1. The plate tectonic movements of the Great Rift; the movement resulted to the formation of a series of islands that aligned along the East Central Africa. The islands that formed were volcanic.
2. Weather and climate patterns changed locally. Areas became more watery or drier in accordance with the rain shadow phenomenon.
3. The global climate pattern was progressive cooling. The ice caps formed at the poles characterized with the freezing of water from the sea. The freezing resulted in to a fall in the sea level. There was emergence of previously submerged Coastal lands.
4. As a result of the above factors (1, 2, and 3), Africa and Eurasia got reconnected. Continental Drift played a significant role.
5. The Mediterranean Sea route provided a migration path for the primates of Africa and Eurasia as they were reconnected.
6. Tropical forests gave way to sparse and dry grasslands and woodlands.
Hypotheses of primate origin:
1. Arboreal Theory – This theory concludes that primates developed from their ancestors through an adaptation to arboreal life. They developed some traits and habits like more reliance on sight rather than smell, use of developed hands and feet for the purposes of holding and grasping, crawling action on branches for obtaining food and fruits from trees etc.
2. Prosimian Theory – Placental animals came into existence during the Eocene Epoch. The mammals had huge bodies and bigger brain size; they appear in the records (fossil). Lorises, Lemurs, and Tartiars are the representative species of the Prosimian placental mammals. Foramen Magnum- opening in the skull was an important feature of these mammals.
3. Oligocene Epoch Theory – The monkeys evolved during the Oligocene Epoch. The animals remained in the forests eating seeds and fruits. It is believed they had less snout and few teeth.