By:E. Christopher Kirk(Department of Anthropology College of Texas at Austin)© Education





What attributes distinguish primates from various other mammals? This question is of main prominence to Biological Anthropology - the research of primate adaptation and evolution.

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Why perform we, as humans, conveniently perceive close similarities between ourselves and also various other primates? The simple answer to this question lies in the truth that human beings and all various other members of the order Primates share a suite of characteristics that they inherited from a common ancestor that lived 10s of millions of years earlier. These attributes have actually been preserved in all or most descendants of that last common ancestor of living primates, so it is possible to enumerate a list of "primate hallmarks" that collectively identify members of the order from various other mammals. Understanding the attribute and also evolutionary history of these mutual attributes is important bereason they carry out a good deal of information about the adaptations of the earliest crvery own primates. Undoubtedly, a lot of of the attributes mutual in common by crvery own primates loss into three categories: (1) adaptations for improved vision; (2) adaptations for manual and also pedal grasping; and (3) adaptations for a slow life background.

All living primates have actually forward-encountering eyes (Figure 1; Johnson, 1901; Cartmill, 1992). In this respect, primates even more carefully resemble cats and also owls than many type of various other mammals (e.g., squirrels or gazelles). Having forward-facing eyes gives primates a large field of binocular vision (Heesy, 2004, 2009). In other words, many of a primate"s visual field is perceived by both eyes all at once. By compariboy, in species via laterally encountering eyes, much of the visual field is monocular (Hughes, 1977). A big binocular visual area in primates is probably an adaptation for amplified depth perception, which is facilitated by the binocular visual cues of vergence and also stereopsis (Walls, 1942; McIlwain, 1996; Tovée, 1996). Although monocular cues (e.g., perspective and also motion parallax) might likewise be used to judge the distance to a visual taracquire, binocular cues are specifically valuable for fine depth perception at fairly close varieties (Barlow & Mollon, 1982; McIlwain, 1996).

Comparichild of a primate (a: sifaka, genus Propithecus) and non-primate mammal (b: jackrablittle, genus Lepus) to show distinctions in eye orientation. The sifaka has actually forward-encountering eyes that produce a vast field of binocular vision straight in front of its head. The jackrablittle has laterally encountering eyes that just create a very narrowhead area of binocular vision in front of and also above its head.
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As a group, primates also have actually high visual acuity (Figure 2). Acute vision in primates is the product of numerous discrete visual adaptations. First, primates have actually bigger eyes than many various other mammals of comparable body dimension (Ross & Kirk, 2007). Having large eyes ensures that a big photo is formed on the retina (Walls, 1942; Land also & Nilsboy, 2002). This huge retinal picture may then be sampled by many type of photoreceptors, improving visual resolution. Second, primates have actually a dense concentration of photoreceptors in the central retina (Kirk & Kay, 2004). Accordingly, as soon as a primate directs its gaze toward a things of interest, the retinal image of that object is sampled by a big variety of tiny and also tightly packed photoreceptors. Third, primates have actually eyes that are more entirely encircled by bone than in many various other mammals (Cartmill, 1980, 1992). In strepsirrhines (i.e., lemurs and also lorises), this bony enclocertain takes the develop of a postorbital bar only, while haplorhines (i.e., tarsiers, chimpanzees, apes, and also humans) likewise possess a postorbital plate (Figure 3). These bony attributes assist stabilize the eyes by insulating them from activities of chewing muscles nearby to the orbit (Cartmill, 1980; Heesy, 2005; Menegaz & Kirk, 2009).

In this plot, day-active haplorhines autumn over the array of all other mammals, while night-active haplorhines (i.e., owl monkeys and tarsiers) and all strepsirrhines overlap the top range of non-primate mammals. Visual acuity was measured in behavioral experiments or calculated based upon eye morphology and ganglion cell density. File taken from: Calderone et al. (2003); Kirk & Kay (2004); Pettiprospered & Manger (2008); Veilleux & Kirk (2009); Pettiprospered et al. (2010); and also Shinozaki et al. (2010).
Anterior views of the crania of a haplorhine primate (a – squirrel moncrucial, genus Saimiri), a strepsirrhine primate (b - bushbaby, genus Galago), and a non-primate mammal (c – raccoon, genus Procyon). The squirrel monkey orlittle is enclosed posteriorly by a postorbital plate (a, arrow). The bushbaby orbit has a postorbital bar (b, arrow) yet lacks a postorbital plate, leaving the orbit open posteriorly. The raccoon orbit lacks both a postorbital bar and also postorbital plate, leaving the orbit open superiorly (c, arrow) and also posteriorly. In both the bushbaby and also the raccoon, the braincase is visible behind the orlittle.

