Why Are Human's Primates?
First, primates have excellent vision. They have forward-facing eyes that sit close together, which allows the eyes’ fields of view to overlap and create stereoscopic, or 3-D, vision. (In contrast, for example, a cow or giraffe has widely spaced eyes and therefore poor depth perception.) Related to this great eyesight is the presence of a post-orbital bar, a ring of bone that surrounds the eyeball. Many primates also have a completely bony socket that encloses the eye. This bone probably protects the eye from contractions of chewing muscles that run down the side of the face, from the jaw to the top of the head. Many mammals that rely less on vision don’t have a post-orbital bar. If you poked a dog in the side of its head near the temple, you would feel muscle and the eye but no bone (and you would probably be bitten, so please don’t do that). Because primates depend on their vision so much, they generally have a reduced sense of smell relative to other mammals.
Primates are also very dexterous. They can manipulate objects with great skill because they have opposable thumbs and/or big toes, tactile finger pads and nails instead of claws (although some primates have evolved so-called grooming claws on some of their toes). Primates also generally have five fingers/toes on each hand/foot. This is actually a very ancient trait. The earliest mammals had five digits, and over time, many mammalian lineages lost a few fingers and toes while primates kept all of them. Primates also retain collar bones, which allow for greater mobility in the shoulder; mammals that strictly walk on all fours, such as horses, lack collar bones so their limbs are more stable and don’t slip to the side while running.
And in general, primates tend to have larger brains than other mammals of a similar size. They also have smaller litters—often just one baby at a time—and longer periods of gestation and childhood.
Scientists are still trying to understand why primates’ unique set of features evolved. Some researchers think the earliest primates lived in trees, so good vision and dexterity would have been helpful in judging distances between branches or for climbing around. Others, such as Boston University’s Matt Cartmill, have suggested that these traits emerged because early primates might have been insect predators and needed clear eyesight and quick hands to grab prey. Both factors, as well as many others, could have played a role.