"Message from The Eocene" is a 1964 science-fiction novel by Margaret St. Clair, published as one half of an Ace Double, with the other side being a collection of short stories, also by Margaret St. Clair.
I probably would have understood this book a little bit better if it had a different title. The word "Eocene" is a misnomer, because this book actually starts in the Hadean Age, almost 100 times further back in time than the Eocene. The earth is just forming, and the surface is a mixture of lava and poison gasses. There is intelligent life there, aliens with radically different physiologies who extract oxygen from minerals, and have a complicated sociopolitical organization, with warring city states existing on mountains above a sea of lava. And the reason that these aliens exist so early in life's history is that they were brought there by another race of even more powerful aliens, the Vaeaa, cosmic overlords who wish to destroy all spiritual knowledge. Our story begins when our protagonist, Tharg, one of the early aliens, is waylaid by the Vaeaa because he is carrying a spiritual tome that goes against their materialistic philosophy.
And this is all in the first 25 pages of a 110 page story.
The book than settles down into two further episodes: after literally billions of years existing as a disembodied intelligence, the rise of sapient humans brings Tharg back to consciousness. Specifically, he tries to reach psychic contact with a family of Quakers in early Victorian England, but their psychic reaction turns the benevolent Tharg into a poltergeist, and they eventually move away, leaving Tharg to slip back into unconsciousness. Some time later, he possesses a mine in New Caledonia, filling it with interlocking geometric shapes, in order to get the miners to dig up the lost spiritual tome, which has been waiting there for the past few billion years. When he recovers it, however, it bursts into flame.
In the final conclusion of the book, Tharg tries to get a spaceship to recover a new book of spiritual wisdom from space, and this also makes a Tibetan monk go insane, a sea captain see mermaids, and a scientist to discover monopoles, but at the end, humanity is given a guide to spiritual transhumanism.
There is a lot going on here. This story was unexpected from the beginning. I have read lots of different time eras in science-fiction, from the contemporary, to the far future, but starting a science-fiction story at the dawn of the planet earth was an interesting start. Especially since it manages to describe, in just a few pages, a biologically and socially unique alien life form, and after it has finished doing that...we then have 20 pages of Victorian ghost story. Given the usually sharp and ready knives of the editors at Ace, it was somewhat unexpected to have one-fifth of the novel taken up by a 19th century ghost story that didn't directly add to the plot. Margaret St. Clair somehow managed to add rich and complicated descriptions of character, scenery and motivation into a 110 page book that takes place in four very different situations.
Although this book was very imaginative, fitting so much into such a short book did make me wonder what the main idea was. I would say, that like several other Ace Doubles from the early 1960s I read recently, such as The Light of Lilith, The Darkness Before Tomorrow, and The Ladder in the Sky, the book had a concept of transhumanism and growth in human potential. A further topic to examine, although it is difficult to say from just one work, is how much Margaret St. Clair being a female writer in a male dominated field could have given her a different perspective.