Toes of a pahoehoe flow advance across a road in Kalapana on the east rift zone of Kilauea Volcano, Hawaii. Credit: J. D. Griggs / U.S. Geological Survey.
A primate is a mammal that belongs to the order Primates. Included in this order are prosimians, monkeys (Old World and New World), and apes, the latter group including hominids. There are approximately 350 extant species of primate.
Primates are characterized by:
• relatively large brain-to-body mass ratio
• advanced stereoscopic color vision with large, forward-facing eyes
• distinctive eye orbit structure
• five fingers on each hand, with sensitive pads
• nails instead of claws
• aposable thumbs
• specialization of the hands (and sometimes feet) for grasping
• hind limb dominated locomotion
• generalized dental pattern
• unspecialized body plan
• neoteny and a long rearing period for offspring
Primates are by and large an arboreal order whose members usually climb by grasping. The skull has a bar, which makes a posterior or lateral margin of the orbit. The shortened face is accompanied by a diminished sense of smell. However, tree-life has led to the development of excellent eyesight, typically with binocular vision.
The first primates appeared at the beginning of the Eocene (55–36 million years ago). There were two varieties of Eocene primates, similar to modern lemurs and tarsiers. The first apes appeared in the Oligocene (about 27 million years ago) and the first hominids (human-like) primates 5 to 4 million years ago.