Rats

In Poul Anderson's "In Memoriam," in the long post-human future of Earth, rats variously evolve into grass-eaters, predators, winged raptors and flippered water-dwellers. Some climb into trees, develop hands, return to the ground and grow bigger bodies and brains but never use fire or complex tools.

I used to think that human beings evolved because some quadrupeds evaded predators by climbing into trees where they were safe and free to chatter and grasped branches, thus developing opposable thumbs. Then, their descendants descended to the plains walking upright in groups with forelimbs freed to manipulate their environment. This account at least had the merit of plausibility.

The Time Patrol Academy is:

"...in the Oligocene period, a warm age of forests and grass-lands when man's ratty ancestors scuttled away from the tread of giant mammals." (Time Patrol, pp. 5-6)

Those "...ratty ancestors..." would have been the pre-arboreal quadrupeds. Bryan Talbot's The Tale Of One Bad Rat (see here) also states that human beings are descended from something like rats. And this may be the case. However, I am now told that what evidence there is indicates that bipedalism preceded opposable thumbs.

Cosmic Perspectives

Olaf Stapledon was at home in realms that other writers do not know about:

the hypertime in which the Star Maker experiments with spatiotemporal creations;

universes composed only of sounds;

the conscious nebulae before they condensed into galaxies;

the galactic mind reviewing in one sentence the history of the Solar System, thus the entire contents of a previous volume;

the Solar System after orbits have been disrupted, planets have been destroyed and later human species have colonized Neptune -

- as was Poul Anderson, e.g., intergalactic space and a cosmic cycle.

In "In Memoriam," in the post-human future:

some octopodidae live longer and care for their young;
later, they work rock, shell, bone and coral with their tentacles;
their language is gestures and color changes;
they learn from their elders and experience;
they practice art and religious rituals;
confined to salt water, they remain in their Stone Age;
their cultures adapt to local conditions and stop innovating;
castes congeal and predetermine individual lives;
intelligence atrophies;
the species loses its ability to cope with environmental changes;
but it survives for twelve million years, much longer than humanity;
it is described in a single paragraph.