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Simian

Simians, also known as anthropoids or higher primates, are the primates of the infraorder Simiiformes, which encompasses all monkeys and apes, including humans.^[1] This diverse group, comprising over 300 species, originated approximately 43 million years ago during the Eocene epoch and is characterized by advanced features such as forward-facing eyes enabling stereoscopic vision, grasping extremities, and relatively large brains relative to body size.^[2]^[3] Taxonomically, Simiiformes falls within the suborder Haplorhini of the order Primates, diverging from the tarsier lineage around 60 million years ago.^[1] The infraorder is divided into two main parvorders: Platyrrhini (New World monkeys, with flat noses and typically prehensile tails) and Catarrhini (Old World monkeys, apes, and humans, with downward-pointing nostrils).^[3] Key anatomical traits distinguishing simians include a rounded head with a mobile neck and expressive face, fused frontal bones in the skull, dental formulae of 2.1.3.3 in Platyrrhini and 2.1.2.3 in Catarrhini, and flattened nails on digits (except in Callitrichidae) rather than claws.^[3]^[4] These features support their predominantly arboreal or terrestrial lifestyles, complex social structures, and enhanced sensory capabilities, particularly vision over smell.^[3] The term "simian" derives from the Latin simia, meaning "ape," reflecting the group's resemblance to these animals in scientific nomenclature.^[5] Evolutionarily, simians represent a monophyletic clade with a near-constant speciation rate, making them one of the most intensively studied groups in biology due to their genetic diversity and proximity to humans.^[2] Fossil evidence traces their radiation to Africa and South America, with subsequent diversification leading to the rich biodiversity observed today across tropical and subtropical habitats.^[3]

Definition and Overview

Etymology and Terminology

The term "simian" derives from the Latin simia, meaning "ape," which traces back to the Greek simos, denoting "snub-nosed" or "flat-nosed," reflecting early observations of primate facial features.^[6]^[5] It first appeared in English around 1600, initially used to describe traits or behaviors characteristic of apes and monkeys in scientific and descriptive contexts.^[6] In the 18th century, Swedish naturalist Carl Linnaeus formalized the term within taxonomy in his Systema Naturae (1758), where he established the order Primates and included the genus Simia to classify various apes and monkeys alongside Homo (humans), Lemur (prosimians), and Vespertilio (bats).^[7] This usage marked an early scientific application, grouping Simia species based on shared anatomical similarities to humans, though Linnaeus's system did not yet distinguish higher primates sharply from more primitive forms.^[7] By the 19th century, as comparative anatomy advanced, "simian" evolved to denote the higher primates (Anthropoidea or Simiiformes in modern terms), explicitly contrasting them with prosimians like lemurs, which were seen as more primitive.^[8] This distinction gained prominence in works such as Thomas Huxley's Evidence as to Man's Place in Nature (1863), where he described human brain convolutions and skeletal features as exhibiting "distinctly Simian peculiarities," emphasizing evolutionary links between humans and apes over prosimians.^[9] In 19th-century texts on comparative anatomy, such as those by Richard Owen, the term frequently appeared to highlight shared traits like forward-facing eyes and grasping hands among simians, reinforcing their separation from prosimian lineages in evolutionary discussions.

