Help ID this SciFi story/book (321st in a series)

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.

Help?

I don’t know this one, but I suspect that AndyL will be along eventually to solve the mystery.

The first part of this, about two cities, one with immortal citizens and one with telepathic ones, is a very good fit to Clarke’s “The City and the Stars” (aka “Against the Fall of Night”). The bit about control normals sounds a bit like Heinlein’s “Beyond this Horizon” though, and I can’t quite place the last bit. Is it possible you’ve combined several stories into one?

Edited to add - Benford wrote a sequel called “Beyond the fall of Night” which was published in 1991 - I think that might be it (one review referred to “Ur-humans” which sound like your control normals.

I had originally thought it was “Beyond the Fall of Night”, but the blurb/reviews I’ve read do not match my recollections. I’m pretty sure I’m not conflating different stories in my head. I distinctly remember the “ur-human” as a female, not a male named Alvin.

Thanks for the try, though.

The Ur-Human in “Beyond the Fall of Night” is a female Cro-Magnon named Cley (Alvin is the lead in the original “Against the Fall of Night”). The two cities certainly sound like Diaspar and Lys from “Against the Fall of Night” (http://en.wikipedia.org/wiki/Against_the_Fall_of_Night), but I see if I can think of any other books that would fit your description…

That’s got to be it then. Thanks.