Humans are a member of the order primates. What are the characteristics which define this order, and what are some of the selective pressures that may have lead to them?
Mammals classified into the Order Primate include prosimians and simians. Most reside in tropical or subtropical climates except for humans which inhabit every continent. The ancestral primates live in trees in tropical forests. Prosimians including lemurs and tarsiers and lorisiforms, most resemble ancestral primates. Simians include such animals as monkeys, apes and humans. Humans are catarrhine or narrow nosed simians that spread successfully outside of Africa, South Asia and East Asia. Great apes and baboons are mainly terrestrial rather that arboreal which many other species of primates are. Primates evolved large brains compared to other mammals, stereoscopic vision that is well developed and most have opposable thumbs. The opposable thumbs allow primates to manipulate their world and grasp and use tools. Sexual dimorphism appears in many species where males are larger and have a different coloration than females. Some primates have prehensile tails. Primates have slower rates of development than other mammals, a longer juvenile period and a long lifespan. About 65mya, the oldest primate is found in the fossil record of Plesiadapis. Primates show an evolutionary trend toward a reduced snout and Old World monkeys are distinguished from New World monkeys by nose structure and teeth arrangement. New World monkeys have sideways facing nostrils and Old World, face downward. Teeth have evolved and Old World monkeys have 8 premolars, while 12 exist in New World monkeys. Nocturnal primates lost color vision, while many species that feed on frutis and plants, have color vision. Selective pressures that might have been at work, included limited food and competition between other species of mammals to survive in the tropical environment.