I love it when an author has a big, hard-science concept and explores it thoroughly, yet creates characters and a story arc eventful and rich with humanity. This is an apocalypse story, and a Zombie story... but not. I mean, no Zombies, but an "us against them" theme that strikes that chord. I want to avoid spoilers, but Neanderthals are coming back, and they are noble--- and deadly. It's a genius idea, and Jeff Carlson has created a marvelous species here. There is a lot of science stuff that I find fun, and believable. The central idea, the "brain scramble" is not so believable, but makes for a GREAT story and a big problem to be solved.
My three star rating is kind of meld of my personal reaction to the way the story unfolds and a less idiosyncratic disappointment in what I feel to be a slapdash ending that just ties everything up in a sloppy knot.
My personal problem with the story is that it is so darned unhappy. Sure, what's to be happy about when civilization is falling apart? But people are not always frustrated, stymied, arguing and troubled. This is a gloomy story, and the tension and pressure and unhappiness are unleavened by the natural human ability to have some fun once in awhile (the "Bugle" character notwithstanding). So I found the book tiring after awhile.
The ending? China is involved in this story, and it is the exception to the fleshed out characterization. It is a HUGE straw man, offstage, menacing, unexplained, and finally swept under the rug. The action scene underground at the base? Exciting, well written, and ridiculous.
So for those who can take it, I say 4 stars. For those of you who found the movies "Pursuit of Happyness" (sic) and "Flash of Genius" worthy but ultimately too much pain for the gain: this might be reminiscent; stay away.