Nonhuman primates are every species in the order Primates except humans. That includes lemurs, lorises, tarsiers, monkeys, and apes, totaling over 500 recognized species spread across tropical and subtropical regions of Africa, Asia, and Central and South America. They are our closest biological relatives, sharing key traits like forward-facing eyes, grasping hands, and large brains relative to body size.
How Nonhuman Primates Are Classified
Living primates split into two major branches. The first, called strepsirrhines, includes lemurs, lorises, and bushbabies. These are sometimes called prosimians, and they tend to be smaller, often nocturnal, and retain older evolutionary traits like a stronger reliance on smell. Many have a specialized grooming claw on one toe, and their body temperatures run slightly lower than other mammals of similar size. Some, like dwarf lemurs, can even enter torpor or hibernation when food and water become scarce.
The second branch, haplorrhines, contains tarsiers, monkeys, and apes. This group includes far more species, occupies a wider geographic range, and plays a larger ecological role in most habitats. One simple physical difference: strepsirrhines have wet, naked noses (think of a dog’s nose), while haplorrhines have dry, furry ones.
Monkeys: New World vs. Old World
Monkeys make up the bulk of nonhuman primate species and fall into two groups based on where they evolved. New World monkeys live in Central and South America. Many have prehensile tails that function almost like a fifth limb, and their thumbs sit in the same plane as their other fingers rather than being truly opposable. Old World monkeys live in Africa and Asia. They have downward-pointing nostrils, opposable thumbs, and fingernails on all digits, but nearly all still have tails, which distinguishes them from apes.
The rhesus macaque, an Old World monkey, holds the largest natural range of any nonhuman primate. It spans more than 6,000 kilometers across 11 countries, from Afghanistan through India, Southeast Asia, and into China. Rhesus macaques thrive in tropical forests, temperate climates, semi-desert, and swamps at elevations from sea level to above 4,000 meters.
Apes: Lesser and Great
Apes are divided into two families. The lesser apes, gibbons and siamangs, are specialists in swinging through trees using their extraordinarily long arms. Their arms are so disproportionately long that when forced to move on the ground, they walk upright on two legs because their hands would drag otherwise. Despite their acrobatic skill, they haven’t shown the same level of complex problem-solving seen in their larger relatives.
The great apes, including chimpanzees, bonobos, gorillas, and orangutans, are the nonhuman primates most closely related to us. They lack tails entirely, possess enormous hand strength, and demonstrate remarkable cognitive abilities, including tool use and complex learning. Chimpanzees and gorillas walk on their knuckles and the soles of their feet but also climb actively. Orangutans are the most arboreal of the great apes, moving cautiously through the canopy with all four limbs gripping branches simultaneously.
How Close Are They to Humans?
The commonly cited figure is that humans and chimpanzees share about 98.5% of their DNA, but that number only counts single-letter changes in the genetic code. When researchers also factor in insertions and deletions (stretches of DNA present in one species but missing in the other), the actual shared sequence drops to roughly 95%. That remaining 5% accounts for the biological differences between the two species, yet 95% overlap is still remarkably high, reflecting a common ancestor that lived around 6 to 7 million years ago. Even Old World monkeys like rhesus macaques shared a common ancestor with humans roughly 25 million years ago.
Brain size relative to body weight is another point of comparison. Common marmosets, small New World monkeys, have brains that represent about 2.7% of their body weight, a ratio equivalent to what’s seen in humans. This doesn’t mean marmosets think like people, but it illustrates how primate brains in general are unusually large for their body size compared to most other mammals.
Social Lives and Group Structures
Most nonhuman primates live in permanent social groups, but the structure of those groups varies enormously. The ancestral state was likely solitary and nocturnal. As species shifted to daytime activity, the pressure to avoid predators favored living in loose aggregations, which over time became more cohesive groups.
Today you can find nearly every arrangement. Some species form large troops with multiple adult males and females. Others live in single-male groups where one dominant male mates with several females. Gibbons and a handful of other species live in bonded pairs, likely because their food sources are spread out and direct paternal care or protection from infanticide makes monogamy advantageous. A few, like orangutans and some lemurs, remain largely solitary, with females scattered across the landscape and males roaming between them. Even within a single species, social structures can shift depending on local food availability and predation risk.
Why They Matter to Medicine
Because nonhuman primates share so much biology with humans, they play a unique role in biomedical research that no other animal model can fill. They are the only animals that can reproduce the full biological process of many human infections, making them essential for developing vaccines against viruses like RSV, Ebola, dengue, and Zika. Early research with monkeys in the 1970s led directly to the development of deep brain stimulation for Parkinson’s disease. More recently, primate studies mapped how to deliver a modified poliovirus into brain tumors as an immunotherapy for glioblastoma, one of the most aggressive cancers.
Nonhuman primates also contribute to research on Alzheimer’s disease, type 2 diabetes, obesity, reproductive health (including miscarriage and premature birth), and organ transplant tolerance. The NIH-supported BRAIN Initiative relies on tracking individual neuron activity in monkeys performing tasks to build a working map of how the brain controls movement, cognition, and emotion.
Conservation Threats
Roughly two-thirds of all primate species are threatened with extinction. Of the 491 species assessed by the IUCN Red List as of 2020, 319 (about 65%) fell into the categories of Vulnerable, Endangered, or Critically Endangered. The primary drivers are not exotic or surprising: hunting and logging affect over 80% of threatened species, and agricultural expansion affects nearly as many. These two pressures, grouped under biological resource use and agriculture, touch more than 390 species each.
The threats are concentrated in tropical regions of South America, Africa, and Southeast Asia, precisely where primate diversity is highest. Habitat loss from farming fragments forests into smaller patches, isolating populations and reducing genetic diversity. Hunting for bushmeat or traditional medicine removes individuals faster than populations can recover, especially in species with slow reproduction rates. Many primates give birth to only one offspring at a time after lengthy pregnancies, making population rebounds difficult even when protections are put in place.