Interstellar Isolation And Re-Integration

On an interstellar scale, human communities will develop in complete isolation, then possibly re-interact many generations later. Robert Heinlein's Future History, having begun with a short story set in 1952, ends with two stories set inside a generation ship whose crew have lost all knowledge of their mission and purpose and whose fate is unknown back in the Solar System.

In Poul Anderson's Psychotechnic History, when the Traveler becomes the first Nomad ship, its crew have no idea where Earth is. However, later in this future history series, agents of the Stellar Union Coordination Service based on Earth know, and also disapprove, of the many Nomad ships that are out there. There must have been a moment when contact was re-established although this is not among the stories that Anderson has told us. However, one lost colony is found in Virgin Planet. Other branches of humanity might remain separate indefinitely, eventually becoming distinct species.