This book is, without a doubt, hard science fiction. It is very heavy on the science. Most of the major characters are scientists, and many chapters have extensive dialogues focused on the scientific details of what's happening. Heck, there's even a glossary of scientific terms at the end of the book, to help readers who need a primer on some of the words used throughout these passages.
So one of my comments on this book is that it might appeal the most to fans of science fiction, especially if you already have at least a passing knowledge of genetics. I'm not sure if this book would go over as well with a broader audience, I could see it maybe being a bit tough to swallow for people with little or no knowledge or interest in science.
The central part of the plot involves evolution, and deals with the idea of humankind entering a new phase of development. Something bizarre has been happening around the world, with women experiencing miscarriages early in their pregnancies, followed by a second pregnancy producing babies that don't live long.
Much of this is shown from the point of view of Kaye Lang, herself a molecular biologist who becomes involved in the effort to understand the situation. Her own specialty is retroviruses. She thinks that parts of human D.N.A. that have no apparent purpose, may actually contain the seeds of a new evolutionary leap.
Despite the efforts of governments to control and study the situation, there is strong public response, sometimes leading to violence and riots. Some people seem to blame the scientists or the government for what's happening, even though this thinking is not very rational.
Now, this type of story isn't completely new to science fiction. Stories about humankind witnessing its own evolutionary change, accompanied by feelings of fear and anger, has been done in a variety of ways. Examples include the old classic story, "The Man Who Evolved" by Edmond Hamilton, or the "Star Trek: The Next Generation" episode "Transfigurations." But, I'll give Bear a lot of credit for doing it better, in many ways. Bear's reputation as a hard science fiction writer is well deserved. The details and the science in the story make this by far one of the most believable, well thought-out presentations of this kind of idea in science fiction.
There's a near part of the story involving a character named Mitch Rafelson, and some frozen Neanderthals. It seems that an evolutionary change like the one happening in the present day may have happened once before, a very long time ago, possibly when modern humans first arrived on the scene.
All this leads to a debate about how evolution works, whether it's a gradual process or if it can happen with a sudden leap, and if so, what is the mechanism by which that leap occurs. Ideas about D.N.A., retroviruses, how they arise and what purpose they may serve all come into play.
Apparently, at the time Bear wrote the novel, this was a debate that hadn't been resolved yet. I'm not sure what the current state of this discussion is in the scientific community.
A good book, one I'd recommend if you like your hard science fiction with plenty of emphasis on the science, or perhaps if you want to learn more about genetics.