Final answer:
The two models explaining how primates spread across the world include the hypothesis that New World monkeys reached South America from the Old World by drifting on rafts or land bridges and the models for early human migration: the 'out of Africa' model and the 'multiregional evolution model'.
Step-by-step explanation:
The two models for how primates spread across the world address the distribution of nonhuman primates and the evolution of early humans. The first, concerning nonhuman primates, suggests that anthropoid monkeys, which evolved from prosimians during the Oligocene epoch, reached the New World (South America) through drifting on log rafts or crossing land bridges from the Old World (Africa and Asia). Due to reproductive isolation, New World monkeys and Old World monkeys underwent separate adaptive radiations over millions of years. The second model relates to the spread of early humans. The "out of Africa" model suggests that modern humans first emerged in Africa and then displaced other human species worldwide, while the "multiregional evolution model" proposes that Homo sapiens evolved from Homo erectus in multiple regions around the same time.
Both models for primate and early human spread conform to the rules of natural selection and the influences of gene flow, genetic drift, and mutation. However, for simplicity's sake, both models do not account for the influence of individual genes. Studies of primate biogeography have been greatly influenced by factors such as natural catastrophic events, resulting in rapid adaptation and speciation, especially in isolated habitats like island chains.