Latin primas, first
The primates probably descend from an arboreal insectivorous group which explains why most primates are primarily arboreal, some of whom (baboons, chimpanzees, gorillas, and humans) are secondarily terrestrial. Primates are predominantly omnivorous, but include folivores (Colobinae, Propithecus) and insectivores (Daubentonia).
The primates have an increased reproductive efficiency that includes longer gestation, more efficient placentation, reduced number of offspring (usually single, although twins occur) and increased parental care. Many species form hierarchical social groups.
The dental formula is: incisors 0-2/1 or 2, canines 0-1/0-1, premolars 2-4/2-4, molars 3-4/3-4, for a total of 18-36. The molars usually have 3 to 5 cusps, brachyodont in primitive forms and bunodont in advanced, indicating a more mixed diet. Premolars are usually bicuspid.
The mandible forms one part in advanced groups, while the orbit and temporal fossa are separated by a postorbital bar or plate. The rostrum is often short; the braincase relatively large.
Digits are usually prehensile, with hallux and pollex opposable; the hallux may be absent. The tail is long to absent; prehensile in Cebidae. The limbs are plantigrade and usually pentadactyl, with claws or nails, and a nail always on the hallux. A well-developed clavicle permits shoulder rotation through angles in 2 planes; radius and ulna are separate, and some forms possess a short olecranon process. The stomach is simple except for example Colobinae, and a caecum is present. The uterus is simplex or bicornuate; the penis usually pendant, with a baculum in most genera; the testes are subdermal or scrotal. The eyes are well-developed, with good binocular vision. Most of these characters are due to the fact that the ancestors of the primates were adapted to arboreal life, which required the ability to climb by grasping a limb, general body agility and coordination, high development of vision and high degree of development of the brain.
The systematics of the order Primates have been subjected to a lot of discussion and several classifications have been made. Three living monophyletic groups can easily be recognized: The lemurs, including the Lorisoidea, the anthropoids (monkeys and apes) and the tarsiers. Problematic are the interrelations between them. Nowadays the following three classifications can be encountered:
- Simpson, 1945 established a) the Prosimii (lemurs and tarsiers) vs. the Anthropoidea, resurrected by Schwartz, 1986,
- Pocock, 1918 and Hill, 1953 made b) the Strepsirhini (lemurs) vs. the Haplorhini (anthropoids and tarsiers) and
- Gingerich, 1976 and Schwartz et al., 1978 made c) The Plesitarsiiformes (tarsiers) vs. the Simiilemuriformes (lemurs and anthropoids).
We will follow classification b, the Strepsirhini (lemurs) vs. the Haplorhini (anthropoids and tarsiers), because it is the classification nowadays by most scientists accepted.
An ancestral group of arboreal insectivores developed into four groups (the extinct Paromomyiformes , the tarsiers, the lemurs and the anthropoids), which are represented in the order Primates by three suborders of which two are extant (Order Primates), 1) suborder Paromomyiformes (also called Plesiadapoidea or Plesiadapiformes, early tertiary forms with rodent-like adaptations), 2) suborder Strepsirhini (subdivided into infraorder Adapiformes [extinct, mostly Eocene in age and earliest representatives of the Strepsirhini] and infraorder Lemuriformes [lemurs and lorisids] and 3) suborder Haplorhini (subdivided into infraorder Omomyiformes [extinct representatives of the Haplorhini], infraorder Tarsiiformes [tarsiers], infraorder Platyrrhini [South American monkeys and marmosets; the nostrils open to the sides] and infraorder Catarrhini [Old World monkeys, apes and men; the nostrils open downward].
The tree screws (Tupaiidae) were long thought to be close to the ancestry of primates and were often placed together with the lemurs (Le Gros Clark, 1936 and Simpson, 1945). A list of differences with the lemurs separates the tree screws and they are placed within the order Insectivora, although they probably were very close to the ancestry of higher mammals as a whole.
The habitat of primates is primarily tropical and the principal areas of primate distribution are Africa (including Madagascar), Southeast Asia, India and Central and South America (dist-primates).