Scope and Distinctions from Other Primates

Simians, also known as anthropoids or higher primates, are the primates of the infraorder Simiiformes (synonym Anthropoidea) within the suborder Haplorhini of the order Primates, encompassing the parvorder Platyrrhini (New World monkeys) and the parvorder Catarrhini (Old World monkeys of superfamily Cercopithecoidea and apes of superfamily Hominoidea).^[5]^[10] This grouping represents the higher primates, characterized by advanced adaptations that distinguish them from more primitive primate lineages. In scientific taxonomy, humans (family Hominidae) are included as part of the Hominoidea within Simiiformes, reflecting their shared evolutionary ancestry with other apes and monkeys.^[1] However, in non-technical or colloquial usage, the term "simian" often excludes humans, referring specifically to non-human monkeys and apes to emphasize their animal-like traits.^[5] Key distinguishing traits of simians from strepsirrhines (the suborder including lemurs, lorises, and galagos, often called prosimians) include a fused mandibular symphysis for enhanced jaw strength, a dry nose without a moist rhinarium, and a complete bony postorbital septum that fully encloses the eye orbit for improved vision protection.^[11] Simians exhibit reduced reliance on olfaction, with smaller olfactory bulbs in the brain, and instead emphasize visual acuity through forward-facing eyes providing stereoscopic vision and trichromatic color perception in many species.^[12] Additionally, simians possess grasping hands and feet with flat nails on all digits (rather than claws), enabling precise manipulation, and they tend to have larger body sizes and brains relative to strepsirrhines, supporting more complex social behaviors and diurnal lifestyles.^[13] Debates on human inclusion in simians arise from tensions between cladistic and traditional classifications. In cladistic phylogeny, which prioritizes monophyletic groups based on shared derived traits and common ancestry, humans are unequivocally simians as descendants of the Simiiformes infraorder, forming a clade with apes and monkeys.^[14] Traditional views, influenced by Linnaean hierarchies and cultural distinctions, sometimes segregate humans into a separate category (e.g., as the sole member of Hominina), treating simians as non-human to highlight perceived exceptionalism in cognition and bipedalism.^[15] This distinction persists in some older literature but is increasingly rejected in modern systematics favoring evolutionary continuity.^[1]

Evolutionary History

Origins and Fossil Record

Simians, members of the infraorder Simiiformes (also known as Anthropoidea), first appeared during the Eocene epoch, with the earliest undisputed fossils dating to around 45 million years ago in Asia. The genus Eosimias, discovered in middle Eocene sediments of the Yuanqu Basin in Shanxi Province, China, represents one of the oldest known simians, characterized by its small size (approximately 100 grams), a dental formula of 2.1.3.3, and postcranial elements indicating agile arboreal locomotion. These traits suggest Eosimias occupied an intermediate position in the transition from prosimian-like ancestors to more derived simians, supporting an Asian origin for the group followed by dispersal to other continents.^[16] Fossil evidence from Africa indicates that simians had reached the continent by the late middle Eocene, approximately 37 million years ago. In northern Africa, particularly Algeria, specimens of Biretia piveteaui from the Bir El Ater locality exhibit primitive simian dental features, such as reduced upper premolars and a hypocone on molars, which align with early simian morphology while retaining some prosimian resemblances. The fossil record of simians becomes more robust in the late Eocene and early Oligocene, highlighting their diversification. A pivotal discovery is Aegyptopithecus zeuxis from the Fayum Depression in Egypt, dated to approximately 33 million years ago, which includes nearly complete skulls and postcrania revealing catarrhine affinities, such as a short face and Y-5 molar pattern, marking it as a stem Old World monkey or ape. This find, along with contemporaneous parapithecids and propliopithecids from the same region, underscores Africa's role in the subsequent radiation of simian lineages.

Key Evolutionary Developments

One of the pivotal events in simian evolution was the divergence of Platyrrhini (New World monkeys) from Catarrhini (Old World monkeys and apes), estimated at approximately 43 million years ago during the Eocene.^[17] This separation likely occurred through trans-Atlantic rafting, where ancestral simians from Africa were transported across the widening South Atlantic Ocean on floating vegetation mats, facilitated by favorable paleoceanographic conditions such as equatorial currents and lower sea levels.^[18] Following this dispersal, Platyrrhini underwent adaptive radiation in South America, diversifying into various arboreal niches; recent fossils, such as the eosimiid Perupithecus from Amazonia (~36 million years ago), indicate early colonization and support multiple dispersal events.^[19] Meanwhile, Catarrhini continued to evolve in Afro-Eurasia, setting the stage for further simian complexity.^[20] During the Miocene epoch (23-5 million years ago), the superfamily Hominoidea (apes) emerged and radiated prominently, marking a significant adaptive shift within Catarrhini. Early hominoids, such as those from the Proconsul group, exhibited key innovations including taillessness, which reduced weight for suspensory locomotion, and enhanced shoulder mobility supporting brachiation—arm-swinging travel through forest canopies. These traits, evident in middle Miocene fossils from Europe and Africa, facilitated exploitation of higher forest strata and vertical clinging, contributing to the ecological success and morphological diversity of apes amid expanding tropical woodlands.^[21] This radiation contrasted with the more quadrupedal cercopithecoids, highlighting hominoid specialization for orthogrady and suspension. The human lineage diverged from other apes around 6-7 million years ago in Africa, initiating the hominin clade with bipedalism as a defining derived trait.^[22] Fossils like Sahelanthropus tchadensis suggest early upright posture, inferred from the anteriorly positioned foramen magnum, enabling habitual terrestrial walking while retaining some arboreal capabilities.^[23] This locomotor adaptation, evolving post-split from the chimpanzee lineage, likely arose in response to changing savanna environments, freeing the hands for tool use and enhancing energy efficiency over long distances.^[22] Bipedalism thus represents a simian innovation that profoundly influenced subsequent cognitive and cultural developments in the hominin line.

