I was shocked to learn that about 98.5% of the genes in people and chimpanzees are identical. This finding means chimps are the closest living biological relatives to humans, but it does not mean that humans evolved from chimps. What it does indicate is that humans share a common ancestor with modern African apes (i.e., gorillas and chimpanzees), making us very, very distant cousins. We are therefore related to these other living primates, but we did not descend from them.Modern humans differ from apes in many significant ways. Human brains are larger and more complex; people have elaborate forms of communication and culture; and people habitually walk upright, can manipulate very small objects, and can speak.For the most part, nonhuman primates are research subjects because they are so similar to humans, and the principal reason for this similarity is simple: humans “are” primates. Current ideas are that the first primates appeared more than 60 million years ago. In contrast, the common ancestor of humans and African apes lived only about 5-8 million years ago; so, for more than 50 million years, humans and the African apes have shared primate ancestry. Shared ancestry is a major reason why human and nonhuman primates have many characteristics in common — tool use, long-lasting social relationships, and complex communication systems. By learning about nonhuman primates we may come to learn more about ourselves. For example, humans walk upright, on two limbs — we are bipedal. Why might humans have evolved to be bipedal, when the vast majority of nonhuman primates are quadrupedal? Individuals of certain nonhuman primate species, however, are bipedal for some activities. By studying those species of nonhuman primates that are occasionally bipedal, and discovering the circumstances in which they display bipedality, we may gain some understanding of the factors that promoted the evolution of bipedality in humans. Human and nonhuman primates also share physiological characteristics. For example, the way in which the brains of rhesus monkeys and humans are organized is similar. One brain area that has been studied extensively is the visual system. Neuroanatomical studies of the nonhuman primate brain have been extremely useful in helping us to understand how the human brain functions and how we see. In this way, nonhuman primates serve as models of particular processes that would be extremely difficult or impossible to study in humans. Study of nonhuman primates has also contributed to our understanding of basic biological phenomena such as reproduction; to better understanding of diseases such as AIDS; and to the development of drugs, treatments, and vaccines for the promotion of better health for human and nonhuman primate alike. In fact, research conducted with nonhuman primates has contributed to Nobel-prize-winning research: development of yellow fever vaccine. Of course, nonhuman primates are also studied because they are fascinating animals. They live in a wide range of habitats, and show many interesting differences in behavior and life styles. For example, in some species like squirrel monkeys, many adult males and many adult females live together the year round in a troop that also contains infants and juvenile animals. In other species, like titi monkeys that live in the same area as squirrel monkeys, a single adult male and a single adult female live together with their offspring.