In the relatively near future, thanks to a series of escalating crises within the solar system, a long-term insurance policy for the survival of humanity is secretly put in place by the upper echelons of the A.I. administration. Because FTL is found to be impractical or even impossible, a sub-light speed solution is required, one that doesn't rely on the fickle nature of civilization.
The solution is a series of shape-shifting, space-faring probes, equipped with artificial intelligence and nanotechnological constructors. Made to be fully independent, they are given the simple directive to travel the space between stars and find suitably habitable planets. Once they land, they're to scan the environment and engineer a population of humans that are adapted to the environment. Once their humans are complete, they're to use resources available to construct more probes like them and all continue on their way. These probes would come to be known as Mothers.
As time marched on, the probes would advance their eternal mission at a good clip, and by a hundred thousand years the Orion Arm of the galaxy was home to thousands of planets populated by derived humans while the light of humanity's original birthplace slowly faded.
Time wasn't kind to the Mothers. Their otherwise austere AGI personalities developed quirks, which grew and mutated into full-on psychoses in some. Occasionally, Mothers encountered one another, and the interaction that followed would be constructive... or catastrophic. Alliances and friendships formed, but also petty feuds and wars. Sometimes a Mother encountered a planet already visited and populated, and reacted kindly or jealously.
Those humans of populated planets remained for the most part isolated and primitive, but under the right conditions sometimes developed civilization and even more rarely became aware of the true nature of their existence, reaching out to their separated kindred. Others lapsed into barbarism, eventually going extinct.
Some Mothers decided to play active roles in the development of their creations, but the vast majority just abandon them and move on their way. Others, some of the more eccentric Mothers, would develop funny ideas about perfecting their craft and turn their human-derived creations into bizarre and contorted art projects, or perform cruel social experiments for the sake of entertaining themselves.
Thus would would be the way of things for 3 million years.
ERA II (ongoing)
One fateful day, something incredible happened: the first ever FTL emanations radiated from an obscure corner of the galaxy. The Mothers, humanity's eternal wards, powerful and myriad as they were, suddenly became obsolete. While they wouldn't know it at the time, they would soon learn to fear.
Those who discovered FTL crawled from what once was known as Earth, that forgotten birthplace of humanity. A Mother had stumbled across this old world and constructed a standard population with rote efficiency, moving on without second thought. With the benefit of ruins and relics of those precursors, the new earthlings' science would advance to heights previously thought unimaginable.
They came to view themselves as the sole inheritors of humanity's legacy and the galaxy as a whole, and upon discovering the Mothers and their far-flung cousins this dogma would translate into an outright resentment of everything not Earth-human. They rejected artificial intelligence in all its forms (seeing it as the precursors' mistake), instead genetically engineering themselves to what they saw as their peak condition, each member of their race a super-soldier in their own right.
Thus would begin the rapid systematic slaughter of Mothers and galactic human life everywhere, all at the hands of those Earth supremists.
But all was not lost, as some human civilizations found themselves able to resist, and stole the FTL technology from the invaders. Those of us precious few in turn took it upon ourselves to save as many of our surviving cousins as possible from genocide at the hands of the Solists.