In 2003 British science fiction author Stephen Baxter published his long (592 pages) novel “Evolution.” It is named for its central character. In addition to the Prologue and Epilogue it consists of 19 novella-length chapters in three sections: Ancestors, Humans, and Descendants. The novel covers a period of some 165 million years. Each chapter deals with a different time period, the first one in the farthest reaches of the past, the last in the far distant future.
At the centerpoint of the novel, a few years from now, Life As We Know It is at its peak. Civilization is brought down by a perfect storm of pestilence and terrorism. Those few who survive retreat into the forest.
Until that point, the novel had celebrated the increasing complexity of the mind, the growth of intelligence, the creation of civilization. A success story.
Nature, Evolution (for they are the same), had arrived at the conclusion that while intelligence up to a point was useful in transmitting mitochondria to the next generation, in the longer run it posed a danger. So intelligence gets selected out as no longer a benefit to survival of the mitochondria. And the mitochondria do survive. But we do not.
That long downward fall is one of the bleakest things I have ever encountered in a work of fiction.