Primates

A limited set of common characteristics cannot strictly define primates, as they do not exhibit a high degree of specialization. This makes them particularly interesting to humans, as we are also part of this diverse animal group, including prosimians, monkeys, and apes. As humans, we are not only a component of nature but also share close biological connections with these primates.

The order Primates within the class Mammalia encompasses a variety of species, from lemurs and tarsiers to monkeys, apes, and humans. Most non-human primates inhabit the subtropical and tropical regions of Africa, Asia, and South America. Their sizes vary significantly, ranging from the tiny mouse lemur, weighing just 30 grams (approximately 1 ounce), to the substantial mountain gorilla, weighing up to 200 kilograms (around 441 pounds).

1.0What are Primates?

• Primates are complex, multicellular organisms belonging to the kingdom of metazoans. They feature an internal skeleton categorized under the phylum Chordata and have a segmented vertebral column, classifying them as vertebrates. Like other mammals and birds, primates are homeotherms, maintaining their body temperature within a narrow range. As mammals, primates exhibit several distinctive traits, including mammary glands, nurturing offspring through suckling, a body covered with hair, and viviparous. They are warm-blooded and exhibit heterodontism, featuring a single dental arch connecting with the skull's squamosal bone. Primates have a thoracic diaphragm that anatomically separates the thorax from the abdomen, and they are diphyodont, meaning they develop two sets of teeth over their lifetime. The anatomical structure of primates remains relatively primitive and generalized, showing few specializations and retaining many characteristics of the earliest mammals, particularly those ancestral to primates.
• Primates constitute the seventh largest order among mammals, encompassing extinct members, with 51 genera and 168 species. The New World hosts 16 of these genera and 50 species. This order is often regarded as highly significant among mammals. It is undisputed that humans belong to the primate order, although the rationale for grouping humans with creatures like the tree shrew, loris, and aye-aye is not commonly understood. The majority of non-human primates are found in tropical regions. Due to their cultural and biological adaptability, humans can thrive in various biomes.

2.0Evolution of Primates

• The earliest ancestors of primates, often referred to as proto-primates, bore a striking resemblance in size and appearance to squirrels and tree shrews. Though fragmentary, fossil evidence, primarily found in North Africa, suggests their existence. 
• Despite contention among primatologists, Plesiadapis is considered by some to be a proto-primate. It is one of the earliest-known primate-like mammals with a relatively robust fossil record. 
• Dating approximately 55 million years ago, these Plesiadapiforms shared specific dental and skeletal characteristics with true primates. They inhabited regions of North America and Europe during the Cenozoic era and persisted until the end of the Eocene period.
• The earliest true primates originated in Africa, Asia, and North America during the Eocene Epoch. These early primates resembled today's prosimians, such as lemurs. During the Oligocene Epoch, anthropoid monkeys evolved from prosimians. By around 40 million years ago, evidence shows that monkeys had spread to both the New World (South America) and the Old World (Africa and Asia). 
• New World monkeys are termed Platyrrhines, reflecting their broad noses, while Old World monkeys are known as Catarrhines, noted for their narrow noses. As the continents of South America and Africa drifted apart, the Platyrrhines arose. New World monkeys first appeared roughly 30 million years ago, with Africa likely being their original homeland. 
• This geographic separation led to reproductive isolation, resulting in distinct evolutionary paths for New World and Old World monkeys. Old World monkeys include arboreal and ground-dwelling species, whereas New World monkeys primarily live in trees.
• Apes evolved from Catarrhines in Africa during the Miocene epoch of the Cenozoic Era, starting about 25 million years ago. Generally larger than monkeys and lacking tails, apes are considered more intelligent due to their relatively larger brain sizes in proportion to their body size. 
• Primates have traditionally been described as evolving adaptations for tree-dwelling, while other mammals adapted to terrestrial or aquatic environments.

3.0Primates Family

Primates are classified into two subgroups: Strepsirhini, which includes lemurs and lorises, and Haplorhini, which encompasses tarsiers, monkeys, apes, and humans.

4.0Classification of Primates

Strepsirhines - Twist nose

• Strepsirhines are generally smaller than monkeys and apes. Most of their digits have flat nails and fleshy pads, except for the second toe, which has a grooming claw. However, the aye-aye lemur is an exception, as it has claws on its toes. 
• Their first fingers and toes are opposable, with the big toe notably separated from the others, allowing for a secure grip during movement. Strepsirhines are characterized by pointed snouts with moist noses, prominent whiskers,  and have highly developed senses of smell and hearing. 
• They are primarily nocturnal and terrestrial animals. Their large eyes are positioned laterally and protected by a bony rim surrounding them from the back and side. A reflective layer called the tapetum lucidum, located behind the retina, enhances their night vision. Strepsirhines have relatively more minor brains. 
• They feature a specialized dental arrangement known as a 'dental comb,' where the incisors and canines are forward-facing and joined to create a comb-like structure for grooming. Typically, they have long, bushy tails and are adept climbers and skilled leapers.

