The Earth's climate went through several major changes throughout the Tertiary period that led to the flourishing of primate species and the extinction of other primate species.
The plesiadaptiforms, which are not considered real primates because of the lack of key primate features, went extinct at the end of the Paleocene epoch. At the beginning of the Eocene epoch came the euprimates, considered the first real primates, whose features made them well-adapted to arboreal life. Euprimates had convergent eye orbits, opposable digits, nails, and larger brains than plesiadaptiforms. This coincides with a period of global warming which made for a more tropical and forested habitat. This environment led the evolution of primates in its direction
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This theory considers the movement of life from the ground into the trees as the most important catalyst in the evolution of the ancestral primate. The essential features of the primate evolved because they were necessary and therefore had greater fitness for creatures swinging from branches.
The visual predation hypothesis does not seek to debunk the arboreal hypothesis, but takes note that other arboreal mammals have not evolved in the same way as primates. Cartmill pointed to animals like the squirrel which does not have features such as advanced care of young. Cartmill proposed that there must be instead a different cause for the unique evolution of primates. He hypothesized that the primate's diet of small prey was the most important cause, leading to the characteristic visual capabilities and motor skills.
Sussman's angiosperm radiation hypothesis argues against Cartmill's by claiming it was the importance of fruit as a major food source that influenced primate evolution. Not insects or small prey, but the dependence on picking and eating fruit in trees was the cause for grasping toes, advanced sight, and mental