Taxonomy and Classification

Higher-Level Placement

Simians, formally comprising the infraorder Simiiformes, are classified within the order Primates and the suborder Haplorhini. This placement distinguishes them from the other major suborder, Strepsirrhini, which includes lemurs, galagos, and lorises. Within Haplorhini, Simiiformes represents a monophyletic clade sister to the infraorder Tarsiiformes, encompassing the tarsiers; this relationship is supported by molecular and morphological evidence, including shared synapomorphies such as forward-facing eyes and enhanced visual acuity. Molecular clock analyses, calibrated with fossil constraints, estimate the divergence between Simiiformes and Tarsiiformes at approximately 66 million years ago, marking a key event in haplorhine radiation shortly after the Cretaceous-Paleogene boundary.^[24] Cladistic classifications emphasize the monophyly of Simiiformes, which includes New World monkeys (Platyrrhini), Old World monkeys (Cercopithecoidea), apes (Hominoidea), and humans (Hominidae), reflecting their shared common ancestry. In contrast, traditional Linnaean hierarchies often rank humans separately at higher taxonomic levels, such as in a distinct superfamily or order, which can render "simians" paraphyletic if humans are excluded, as the latter are phylogenetically nested within the group.^[25] This tension highlights ongoing debates in taxonomy between evolutionary phylogeny and historical ranking systems.

Major Subgroups and Families

Simians, or Anthropoidea, are divided into two main parvorders: Platyrrhini (New World monkeys) and Catarrhini (Old World monkeys, apes, and humans). This classification reflects their evolutionary divergence, with Platyrrhini originating in South America and Catarrhini in Africa and Asia. Together, these groups encompass approximately 300 simian species, representing the majority of primate diversity, with ongoing genetic studies in regions like Amazonia leading to recent taxonomic revisions and new species descriptions in the 2010s and beyond. Taxonomic counts can vary due to debates over species versus subspecies status.^[26]^[27] The parvorder Platyrrhini comprises five families: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae, distributed across 24 genera and approximately 189 species as of late 2024 (excluding subspecies). These New World monkeys exhibit high diversity in body size, locomotion, and social structures, adapted to Neotropical forests. For instance, the family Cebidae includes capuchins (genera Cebus and Sapajus) and squirrel monkeys (Saimiri), totaling about 25 species known for their tool use and omnivorous diets. Callitrichidae features marmosets (e.g., genera Callithrix and Cebuella) and tamarins, small-bodied primates with claw-like nails for tree gouging, encompassing around 42 species. Recent genetic analyses have added at least 15 new Platyrrhini species since 2012, primarily in Amazonian genera like Mico (e.g., Mico munduruku in 2019) and Plecturocebus (e.g., Plecturocebus grovesi in 2019), highlighting cryptic diversity in southern Amazonia.^[27]^[27]^[27] The parvorder Catarrhini is further subdivided into the superfamily Cercopithecoidea (Old World monkeys) and Hominoidea (apes and humans). Cercopithecoidea consists of the family Cercopithecidae, which includes 24 genera and 138 species across two subfamilies: Cercopithecinae (e.g., baboons, macaques, and guenons) and Colobinae (e.g., colobus and langurs). This family dominates Old World primate diversity, with species like the olive baboon (Papio anubis) and rhesus macaque (Macaca mulatta) exemplifying terrestrial and arboreal adaptations in African and Asian savannas and forests. Hominoidea encompasses two families: Hylobatidae and Hominidae. Hylobatidae, the lesser apes or gibbons, includes four genera (Hylobates, Hoolock, Nomascus, and Symphalangus) and 20 species, characterized by brachiation and small size, such as the siamang (Symphalangus syndactylus). Hominidae, the great apes and humans, comprises four genera (Pongo for orangutans, Gorilla for gorillas, Pan for chimpanzees and bonobos, and Homo for humans) and eight species, noted for larger body sizes, complex cognition, and knuckle-walking in non-human members. This division underscores the apes' tailless morphology and advanced social behaviors relative to monkeys.^[28]^[29]

