Scattered among the stars, the humans of the Third Imperium began to diversify, finding it easier (as most starfaring races have) to adapt themselves to the planet, rather than the other way around (see Venus). They began to groiw apart, diverging into at least sixty separate variants of the human genome. One of the “core” variants called themselves the Sapens (from the root sapients), and some of this group returned to Earth to keep watch. (This is the party that would rescue the Venusians).
The Sapen watchers floated above the icy wastes in gravitic castles, living in relative leisure (as was actually the norm among the Third Imperium). They waited patiently above the wastes for the ice to melt – and that was a long wait.
Meanwhile, the Third Imperium reached the glory of the Old imperium, dominating most of Orion’s Arm – in relative peace and prosperity – creating a galactic civilization with more in common (to our minds) with comic books rather than hard space opera. They were approaching the Clarke Barrier – where technology might as well be magic. The imperium merged or at least cooperated with other interstellar civilizations to form an swath of relative civilization that spanned most of the Galaxy.
While this happened, the glaciers started to melt upon the backwater planet of their origin.
As the glaciers melted, the last true humans – mostly from the orbital Second Earth, and some from even more unlikely places, were settled by the Sapens upon the thawing Earth as a sort of permanent museum. The still primitive ecosystem was augmented with throwback specimens preserved or recreated without a lot of thought to actual context. It was as if the Sapens priority was “whatever seems cool” rather than any sort of orderly, accurate restoration. And this was not far from the truth.
This would turn out to be the last extant population of recognizable humans – hence the nicknamed the Lastocene. Their era was abundantly strange, with its grab-bag of a biosphere, its strange alien overlords hovering in the skies, occasional refugees and outcasts from the wider Galaxy (whom the Sapens let through in small numbers) and the magic.
The Sapens had a Wish Machine of their own, and were first appalled when the humans found ta way to tap into it – but then decided to see how that went. So, then, the Lastocene resembled a Saturday Morning cartoon in many respects: Humans on Earth, but unburdened by their own history, and empowered with random technology and magic.
Left on their own, humans tend to organize into tribal states, and those states can remain stable indefinitely unless they encounter some broader force. The Sapens worked hard to suppress those forces and this, while the boundaries ebbed and flowed, there was never a dominant faction, and never any substantial advance of civilization. The first inhabitants of the Lastocene would not have been terribly lost in the last days of the Lastocene.
The planet cooled, as it was wont to do, and the glaciers began to expand. The Sapens evacuated those who were willing to take their chances among the stars, and left the unwilling or undeserving to their frozen fates. Here ends the history of humanity upon the Earth.