examine a recently discovered mass grave. Many of the bodies are those of women, and almost all were pregnant.

Lang and Rafelson independently return to the United States, and soon discover that the Centers for Disease Control and Prevention (CDC) has been chasing down a new virus that infects only pregnant women. It turns out to be a reactivation of an endogenous retrovirus - one of those ancient viruses that we appear to be hauling around in our DNA, but that never gets expressed. In Bear's story, they do. And they cause pregnant women to miscarry. But very soon after the miscarriage, the mothers are pregnant again - whether or not they've been sexually active.

The CDC - as well as the increasingly panicked public - sees a horrific viral disease that seems bent on destroying an entire human generation, and many women begin to refuse any physical contact with men. The CDC responds with an attempt to develop a vaccine, and Lang joins the effort because she was the first to document the full sequence of an endogenous retrovirus. She was also the first to hypothesize that they have the potential to be expressed.

Rafelson hears about the virus and begins to wonder if it could have anything to do with the sad little frozen family he discovered in the Italian Alps. He had secretly taken tissue samples from the bodies, and now has them analyzed for the presence of the virus. The analyses come up positive, and very soon after that he is introduced to Kaye Lang.

The press calls the disease Herod's flu, after the first-century B.C.E. Roman-appointed "King of the Jews," who, according to one biblical account, ordered the slaughter of all newborns in Bethlehem in order to thwart the prophecy of the coming messiah. As the novel progresses, Herod's flu gains momentum and social order deteriorates. Rafelson and Lang - who have by this time become lovers - realize that the virus is an evolutionary engineer. Cellular stresses produced by a changing environment induce the expression of the virus. It begins to mine the so-called junk DNA that exists throughout the human genome, producing new combinations of genes in the second fetus to create a new kind of baby, one that is perhaps better adapted to the changing environment.

It is here that the science begins to tail off and the real story begins, as Kaye and Mitch start to contemplate what the next phase of human evolution might be.

For a book with such an entrenched scientific theme, Bear does an excellent job of developing his characters. He gives them depth, even if his descriptions sometimes suggest he is scouring the English language for the most explicit adjectives he can find.

The novel explores the reactions of scientists and lay people in the face of a looming evolutionary change and provides an interesting look at two scientific minds that are naturally curious about the genetic innovation that may very well render them and their kind physically obsolete.

The story has a feeling of inevitability to it - once you are about a quarter to half of the way through it, you have a general sense of how it will end. But it is still possible to enjoy the characters' reactions to the impending conclusion. That is the real story.

In Darwin's Radio, Bear achieves one of the most important functions of science fiction: He explores human reactions and adaptations to an unforeseen circumstance, and in doing so he reveals something about human nature. You can't do that with other fiction genres, and at its best, this is what science fiction offers us. Few do it as well as Bear has.