Physical Characteristics

Anatomy and Morphology

Simians (Anthropoidea), members of the infraorder Simiiformes, exhibit a distinctive cranial morphology characterized by a complete postorbital closure, where a bony septum fully separates the orbit from the temporal fossa, providing structural support for the eyes. This feature is a defining synapomorphy of anthropoids, distinguishing them from strepsirrhines.^[30] Additionally, simian skulls feature a relatively large braincase compared to body size, with encephalization quotients higher than those of prosimians, reflecting expanded neocortical regions; for instance, early Miocene catarrhines like Aegyptopithecus show brain volumes exceeding those of similarly sized strepsirrhines.^[30] The limb anatomy of simians is adapted for arboreal locomotion, with elongated fore- and hindlimbs that facilitate grasping and suspension among branches. All simians possess opposable thumbs and hallux (big toes), enabling a precision grip through flexible joints and broad digital pads, though the thumb is reduced in some New World monkeys like spider monkeys to enhance hook-like swinging.^[31] A notable variation occurs in certain New World monkeys (Platyrrhini), where the tail is prehensile, functioning as a fifth limb with tactile pads and underlying musculature for gripping, as seen in capuchins and howler monkeys.^[31] Simian dentition supports an omnivorous diet through a heterodont formula that varies by subgroup but emphasizes incisors for cropping, canines for puncturing, and cheek teeth for grinding. Old World simians (Catarrhini), including monkeys, apes, and humans, typically have a dental formula of 2.1.2.3 / 2.1.2.3, with two premolars per quadrant facilitating efficient mastication of fruits, leaves, and insects. In contrast, New World monkeys (Platyrrhini) generally follow a 2.1.3.3 / 2.1.3.3 formula, featuring an additional premolar that aids in processing tougher foliage.

Sensory and Locomotor Adaptations

Simians exhibit advanced visual capabilities that distinguish them from many other mammals, primarily through trichromatic color vision achieved via specialized opsin genes. In most simians, particularly Old World monkeys and apes, three types of cone photoreceptors enable discrimination of short-, medium-, and long-wavelength light, allowing for enhanced detection of ripe fruits and foliage in arboreal environments. This trichromacy evolved through duplication of the long-wavelength-sensitive (LWS) opsin gene on the X chromosome, providing routine color vision in males and females alike, unlike the polymorphic system in many New World monkeys where only some females achieve trichromacy.^[32]^[33]^[34] Accompanying this visual enhancement is a reduced reliance on olfaction, marked by the degeneration of olfactory receptor genes concurrent with the emergence of full trichromatic vision. Primates, including simians, have undergone a significant contraction in their olfactory repertoire compared to other mammals, with Old World simians showing particularly accelerated loss of these genes, shifting sensory emphasis toward vision for foraging and navigation. This evolutionary trade-off underscores the prioritization of visual cues over chemical signaling in diurnal, fruit-dependent lifestyles.^[35]^[36] Locomotor adaptations in simians vary by subgroup, reflecting diverse arboreal and terrestrial demands. Monkeys predominantly employ quadrupedalism, utilizing all four limbs for balanced progression along branches or on the ground, supported by elongated limbs and grasping hands and feet. In contrast, apes, especially gibbons, specialize in brachiation, a form of arm-swinging suspension that propels the body between supports using highly mobile forelimbs. Great apes, such as chimpanzees and gorillas, incorporate knuckle-walking during quadrupedal movement, flexing their fingers to support weight on the dorsal surfaces of the middle phalanges while maintaining a stable posture.^[37]^[38] Humans, in particular, exhibit obligate bipedalism supported by adaptations such as a forward-positioned foramen magnum, an S-shaped vertebral column, and a valgus knee angle for efficient upright locomotion.^[39] Enhancements to balance and agility further facilitate simian tree-dwelling lifestyles, particularly through modifications in the vestibular system and shoulder complex. The semicircular canals of the inner ear, which detect angular head accelerations during movement, show morphological variations correlated with locomotor modes; for instance, apes exhibit relatively larger canal radii compared to monkeys, aiding gaze stabilization and postural control in suspensory behaviors. Flexible shoulder joints, characterized by a shallow glenohumeral articulation and robust rotator cuff musculature, enable extensive circumduction and suspension in apes, allowing efficient navigation through discontinuous arboreal supports. These adaptations collectively enhance agility in three-dimensional forest canopies.^[40]^[41]^[42]