In addition to having actually adaptations for intensified depth perception and visual acuity, primates also have actually large visual regions of the brain (Allman, 1999; Kaas, 2005, 2008). Macaque monkeys, for instance, have actually at least 32 functionally unique areas of the cerebral cortex that are devoted primarily to handling visual indevelopment (Van Essen et al., 1992; Van Essen, 2004). These visual areas comprise 50% of the total macaque cerebral cortex, whereas only 3% of the macaque cerebral cortex is devoted mainly to auditory functions. Many type of non-primate mammals have proportionately smaller visual regions of the brain and also fewer functionally unique cortical visual locations (Kaas, 2005, 2008). The comparatively big location of the primate brain devoted to vision is partly explained by the fact that primates need to process a bigger amount of visual "input" to the brain than many other mammals (Kirk, 2006).

Most mammals (e.g., dogs, deer) are unable to pick up and host an item with a solitary hand or foot. By comparison, a lot of living primates share a variety of adaptations that make their hands and feet prehensile (Figure 4). First, the fingers and also toes of primates are typically lengthy compared to the size of the palm or sole, permitting the digits to partially or entirely encircle an object being grasped (Lemelin, 1999; Jungers et al., 2005; Kirk et al., 2008). Second, the hallux is commonly abducted and might therefore be opposed against the single and also the various other pedal digits in order to understand an item (Cartmill, 1985). Third, the palms of the hand also and also the soles of the feet are hairless, permitting these surfaces to much better adright here to objects being grasped. Fourth, primates have actually fingers and toes tipped by hairless apical pads. These apical pads have actually friction ridges and also are richly innervated through mechanoreceptors (Cartmill, 1974, 1985; Hoffmale et al., 2004). Fifth, the fingers and also toes of primates usually have flattened nails instead of clegislations (Soligo & Müller, 1999). As an outcome of these qualities, primates tend to usage their hands and also feet in methods that differ fundamentally from most other mammals (Cartmill, 1985). For instance, while an arboactual squirrel might usage its pointed cregulations to cling to a tree trunk or large branch, arbogenuine primates generally use their prehensile hands and feet to grasp smaller-diameter supports.

