Global cooling marked the start of the Oligocene, an environmental shift that led to changes in the flora and fauna. During this epoch archaic species of the early Cenozoic were replaced by modern herbivores, such as horses, camels and deer, and carnivores such as cats and dogs. Global temperature dropped as much as 10° C, shrinking forests and introducing the grassy plains that would come to dominate in the Miocene. Herbivorous species adapted to eating grass and carnivores adapted to hunting out in the open. The earliest South American primates are known from this epoch. A site in Bolivia dates the arrival of primates in South America to approximately 30 million years before the present. Primate evolution in the Oligocene involved increases in brain and body size, sexual dimorphism, and possibly social complexity. Environment changes caused the near extinction of primates in the Northern Hemisphere. In Asia and Africa, anthropoid primates (the higher primates) seem to have thrived. On the basis of analysis of the fossil remains, these primates are thought to have been arboreal quadropeds that fed on fruit and leaves. These species are the ancestors to the apes of the Miocene.
Source