Distribution and Ecology

Geographic Range

Simians, encompassing the parvorders Platyrrhini and Catarrhini, exhibit a bifurcated global distribution reflective of their evolutionary divergence. The Platyrrhini, or New World monkeys, are endemic to the Neotropical region, occupying territories from southern Mexico through Central America and extending southward to northern Argentina. This exclusive range spans diverse ecosystems across the Americas, with no native presence elsewhere.^[29]^[43] In contrast, the Catarrhini, comprising Old World monkeys and apes (including hominoids), are native to the Paleotropical realm, primarily sub-Saharan Africa and continental Asia, with some taxa reaching North Africa, the Arabian Peninsula, and Southeast Asian islands. Their distribution highlights a concentration in tropical and subtropical zones of the Old World, underscoring ancient biogeographic barriers like the Atlantic Ocean that separated them from New World counterparts. Additionally, human-mediated introductions have established non-native populations in Europe, most notably the Barbary macaques (Macaca sylvanus) on the Rock of Gibraltar, where approximately 230 individuals persist in a semi-wild state.^[44]^[45] The fossil record reveals a more expansive historical range for simians during the Eocene and Oligocene epochs (approximately 56–23 million years ago), when early anthropoid primates—precursors to modern simians—appeared and diversified beyond their current locales. Evidence of these early forms exists in North America, where primates thrived until their regional extinction around 35 million years ago, and in Europe, where Oligocene deposits yield fossils indicating survival through climatic shifts. Such distributions contrast with the later restriction to equatorial regions, influenced by global cooling and faunal turnovers.^[46]^[47]^[48]

Habitat Preferences and Adaptations

Simians, encompassing monkeys and apes, predominantly exhibit arboreal lifestyles, with the majority inhabiting tropical rainforests that support complex canopy structures essential for their survival. Approximately 90% of nonhuman primate species rely on these tropical forest habitats, which provide abundant resources and protection from ground-based predators.^[49]^[50] Key physiological adaptations include fur coloration patterns that enhance camouflage against foliated backgrounds, reducing visibility to predators in dense vegetation.^[51] Additionally, arboreal simians employ heat-dissipating behaviors, such as seeking shaded microhabitats and reducing activity during peak solar radiation, to manage thermal stress in humid, high-temperature environments.^[52] Certain simian lineages have transitioned to more terrestrial habits, particularly in open savanna ecosystems, where species like baboons (Papio spp.) and some macaques (Macaca spp.) spend significant time on the ground. These adaptations include robust limb morphology, with strengthened fore- and hindlimbs that facilitate efficient quadrupedal locomotion over varied terrain and support defensive maneuvers against predators.^[13] Such terrestrial shifts enable exploitation of grassy habitats lacking dense tree cover, contrasting with the arboreal dominance of most simians.^[50] New World monkeys (Platyrrhini) demonstrate remarkable altitudinal versatility, occupying habitats from sea level to high elevations in the Andean cordillera. For instance, the Andean night monkey (Aotus miconax) thrives in cloud forests up to 3,100 meters, where adaptations to cooler, mist-laden conditions include denser fur insulation and behavioral adjustments to maintain thermal balance.^[53] This elevational range highlights simian flexibility in responding to topographic gradients, though extreme altitudes remain limited to specialized species.^[54] Simians display diverse social organizations, ranging from solitary lifestyles to complex multi-male/multi-female troops. Orangutans exemplify solitary living among apes, with adults typically foraging independently and interacting primarily during mating or extended mother-offspring associations. In stark contrast, gelada baboons form some of the largest stable groups in the order, with troops comprising up to 200 individuals organized into hierarchical units such as one-male harems and bachelor groups.^[55]^[56]^[55] Dominance hierarchies structure most simian societies, with rank determined by factors including age, sex, and kinship ties. These systems grant higher-ranking individuals priority access to resources and mates, enforced through ritualized displays like threats, chases, and vocalizations, as well as supportive alliances among relatives or affiliates. For instance, in baboons and macaques, matrilineal kin groups play a key role in maintaining female hierarchies, while male ranks often shift with immigration and coalitions.^[55]^[55]^[55] Variations in group dynamics highlight adaptive flexibility across simian taxa. Chimpanzees inhabit fission-fusion communities of 20 to 150 members, where subgroups dynamically split and reform daily in response to resource distribution and social opportunities, fostering fluid alliances and territorial patrols. Macaques, by comparison, sustain stable, cohesive troops with fixed multi-male/multi-female compositions, promoting enduring bonds and predictable interactions within the group.^[57]^[55]