(a) Two views of the hand also of a primate (sifaka, genus Propithecus), showing the nails, hairmuch less palmar skin via friction ridges, and also big apical pads on the digits. (b) Comparikid of the hand and also foot skeleloads of a primate (bushbaby, genus Galago) and a non-primate mammal (tree shrew, genus Tupaia). From left to right: bushbaby hand; bushbaby foot; tree shrew hand; tree shrew foot. Keep in mind that the bushbaby, prefer all strepsirrhine primates, has a claw on its second pedal digit that it provides for grooming. All other digits of the bushbaby are tipped by nails. Unprefer the bushbaby, the tree shrew has clegislations on all digits, shorter fingers/toes, and lacks an abducted hallux.
The crucial occasions in an organism"s lifeexpectations are very closely interconnected in their timing (Harvey and also Clutton-Brock, 1985). Many kind of non-primate mammals (e.g., shrews and mice) screen the quick advancement, early reproduction, huge litter sizes, and also brief lifespans characteristic of species via a "fast" life background. By comparichild, primates generally exhibit a suite of developmental and also physiological features connected with a "slow" life background (Charnov & Berrigan, 1993; Ross, 1998; Mumby & Vinicius, 2008). First, primates mature gradually. They have lengthy gestation lengths and also lengthy durations of infant dependency. Primates likewise require a lengthy duration of time in between birth and adulthood, and also thus have actually late eras of initially remanufacturing. Second, primates generally give birth to tiny litters (typically single or twin births) and have long interbirth intervals. As a result, the full variety of offspring developed by an individual primate throughout its lifetime has a tendency to be reasonably small. Third, primates tend to be long-lived compared to various other mammals. For example, the maximum lifespan of a computer mouse lemur is 15 years. By contrast, the maximum lifeexpectancy of the similar-sized spiny pocket computer mouse or area mouse is just 2 years and also 4 years, respectively (Turbill & Ruf, 2010). Because many type of behavioral, ecological, and also physiological components are correlated via these life background variables, the question of why primates as a group have progressed slow-moving life histories is facility. However, it is most likely that primate life histories are affected by the reality that the majority of primates are social and have moderate- to large-sized brains (Smuts et al., 1987; Byrne & Whiten, 1988; Kappeler & Pereira, 2003). Unprefer non-social mammals, primates may delay maturation in order to provide a longer duration of time for social learning (Barrickmale et al., 2008). The cognitive requirements of sociality may also choose for raised brain dimension and knowledge (Byrne & Whiten, 1988). Increased brain dimension, in turn, typically only evolves in the context of sluggish life history (van Schaik & Deaner, 2003).
The well-known literature frequently contains misleading information around the characteristic attributes shared by crown primates. For instance, it is not unprevalent to encounter the assertion that all primates have exceptionally big brains. This statement is an overgeneralization. As a team, haplorhine primates carry out tend to have actually exceptionally large brains family member to body size (Martin, 1990). In this respect, haplorhines are unusual yet not distinct, since some other mammals (e.g., whales and dolphins) have comparable or higher levels of encephalization (Allmale, 1999). By contrast, strepsirrhine primates tend to have actually brain sizes that are basically "average" for placental mammals of comparable body mass. Strepsirrhines as a team thus exhilittle a selection of encephalization equivalent to that of many type of other mammalian groups (e.g., carnivores and artiodactyls; Jerison, 1973; Passingham, 1982).

One might also encounter the twin assertions that all primates have a poor feeling of smell and also that only primates have binocular vision. Although haplorhines perform indeed have a reduced olfactory device, strepsirrhines have olfactory anatomy that is not fundamentally various from that of many various other mammals (Cavern, 1973; Smith et al., 2001, 2007). Similarly, binocular vision appears to be almost universal among mammals (Hughes, 1977; Heesy, 2004). What differs between species is not the presence of binocular vision, but rather the size of the binocular visual field. Even rabbits, with laterally dealing with eyes and also 360˚ panoramic vision, have actually a narrowhead area of binocular vision (Hughes, 1977).

With regard to clintends of primate exceptionalism, it must be detailed that there is just one anatomical attribute that is well-known to be present in all living primates but is absent in all other living groups that have actually been stupassed away. In many mammals, the middle ear has a bony floor referred to as an auditory bulla (Figure 5). Primates are unique among living mammals in having an auditory bulla that is entirely formed by the petrosal bone (MacPhee, 1981). While auditory bullae have probably advanced multiple times independently in mammals to protect the contents of the middle ear, the specific identification of the bone that forms the bulla has no clear adaptive meaning.

a: Inferior check out of the basicranium of a primate (lemur, genus Eulemur) reflecting the auditory bullae developed by the petrosal bones. The specimen’s best auditory bulla (presented on the left side) is opened to disclose the middle ear air space and also the ring-shaped bone (ectotympanic) that supports the eardrum. b: Inferior see of the basicranium of a non-primate mammal (cat, genus Felis) showing auditory bullae that build from the ectotympanic bones and also independent centers of ossification (entotympanic bones). The specimen’s left bulla (presented on the best side) is opened to expose the middle ear air room, which is divided right into 2 adjacent chambers by a thin bony septum.
Although crown primates have actually many type of equivalent attributes because they share a prevalent ancestor, most of these attributes have actually not been universally preserved in living species. Humans, for example, no much longer possess an abducted hallux bereason of selection for reliable terrestrial bipedalism. Similarly, callitrichine primates have actually re-advanced clegislations on their digits to facilitate clinging to tree trunks while feeding on exudays. It is additionally necessary to recognize that various other groups of mammals have actually individually progressed many kind of of the features mutual by crvery own primates (a procedure dubbed homoplasy; Futuyma, 1998). For instance, a variety of arbogenuine rodents and marsupials have progressed prehensile feet through an abducted hallux and also big apical pads (Cartmill, 1985; Nowak, 1991). In addition, the characteristic functions of crvery own primates more than likely did not evolve concurrently in the primate stem lineage. Of the many kind of features discussed above, just one (long hands-on digits) is well-known with certainty to be current the earliest possible stem primates - the plesiadapiforms (Bloch & Boyer, 2003; Bloch et al., 2007). Accordingly, if plesiadapicreates are stem primates, then the various other characteristic features of crvery own primates have to have evolved after long digits first appeared.

