These similarities are common and unique in the fact that only primates contain all of them together.

     Many sources list primates as having 4 unique types.

1. Prosimians - rodent-like primates including the aye, aye and lemur.
2. Simians - broken further into two categories such as:
   Old World - Baboons and Mandrills, mainly noted for having no prehensile tail.
   New World - Marmosets and Squirrel Monkeys.
3. Pongidae (Ape) - resembling monkey except they posses no tail, are bulkier, larger and significantly more intelligent.  Mainly vegetarian and some have human-like teeth structures.  The Ape itself can be divided further into two categories:
   Lesser Apes - Gibbons and Siamang.
   Greater Apes - Chimpanzees, Gorillas, Orangutans, Pongidae homologous
   (Homo Habilis).  Homo Habilis is also classified as a human ancestor.
4. Hominidae - Humans.  Noted as the only primate that stands and walks completely erect.

     Most of the reference material found on the Order of Primates suggests a dual Family break listing Hominidae as a single family including humans:

A Hominid is any creature of the family Hominidae (order Primates), of which only one species exists today--Homo sapiens, or human beings. Extinct species of the family are indicated in fossil remains, some of which are now quite well known: Homo erectus, Homo Habilis, and, going further back in evolutionary history, various species of Australopithecus.
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Human SkullMonkey Skull

and the family Pongidae encompassing the orangutan, chimpanzee and gorilla:

The family most closely related to the Hominidae today is the Pongidae, the anthropoid apes including the gorilla, the chimpanzee, and the orangutan. These are believed to have diverged from a common ancestral line during the Late Miocene epoch (11.2 million to 5.3 million years ago). The physical characteristics that distinguish hominids from the pongids are erect posture, bipedal locomotion, rounded skulls with larger brains, small teeth (including unspecialized canines), and such behavioral characteristics as communication through language.
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     However, other evidence claims the members of these two families Hominidae and Pongidae are too closely related to be split and that Hominidae should and does contain Humans, Gorillas, Chimpanzees, and Orangutans.

     In the slow process of evolution, countless generations are needed to acquire these characteristics, so it is extremely difficult to draw a line between pongid types and hominids in the fossil record available. Teeth and skull fragments constitute most of the evidence, and these are often not well enough preserved to show adaptations that might serve to distinguish one species from another.
Copyright (c) 1997 Encyclopedia Britannica, Inc. All Rights Reserved