Diet, Foraging, and Reproduction

Simians exhibit a diverse dietary spectrum, with frugivory serving as the primary feeding strategy for the majority of species, where ripe fruits often constitute 50-70% of the ingested dry matter depending on availability and habitat. This reliance on fruits provides essential carbohydrates, lipids, and vitamins, but is supplemented by foliage such as leaves and flowers, which offer protein and fiber, as well as insects, seeds, and occasionally vertebrate matter. For instance, Old World monkeys like baboons (Papio spp.) are notable omnivores, incorporating a broader range of animal proteins including small mammals, birds, and invertebrates into their diet to meet nutritional needs during periods of fruit scarcity.^[58]^[59] Foraging behaviors in simians are adapted to exploit these varied resources efficiently, often involving group coordination and opportunistic strategies influenced by seasonal resource fluctuations. New World monkeys such as capuchins (Cebus and Sapajus spp.) demonstrate advanced tool use, employing stones to crack nuts or dig for underground tubers and invertebrates, which enhances access to high-energy foods otherwise unavailable. In some species, including certain colobine monkeys in montane forests, foraging involves seasonal altitudinal movements to track fruiting patterns, allowing groups to shift elevations by hundreds of meters in response to resource distribution without permanent relocation. These techniques underscore the cognitive and locomotor flexibility that enables simians to optimize energy intake across dynamic environments.^[60]^[61] Reproductive biology in simians is characterized by relatively long gestation periods ranging from 4 to 9 months, varying by taxon: for example, approximately 5.5 months in macaques (Macaca spp.), 8 months in chimpanzees (Pan troglodytes), and 8.5 months in gorillas (Gorilla spp.). Single births predominate across simian species, with twins rare except in callitrichids like marmosets, reflecting an evolutionary emphasis on investing in fewer offspring with extended parental care. Marked sexual dimorphism, particularly in body size and canine development, plays a key role in shaping mating systems; in highly dimorphic species like gorillas, where males can be twice the size of females, polygynous systems prevail, with dominant silverback males monopolizing breeding access to multiple females in stable groups.^[62]^[63]^[64]

Conservation and Human Interaction

Threats and Status

Non-human simians face severe conservation challenges, with nearly two-thirds of the approximately 530 assessed non-human primate species classified as threatened on the IUCN Red List as of 2025.^[65] This high level of endangerment stems primarily from anthropogenic pressures, with habitat loss due to deforestation affecting the majority of species; for instance, agricultural expansion, logging, and infrastructure development have driven declines in over 60% of primate populations globally.^[66] In Asia, where simian endemism is high, approximately 84% of primate species are threatened, largely by forest conversion.^[67] Beyond habitat destruction, hunting for bushmeat and the illegal pet trade pose acute risks, particularly in Africa and Southeast Asia, where these activities have decimated populations of species like the Cross River gorilla and Tapanuli orangutan.^[68] Disease transmission from humans exacerbates these threats; Ebola outbreaks, for example, have caused up to 90% mortality in western lowland gorilla groups, contributing to a 60% population decline over the past two decades and elevating their status to critically endangered. Similarly, yellow fever has recently impacted species like the northern muriqui in Brazil.^[68] Population trends reflect these pressures, with 75% of primate species experiencing declines, often ranging from 50% to over 80% in the last 40 years due to combined habitat fragmentation and exploitation.^[69] For the Tapanuli orangutan, habitat has shrunk to about 2.5% of its historical range, with ongoing poaching threatening the remaining approximately 800 individuals as of 2025.^[68]^[70] However, conservation interventions offer some optimism; reintroduction programs have bolstered the golden lion tamarin population from near extinction in the 1980s to over 4,800 individuals as of 2025 through habitat protection and captive breeding releases in Brazil's Atlantic Forest.^[71]^[72] The Cao-vit gibbon has also shown stability, with recent group formations indicating potential recovery in protected Vietnamese forests.^[68] The 2025 edition of the World's 25 Most Endangered Primates list continues to highlight acute threats but notes some progress, with a few species removed due to improved conservation outcomes.^[73]

