Time: mid 90s/early 00s, at a guess, I remember that I bought the book in hardback when it came out, but damned if I can find it now.
Setting: Earth, tens of thousands, if not millions of years in the future. Humanity has retracted into two cities. The citizens of one city have given up an imortal life to have some sort of telepathic contact with each other.
Humanity itself has been separated out into different “castes” depending on the genetic modifications of the individual. One subset is a group of control-normals with a minimum of genetic changes, the protagonist is a member of this group. She is recruited by a group?/organisation? to help defend the Earth against some sort of vaguely defined menace from space, the roots of which stretch all the early history of human exploration/expansion and whose existence is somehow tied into humanity’s development.
I cannot for the life of me remember either the author or the title of the book. I had thought, based on my hazy recollections, that it was a relatively well known author, but I’ve been trolling bibliographies and can’t find a synopsis anywhere.