A book about transhumans, not post-humans, as the genetic link is unbroken; posthumans would be uplifted animals or androids, or other, completely new beings, sharing only culture but not biology.
I get that this book, unlike most sci-fi, concentrates on genetical evolution, not technological revolution. Hence, it would be silly to criticize it for that. But criticize it, none the less, I must, even if the real objective of the author was to show us a dystopia to make us pause.
What is shown there, is natural extension of unsustainable, purely capitalist system, with all its worst aspects - squandering of resources, crushing poverty, social darvinism, schizoid technology, slavery, weaponized demographics and deification of evolutional theory. A world that must never happen.
In the foreword, Brian Aldiss conjures the most famous work of H.G. Wells, Time Machine. I much recommend his much less known, but much more critical work, The Shape Of Things To Come, for referencing a similar vision of a doomed world. Wells adds a happy ending to his vision, but what he proposes must lead to the same bleak, impoverished future Dixon had much more somberly described.
However I must say, once story the book loses any contact with civilized life, it becomes really fascinating.