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Mammals are a diverse group of animals with over 4000 species, including rodents. The most significant feature that distinguishes mammals from other animals is their ability to regulate their body temperature, known as endothermy. Mammals evolved around 300-200 million years ago during the Cretaceous period, with early species being small and insectivorous. During the late Cretaceous and early Tertiary periods, mammal evolution experienced an explosion with many new species emerging. Mammals share several traits such as vertical body orientation, four limbs that can be transformed into flippers, internal fertilization, breathing air at the surface, lactation, and bipedalism. There are three subclasses of mammals: placental mammals, marsupials, and monotremes. Humans are primates along with other apes, monkeys, lemurs, and have distinct features such as larger brains, specialized teeth, small litter sizes, and extensive parental care. While humans are primates, they have adaptations that differentiate them from other primates such as bipedalism, larger brains, specialized teeth, small litter sizes, and ability to form complex societies and cultures.