With its 300 or more species, primates represent the third most diverse order of mammals, after rodents and bats. They include lemurs, lorises, tarsiers, monkeys, apes, and humans. For their body weight, the primate brain is larger than that of other terrestrial mammals, with a fissure unique to primates that separates the first and second visual areas on each side of the brain.

In all primates except humans, the big toe diverges from the other toes, together forming a pincer capable of grasping objects. Not all primates have similarly dexterous hands; only the catarrhines (Old World monkeys, apes, and humans) and a few of the lemurs and lorises have an opposable thumb. Fossils of the earliest primates date from at least as far back as the Early Eocene Epoch (54.8 – 49 million years ago).*

   
 
(Left to right: Hanuman Langur, 3 Common Indian Monkeys, Black Monkey)
* Text adapted from the Encyclopedia Britannica, 2003