Suborder Strepsirrhini (non-tarsier prosimians)
Suborder Haplorrhini (tarsiers, monkeys and apes)
The reason why humans are classified in the Order primates is due to assumptions that humankind has evolved from ancient ape-like ancestors. This is largely due to a number of obvious similarities between apes and human. Apes and monkeys have opposable thumbs that can grip powerfully just like humans. The primitive limb structure of primates, one upper limb bone, two lower limb bones, (many mammals have "lost" various bones, especially in this area from fusing of the two lower limb bones), is also similar to human.
Despite evolutionary assumptions, no true transitional form between the apes and humans has been found. In fact, paleoanthropologists have classified ancient humans and apes into two distinct genera. The genus to which humans belongs is homo. Australopithecus, which are believed to be ancient ancestors of humans, were instead fully ape.
All primates have five fingers and nails instead of claws on each hand, just like humans. However, some primates have combination of nails and claws. Although opposing thumbs are not only limited to this order, it is a distinguishing feature of primates. The combination of opposing thumbs, short fingernail, and long, inward-closing fingers helps primates to grip branches, which eventually allows some species to develop brachiation as a significant way of transportation. Forward-facing color binocular vision is also useful for the brachiating primates, especially for finding and collecting food, though recent studies suggest it was more useful in courtship. 
Under the worldview of naturalism, the ultimate goal of animals is to maximize reproductive success. In primates, this goal is usually achieved by what is called K-selection. K-selection, which most of the female primates use, is having few young, and investing a great deal of time and effort to rear these young. On the other hand, male primates take very little interest in helping to rear young. Their ways of maximizing their reproductive success is to father as many young as possible and not to invest much on each one. The way male primates choose to rear young is called R-selection. If the human truly evolved from primates, it is certain that human is an heir to the primate’s K-selection. 
Most primates eat many different foods, but they usually prefer some foods over others. Two main nutrients they need are carbohydrates and protein. Most prosimians get carbohydrates from gum and fruit and protein from insects while most anthropoids get carbohydrates from fruit and protein from insects or young leaves. Primates can be categorized into four groups by their preference of food. These four groups are: insectivore, gummivore, frugivore, folivore. Insectivores, those that eat insects, are very small, because big primates can't maintain their life by eating such a small insect unless they have some special methods to get large quantities of insects, like anteater. Gummivores, who eat plant gum, are also usually small, due to the same reason as insectivores. Frugivores eat fruit and are of about intermediate size. Frugivores have especially wide incisors to scrape out the rinds of fruit from the meat. The leaf eating folivores range from intermediate to large. Primates have to be large in order to get plentiful leaves. As a testament to man's separation from these other primates, however, humans are the only practicing carnivores in the primate category. 
Relationship with human
Evolutionists believe that humans and other primates have both evolved from some primate like ancestor. This commons ancestor is believed to have existed in the Pliocene geologic age between 5 to 8 million years ago.
The large apes of Africa and Asia are analyzed to be the most similar to human. Among all animals in the world, they are most like humans in brain and body form, by having a complex social life, and in many other major and minor features, including the lack of a tail. Comparison of DNA by geneticists is also used to support that Chimpanzees in Africa are our closest living "cousins." It has long been paraded that humans and chimps share over 98% of their DNA, though this figure has been revised downwards in recent years.
Using fossil records, paleoanthropologists have been trying to reconstruct the evolutionary history of humans from this common ancestor, but currently no substantial evidence has been found. This missing transitional form between humans and the other apes is typically referred to as the missing link. 
- African congo
- Primateby multiple authors. Wikipedia
- Primate Behaviour Dr. Bill Sellers. Faculty of Biological Sciences, University of Leeds.
- The Unique Characteristic of Primates National Zoological Park.
- Basic primate ecology: Food, territory, and living in groups Bruce Owen.
- Our Primate Origins: An Introduction Human Ancestors Hall.