Julian May’s 19811 The Many-Colored Land2 is the first volume in May’s four-volume Saga of Pliocene Exile.
Earth has been absorbed into the Galactic Milieu. Planetary society has been rationalized, humans have been provided with an abundance of worlds to settle, and peace reigns. Utopia all round (provided one was not too attached to the languages and religions that the Milieu suppressed as superfluous to need).
In the early 2030s, Theo Guderian made a fascinating discovery. Various factors unique to the French city of Lyon make it possible to send objects six million years into the past. However, retrieving objects from the past to the present day ages them almost instantly by six million years. While robust materials like amber could survive a round trip, living plants and animals could not. His time portal thus appeared to have few practical applications.
It fell to Theo’s widow to arrive at a use for the portal: not everyone is happy with the world as it is in the 21st century. For them, a one-way trip into the past is the only escape possible from the Galactic Milieu. At first covertly, and then officially sanctioned, exiles set off for what they expect will be a pristine and empty Earth3.