On Old Earth, the fourth epoch of the Tertiary period and Cenozoic era, lasting from 23 to 5 million years ago. It was preceded by the Oligocene and followed by the Pliocene. The abundance and diversity of mammals was at a peak. This epoch saw the appearance and flowering of the first apes, though as the climate dried and cooled towards the end of the epoch they were replaced by monkeys. Modern Carnivores (hyenas, raccoons, dogs, cats) first appeared. This epoch saw the spread and diversification of many hoofed mammals (horses, deer, giraffes, camels). The climate was warmer than in the preceding Oligocene and the following Pliocene. It was also drier; the spreading grasslands encouraged a new diversity of hoofed grazers and browsers. South America had its own diverse mammalian fauna as well as some large carnivorous birds. Australia had a similar diversity of large marsupials. Invertebrate life was broadly similar to that of modern times. In the oceans, upwelling of cold polar water made the kelp forest biome possible.
Elsewhere in the galaxy, the Iahi Daon were active in the Orion Arm during the Old Earth's Miocene, though there is no evidence that they reached Earth. According to their accounts the Muuh civilization was largely destroyed by unknown assailants in the mid-Miocene, as was that of the Black Acropolis Builders. Later, surviving Muuh concluded provolution work on the Soft Ones, who began a civilization of their own in the Perseus Arm.