Infraorder- Lemuriformes

• This group includes all lemurs found exclusively in Madagascar and the nearby Comoros Islands. Their body sizes range from the tiny 30-gram pygmy lemur to the sizable 10 kg Indri. Lemuriformes display diverse dietary habits and modes of locomotion. 
• For example, species like the sifaka are skilled leapers, using their elongated hind limbs to grip vertical branches. In contrast, others are moving through the trees on all fours or spending significant time on the ground. Lemuriformes are classified into two superfamilies: Daubentonoidea and Lemuroidea.

Superfamilies - Daubentonoidea

• This superfamily comprises a single subfamily, Daubentonidae, which includes only one genus, Daubentonia (the aye-aye). The aye-aye is a moderately sized, black animal with large ears. It exhibits extreme specialization while retaining some fundamental features that classify it under lemuriformes.

Superfamilies - Lemuridae

• The group includes four families: Chirogaleidae, Indriidae, Lepilemuridae, and Lemuridae. Chirogaleids are the most primitive and most minor of the Lemmuroids. The Indriidae family includes the genera Indri, Propithecus, and Avahi. The Lepilemuridae family consists of the genus Lepilemur. The Lemuridae family contains the genera Lemur, Hapalemur, Prolemur, Eulemur, and Varecia.

Infraorder - Lorisiformes

• Lorisiforms are nocturnal primates found in the forest regions of Africa and Asia. Their diets mainly include fruits, gums/exudates, and insects. Like lemurs, they have a tooth comb and a grooming claw on the second digit. However, in lorisiforms, the tympanic ring in the ear region is fused to the lateral wall, unlike in lemurs where it is suspended in the bulla. This infraorder is divided into two families: Lorisidae and Galagidae.

Haplorrhini - Dry nose/ simple nose

• A new suborder was created to include tarsiers, which were previously classified under Prosimii in the traditional primate classification system. This suborder, called Haplorhini, is considered to comprise the "higher" primates. Haplorrhini includes tarsiers, monkeys, apes, and humans. Scientists believe that haplorrhines first appeared in the Eocene epoch around 50 million years ago and are the ancestors of today's monkeys and apes. The living haplorrhines are divided into three infraorders: Tarsiers, Platyrrhini( new world monkeys), and Catarrhini (old world monkeys). Together, Platyrrhini and Catarrhini are referred to as anthropoids.

Tarsiiformes

• These primates are among the most minor and most unusual of all living primates, found in Southeast Asia. They exhibit a mix of prosimian and anthropoid traits. Some of their primitive features, shared with lower primates, include high-cusped molars, an unfused mandibular symphysis, multiple nipples, and grooming claws on the second digits of their feet. Tarsiers have exceptionally long legs and ankles, and their eyes are unusually large, even more significant than their brains. This infraorder has only one family, Tarsiidae, and currently includes three living genera: Carlito, Cephalopachus, and Tarsius.

Platyrrhini

• Platyrrhines, known as New World monkeys, encompass all anthropoid primates living in Central and South America. The term "platyrrhini" refers to their noses' broad, flat shape, with outward-directed nasal openings distinguishing them from old world monkeys. Living platyrrhines are small to medium-sized anthropoids. They lack the hypoconulid on the first two lower molars present in most old world monkeys and do not have ischial callosities. Most platyrrhines have well-developed tails, some prehensile and function like a fifth limb. Their thumbs have imperfect opposability, and their finger grips need to be better developed, while their large hallux are strongly opposable. Some species have special scent glands, predominantly arboreal and diurnal.
• Traditionally, platyrrhines were divided into two families: Cebidae and Callithricidae. However, recent molecular systematics have reclassified them into three families: Pitheciidae, Atelidae, and Cebidae, comprising 19 living genera.

Caterrhini

• The catarrhines include old world monkeys, apes, and humans. "Caterrhini" refers to the shape of their noses, similar to "Platyrrhini." Catarrhine nostrils are narrow, close together, and face downward, unlike platyrrhine monkeys' round, laterally facing nostrils. Generally, catarrhine monkeys are more prominent than platyrrhine monkeys. Catarrhines have only two premolars in each quadrant, compared to three in platyrrhines. They also have a bony tube between the eardrum and outer ear. Most catarrhines are diurnal and possess a well-developed grip with an opposable thumb and big toe, except in humans. This infraorder is typically divided into two superfamilies: Cercopithecoidea (old world monkeys) and Hominoidea (apes and humans).

Ceropithecoidea

• Cercopithecoids, commonly known as old world monkeys, are the most numerous and diverse group among the living catarrhines. They are found across Africa, Asia, and parts of Europe. This group comprises a single family, Cercopithecidae, which is divided into two subfamilies: Cercopithecinae and Colobinae.

Hominoidea

• The superfamily Hominoidea includes humans and apes. Living hominoids share the characteristic of lacking tails and having relatively primitive rounded molars. In the updated systematic classification, Hominoidea consists of two families: Hylobatidae and Hominidae.