Role in Research and Culture

Simians have played a pivotal role in biomedical research, particularly in vaccine development and genetic studies. Rhesus macaques were instrumental in the testing and production of the polio vaccine during the mid-20th century, with the Research Institute for Viral Vaccines (RIV) utilizing approximately 5,000 of these monkeys annually for safety testing and virus propagation in kidney cell cultures.^[74] This work contributed significantly to the successful eradication of polio in many regions, as the vaccine's efficacy was validated through these primate models before human trials. In genetics, comparisons between the human and chimpanzee genomes have revealed about 98.8% sequence similarity, highlighting key differences in gene regulation and expression that underlie human-specific traits like enhanced cognitive abilities.^[75] The chimpanzee genome sequencing project, completed in 2005, has served as a foundational resource for understanding primate evolution and human disease mechanisms.^[76] Beyond science, simians hold profound cultural significance in mythology and popular media, often symbolizing strength, devotion, and societal critique. In Hindu mythology, Hanuman, depicted as a vanara (monkey-like deity), embodies bravery, loyalty, and selfless service as the devoted companion of Rama in the epic Ramayana, influencing festivals like Hanuman Jayanti and temple worship across India and Southeast Asia.^[77] In Western media, the Planet of the Apes franchise, originating from Pierre Boulle's 1963 novel and adapted into films starting in 1968, portrays intelligent apes in a dystopian society, exploring themes of human hubris, racism, and environmental degradation through ape characters that challenge anthropocentric views.^[78] These depictions have shaped public perceptions of primates, blending entertainment with commentary on human-animal relations. Ethical considerations surrounding simian use in research have evolved markedly since the 1980s, driven by animal welfare advocacy and regulatory reforms that prioritize the 3Rs principle (replacement, reduction, refinement). In the United States, the 1985 amendments to the Animal Welfare Act imposed stricter oversight on primate experimentation, mandating institutional animal care committees and promoting alternatives like in vitro models to minimize invasive procedures.^[79] This shift reflected growing recognition of primates' cognitive and emotional capacities, leading to reduced reliance on chimpanzees in invasive studies by the early 2000s and increased funding for non-animal methodologies in toxicity testing and neuroscience.^[80]

