Renewal — Hylton Smith

The Nexus Odyssey is comprised of The Darwinian Extension Trilogy and the concluding volume Renewal. It begins forty three years after the trilogy and can be read as a stand alone book.
The City ship Phoenix is into the fifth decade of its journey from Mars to the 55 Cancri system.
The three species aboard are shocked to find the planet Nexus is nothing like they expected it to be. A deadly struggle ensues between the crew and a lifeform which tries to control them. The planet's orbit has stabilised, but this was due to an event external to its solar system. The explanation is finally discovered and affects the fate of all three species, as well as revelation of the truth behind the elusive Progenitors.