Book scenario

The scientific world is watching an asteroid that is passing close to earth, at the same time a small rock from space hits earth from the opposite direction, it then disappears, the scientists think it burned up in the atmosphere, but there was an alien life force living on it, and when it landed the alien left it and took over the body of a local woman.  The husband eventually noticed the change and confronted it. They talked at length and the alien decided to let the human go, but it had to live somewhere, so it traveled back in time to when life on earth was just beginning and it took over a life form there.  It directed the lifestyle of that animal and it grew as the alien wanted, the alien lives eternally, and still lives in the body of that animal, through al it's changes, the alien also inhabits all the bodies of that species. That book 1 in a nutshell.