Question: What makes primates unique from other mammals? 

Answer: There are several characters that distinguish members of the Order Primates from other mammals, but different members of the Order may or may not display all of these characteristics as they have evolved in different directions throughout their evolutionary history:\n\n1.  opposable thumb and big toe (opposable thumb is best developed in humans.  thumb is completely lost in spider monkeys)\n\n2.  flat nails instead of claws  (marmosets have re-developed claws to assist in climbing)\n\n3.  hindlimb-dominated locomotion (best developed in humans and sifakas, forelimbs have become dominant in gibbons)\n\n4.  relative reduction in the olfactory sensory system as compared to other mammalian orders (very noticeable in South American primates, snouts have become enlarged again in the baboons, are still well developed in many lemur species)\n\n5.  increased reliance on visual sensation (primates in general probably have the best vision and best colour sensation of any group of mammals)\n\n6.  tendency towards smaller litter size, longer gestation times and extended juvenile growth with accompanied increase in material care (most developed in humans with 18+ years of parental care and litter sizes of even two being unusual)\n\n7.  relatively large brains (again, most developed in humans)\n\n8.  reduced number of teeth, with a maximum of two incisors, one canine, three premolars and three molars in each jaw quadrant (humans have further reduced this to two premolars, and the third molars (aka wisdom teeth) often do not develop at all)\n\nSo there is no one trait that automatically designates a mammal as being a primate.
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