Hylobatidae

• The hylobatids, which include siamangs and gibbons, are the smallest and most abundant of the apes. Due to their diminutive size, they are sometimes called "lesser apes." Hylobatids are found in northeast India and the tropical forests of Southeast Asia.

Hominidae

• This family encompasses the great apes and humans: chimpanzees, orangutans, gorillas, bonobos, and humans. Hominidae is characterized by several key features, including a larger brain relative to body size, sexual dimorphism, relatively larger body size, semi-upright posture, and the consequent differences in skeletal structure.

5.0Characteristics of Primates

According to Mivart, primates are defined as follows: "Primates are clawed, clavicle-bearing placental mammals, characterized by having orbits encircled by bones; three types of teeth at least at one stage of life; a brain that consistently includes a posterior lobe and calcarine fissure; at least one pair of extremities with opposable innermost digits; a hallux with a flat nail or none; a well-developed caecum; a pendulous penis; scrotal testes; and always two pectoral mammae." The main characteristics of primates include:

• Pentadactyl hands and feet:  Except for spider monkeys, which have four fingers on each hand and five toes on each foot, all primates have retained the ancient mammalian trait of pentadactylism.
• Increasing refinement of hands and feet for grasping objects: This characteristic has been a significant hallmark of primate evolution. There is a noticeable trend toward increasing manual dexterity. This feature has been further enhanced by developing susceptible tactile pads on the tips of fingers and toes, which contrasts with most other mammals.
• Presence of flat nails: Primates have flat nails that protect their fingertips, along with dermatoglyphs (fingerprints) on their fingers and toes.
• Presence of opposable thumbs: Primates possess opposable thumbs, allowing them to perform both power grips and precision grips.
• Presence of clavicle and the generalized limb structure of early mammals: Primates have retained the primitive mammalian limb structure, which includes one upper limb bone (such as the humerus in the forelimbs and the femur in the hind limbs) and two lower limb bones (such as the radius and ulna in the forelimbs and the tibia and fibula in the hind limbs). Strong clavicles and highly flexible shoulder joints enable most primates to use their arms efficiently for climbing trees. In contrast, many other mammalian orders have lost various bones, often fusing the two lower limb bones, as seen in horses.
• Erect posture: All primates exhibit a pronounced tendency towards upper body erectness. This is evident in their sitting and standing postures and their occasional bipedalism.
• Reduction in the sense of smell: The significance of the sense of smell has diminished in primates. As a result, the olfactory center of the brain, known as the rhinencephalon, has proportionally decreased in size. Additionally, the skeletal structures related to smell, such as snout length and facial protrusion, have steadily and progressively reduced.
• Increased dependence on visual sensation: Primates' orbits (eye sockets) are highly front-facing, which increases the overlap of visual fields, thereby enhancing binocular vision and depth perception, resulting in stereoscopic vision. All living members of the primate order possess stereoscopic vision. Additionally, most living diurnal primates likely have some form of color vision, which is very advantageous for a daytime lifestyle.
• Relatively large brain, as compared to body size: There is a continuous trend of brain development, marked by increasing elaboration and differentiation of the cerebral cortex. The enlargement of the cerebral cortex is associated with improved manual dexterity and enhanced hand-eye coordination, both of which are significant aspects of primate evolution.
• Reduction in number of teeth:  Primates exhibit a reduced number of teeth compared to primitive mammalian dentition. Typically, they have a maximum of two incisors, one canine, three premolars, and three molars in each jaw quadrant. They maintain a simple, generalized cusp pattern on molars, enabling them to adapt to various food sources.
• Presence of mammary glands: Most primates have one pair of mammae on the chest.
• A trend towards smaller litter size, longer gestation periods and prolonged period of juvenile growth: The gestation period lengthens as we move from more primitive small prosimians like Microcebus and Galago, which have gestation periods of at least four months, to humans, where it extends to nine months. A longer gestation period results in offspring being born more mature, increasing their chances of survival. With reduced litter sizes, more individual attention is given to each offspring, allowing for greater mobility with the young clinging to the mother. Additionally, there's a trend of increasing postnatal growth period from lower to higher primates, ranging from less than 1 year among nocturnal prosimians to approximately 15 years in humans.
• Increase in the natural life span: There is a noticeable trend of significant increases in the natural lifespan of all primates. For instance, tiny mouse lemurs can survive for up to 8 years, whereas rodents of similar size typically live no more than 2 years. Larger primates, such as baboons, with males weighing up to 90 lbs, may live for up to 30 years, whereas domestic dogs of comparable size have a lifespan of only 10-12 years on average. Humans, with an average lifespan of 70 to 80 years, also demonstrate this trend.
• Complex social behavior: Primates demonstrate complex and diverse forms of communication through vocalization, visual displays, and specific social behaviors like infant care and grooming. It's accurate to say that primates are exceptionally social creatures among mammals.