In fact the first trilogy was not really a trilogy in the same way this second is. The universe to which Brin takes us is one in which the 5 galaxies are populated by many alien races, and have been for some time. There are a number of facts given which set the scene for all that follows. No intelligent species has ever reached full sapiency without help. That is uplift, by another species.
Humans appear to have no patrons.
Brin managed to finish the first series without ever seriously addressing the conundrums listed above. This second series of three books, linked much more closely than the first, sets about answering them with a new set of questions. In the first book we find ourselves on Jijo, A planet which should be fallow, preparing itself to be re-settled with sapient beings in the indeterminate future. But wait, we have 6 'sooner' races who have illegally settled this world, looking for a better way to live and die. Qhueuns Glavers, and the centaur like Urs.