adaptations - features of an organism that rise the likelihood that it will successfully survive and develop healthy and balanced offspring

abducted - disinserted ameans from the midline; an abducted hallux diverges from the various other toes and facilitates the capability of the foot to master an item, such as a branch

arboreal - living mainly or exclusively in trees

artiodactyls - hoofed mammals through an also number of toes; e.g., gazelle, deer, cows, pigs

auditory bulla - a bony importance beneath the cranium of most mammals that forms the floor of the middle ear space

binocular - viewed by both eyes simultaneously

bipedalism - locoactivity on 2 limbs

callitrichine - member of a household of small Neotropical primates, consisting of marmosets, pygmy marmosets, tamarins, and also lion tamarins

cerebral cortex - the external extending of the biggest part of the brain (cerebrum) in the majority of mammals; typically described as the brain"s "grey matter"

crvery own primates - the group that has all the descendants (both living and also extinct) of the last prevalent ancestor of living primates

depth perception - the capability to judge the distance to a things utilizing visual cues

encephalization - brain size measured loved one to body size

exudates - substances, such as gums or resins, that drainpipe from damaged plant tissues

friction ridges - fine ridges developing on the surconfront of the hairmuch less palms, soles, and also digits of some mammals; these ridges increase adhesion in between the skin and also an item being grasped

gestation - the period of mammalian breakthrough from conception to birth

hallux - the excellent toe

haplorhines - a team of connected primates that has tarsiers, chimpanzees, apes, and humans

homoplasy - the independent advancement of comparable features in sepaprice evolutionary lineages; these attributes may evolve via similar developmental pathmethods ("evolutionary parallelism") or various developpsychological pathmeans ("evolutionary convergence")

infant dependency - the period of mammalian advance from birth to the suggest at which the offspring may efficiently survive independent of its mother

interbirth interval - the average time between succeeding births

laterally - encountering amethod from the midline of the body

life history - the series of successive transforms that an organism undergoes in the time of its advancement from conception to death

manual - of, or relating to, the hands

mechanoreceptors - cells in the skin concerned the feeling of touch that respond to mechanical stimuli (e.g., pressure, stretch, vibration)

middle ear - an air-filled room between the ear drum and cochlea (body organ of hearing) that contains the three tiny bones (malleus, incus, and also stapes) that transmit acoustic vibrations to the inner ear

monocular - viewed by just one eye

activity parallax - a monocular cue for depth perception in which close to objects show up to move farther family member to an observer in motion than do much objects

orbit - the bony eye socket

pedal - of or relating to the feet

perspective - a monocular cue for depth perception in which parallel lines appear to converge towards the horizon

petrosal - a bone that surrounds the inner ear in all mammals and also develops the finish auditory bulla in crvery own primates

photoreceptors - light-sensitive cells (i.e., rods and also cones) in the retina

prehensile - referring to the capability to organize and manipulate an item using a solitary appendage

Primates - a mammalian taxonomic group (order) that has the living lemurs, lorises, tarsiers, primates, apes, and humans

plesiadapiforms - a group of extinct mammals that may be stem primates; all known plesiadapicreate species are even more than 38 million years old

retinal image - the picture projected onto the retina by the eye"s cornea and lens

social - living in semi-steady teams identified by established relationships in between individuals that are based upon recurring interactions

stem - describes the extinct members of an evolutionary family tree that branched from the family tree prior to the appearance of the last common ancestor of the living members of that lineage

stereopsis - the perception of depth and three-dimensionality that is created once the brain combines the slightly various imperiods from the left and appropriate eye; one binocular cue for depth perception

strepsirrhines - a group of related primates that consists of lemurs and also lorises

vergence - the activity of the eyes towards the midline when focusing on near objects; one binocular cue for depth perception

visual acuity - the ability to detect visual details; likewise dubbed "readdressing power"

visual field - the complete area of area perceived by an organism"s eyes

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