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How did humans evolve into the big-brained, bipedal ape that we are today? This article examines the fossil evidence of our 6 million year evolution.
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Sahelanthropus tchadensis - Smithsonian's Human Origins
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Chimps, Humans and Monkeys: What's the Difference?
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[PDF] 2024-2025 Report of the IUCN Species Survival Commission and ...
the taxonomic list of primates maintained by the Primate Specialist Group (PSG) rec- ognises 539 species and 723 taxa (species and subspecies) in 83 genera ...
[23]
Taxonomy and systematics of the Neotropical primates: a review ...
... species of woolly monkeys (Atelidae, Platyrrhine, Primates) are there? The first molecular analysis of Lagothrix flavicauda, an endemic Peruvian primate species ...
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[25]
Primitive Old World monkey from the earliest Miocene of Kenya and ...
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[26]
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The Old World monkeys (cercopithecids) are divided into two subfamilies, the cercopithecines (which have check pouches) and the colobines (leaf-eating ...
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New World Monkeys | Learn Science at Scitable - Nature
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[29]
The Grasping Hand: Primate Fingers -> Opposable Thumbs | AMNH
The spider monkey's prehensile, or grasping, tail can support its entire body weight. Each monkey has a unique pattern of lines on its tail, like a fingerprint, ...Missing: simian | Show results with:simian
[30]
Color vision diversity and significance in primates inferred from ...
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[31]
Evolution of Primate Sense of Smell and Full Trichromatic Color Vision
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[32]
The Genetic and Evolutionary Drives behind Primate Color Vision
In Old World primates, a duplication of the LWS opsin gene occurred giving rise to a “red-sensitive” or L pigment and a “green-sensitive” or M pigment. Their ...
[33]
Loss of Olfactory Receptor Genes Coincides with the Acquisition of ...
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[34]
Evolution of Primate Sense of Smell and Full Trichromatic Color Vision
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Primate Locomotion | Learn Science at Scitable - Nature
A few body adaptations, for example prehensile tails, arm-hanging anatomy, and ischial callosities, are associated with a primate posture, but most are not.
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Feb 8, 2018 · The African apes are committed to KW during quadrupedality and even adopt this type of locomotion when moving atop large branches (Doran, 1992, ...Muscle-Tendon Unit: Size... · Anatomy Of Knuckle Walking... · The Knuckle-Walking Complex
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The primate semicircular canal system and locomotion - PMC - NIH
The semicircular canal system of vertebrates helps coordinate body movements, including stabilization of gaze during locomotion.
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The evolution of the vestibular apparatus in apes and humans - PMC
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Monkeys | Research Starters - EBSCO
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Catarrhini - an overview | ScienceDirect Topics
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Barbary macaques: Europe and north Africa's last monkeys
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Primate Populations and Their Interactions with Changing Habitats
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[PDF] Terrestriality across the primate order - University of Michigan
Apr 30, 2024 · In Figure 2, we detail the phylogenetic distribution of terrestriality among extant primates, using the ordinal classification of terrestriality.
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Adaptive Significance of Coloration in Mammals - Oxford Academic
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Keeping cool in the heat: Behavioral thermoregulation and body ...
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[50]
Andean Night Monkey, Aotus miconax
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(PDF) High Altitude Primates - ResearchGate
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Primate Sociality and Social Systems | Learn Science at Scitable
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Orang-utan - Wisconsin National Primate Research Center
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Behavioral Ecology: Social Organization in Fission–Fusion Societies
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[PDF] The nutritional consequences of foraging in primates
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(PDF) Primate Nutritional Ecology - Academia.edu
... primates consume both plants and animals. Harding (1981), for example, undertook a review of the diet of 131 species of wild primates and found that 90% of ...
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Digging up food: excavation stone tool use by wild capuchin monkeys
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[58]
Seasonality, extractive foraging and the evolution of primate ...
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Life history - NC3Rs Macaque Website
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Gestation Period and Twinning in Chimpanzees - Science
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The IUCN Red List is updated. For each Red List update, IUCN provides summaries of the numbers of species in each category, by taxonomic group and by country.
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'Running out of time': 60 percent of primates sliding toward extinction
including drills and gibbons, lemurs and tarsiers, bush babies and spider monkeys — face the ...
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The Current Status of the World's Primates: Mapping Threats to ... - NIH
Oct 31, 2021 · Within the four geographical areas where primates are found, i.e., the Americas, Africa, Asia, and Madagascar, 36%, 37%, 73%, and 87% of primate ...
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[PDF] Primates in Peril
Primates in Peril: The World's 25 Most Endangered Primates 2023–2025. IUCN SSC. Primate Specialist Group, International Primatological Society, Re:wild,.
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Primate extinction looms, but hope remains, scientist says
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A look back: 40 years after the first GLT reintroductions
Apr 11, 2024 · The reintroduction of captive-born primates into the wild was an uncommon practice at the time. Many say the actions taken 40 years ago saved ...
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A Brief History of Polio Vaccines - Science
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Comparison of human and chimpanzee genomes reveals striking ...
Aug 31, 2005 · The first comprehensive comparison of the genetic blueprints of humans and chimpanzees shows our closest living relatives share perfect identity.
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Comparing the human and chimpanzee genomes - PubMed - NIH
The chimpanzee genome sequence is a long-awaited milestone, providing opportunities to explore primate evolution and genetic contributions to human physiology ...Missing: significance | Show results with:significance
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The god Hanuman: A symbol of bravery and service
He is a Vanara – a monkey, ape, or forest-dweller. Vanaras are shapeshifters and are still considered by some believers to be demi-gods.
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Fifty Years of Politics Surrounding the Apes Franchise
Jul 11, 2014 · These two films focus on the perspective of the apes, with chimp Caesar (played by Andy Serkis) as your undeniable star. The ape actors took off ...
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The Evolving Regulatory Environment - International Animal ... - NCBI
The public wants to benefit from scientific advances, but also wants to be reassured research does not impose unnecessary suffering on animals, said MacArthur ...
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A History of the Development of Alternatives to Animals in Research ...
Oct 28, 2024 · As the 1980s opened, supporters of alternatives devoted significant attention in Britain and the United States to the area of toxicity testing.