Table of Contents
Primates are the mammalian group to which humans belong. Understanding the relationships among primates clarifies what humans share with other species and what is unique about us. This chapter focuses on the diversity of primates, their classification, and how evolutionary relationships are inferred, setting the stage for placing humans within this broader group.
What Counts as a Primate?
Primates are a well-defined order of mammals. They share a characteristic combination of traits, such as:
- Grasping hands and often feet, usually with opposable thumbs or big toes
- Nails rather than claws on most digits
- Forward-facing eyes and stereoscopic (3D) vision
- Large brains relative to body size
- Prolonged development and complex social behavior
These features occur in different degrees among primates but collectively mark the group as distinct from other mammals (such as rodents, carnivores, or ungulates).
Major Groups Within the Order Primates
The order Primates is traditionally divided into two major suborders. Modern systematics often uses slightly different terms, but the biological grouping is essentially the same.
Strepsirrhines (Wet-Nosed Primates)
Strepsirrhines include:
- Lemurs (mostly in Madagascar)
- Lorises and pottos (Africa and Asia)
- Galagos (bushbabies, in Africa)
Typical characteristics:
- Moist, naked nose region (rhinarium) similar to that of dogs and cats
- Often a stronger reliance on smell than on vision
- Commonly have a reflective layer in the eye (tapetum lucidum) and large eyes adapted to low light; many species are nocturnal
- A “tooth comb” (forward-projecting lower incisors and canines) in lemurs and many related forms, used for grooming
Strepsirrhines are generally considered to retain more ancestral traits compared with other primates. However, they are not “primitive” in a simple sense; many have specialized adaptations to Madagascar’s unique environments or nocturnal life.
Haplorhines (Dry-Nosed Primates)
Haplorhines include:
- Tarsiers
- Monkeys (New World and Old World)
- Apes (including humans)
Shared features:
- Dry, hair-covered nose region (no rhinarium)
- Greater emphasis on vision; generally better color vision and more forward-facing eyes
- Typically larger brains relative to body size than strepsirrhines
- More complex social systems in many groups
Importantly, tarsiers form a small, specialized group within Haplorhini and occupy an intermediate position in some anatomical and behavioral traits (for example, nocturnal like many strepsirrhines but with several haplorhine-like brain and eye features).
Monkeys and Apes: Two Key Haplorhine Branches
Within the haplorhines, there is a critical split between monkeys and apes. Humans are part of the ape branch, so these relationships are essential for understanding our closest relatives.
New World vs. Old World Monkeys
Monkeys are divided geographically and evolutionarily into:
- New World monkeys (Platyrrhini)
- Live in Central and South America
- Broad, outward-facing nostrils
- Many have prehensile tails (can grasp branches; e.g., howler monkeys, spider monkeys)
- Generally more strictly arboreal (tree-dwelling)
- Old World monkeys (Cercopithecoidea)
- Live in Africa and Asia
- Narrow, downward-facing nostrils
- Non-prehensile tails (if present)
- Often show marked differences between forelimbs and hindlimbs adapted for both arboreal and terrestrial life
- Frequently have sitting pads on the rump (ischial callosities) and often complex facial coloration or ornamentation
Old World monkeys are evolutionarily closer to apes (and thus to humans) than New World monkeys are.
Apes (Hominoidea)
Apes include:
- Gibbons (lesser apes)
- Great apes:
- Orangutans
- Gorillas
- Chimpanzees and bonobos
- Humans
General ape characteristics that distinguish them from monkeys:
- No tail
- More flexible shoulder joints, well-suited for swinging, climbing, and broad range of arm movement
- More upright body posture in many activities
- Larger brains relative to body size
- Generally longer lifespans and more extended development
Gibbons are smaller “lesser apes” adapted to brachiation (arm-swinging) in the forest canopy. The great apes are larger-bodied, with even more pronounced brain enlargement and complex social behaviors.
Phylogenetic Relationships Among Apes
Among the apes, evolutionary relationships can be summarized as a series of branching points:
- Gibbons form the earliest branch among living apes. They share many ape traits but diverged before the other great apes split from each other.
- Orangutans (genus Pongo) diverged next. They are primarily Asian, largely solitary, and highly arboreal.
- The remaining great apes—gorillas, chimpanzees, bonobos, and humans—share a more recent common ancestor among themselves than with orangutans.
- Within this group, gorillas (genus Gorilla) branched off earlier, leaving chimpanzees, bonobos, and humans more closely related to one another.
- Chimpanzees and bonobos (genus Pan) are humans’ closest living relatives.
- Genetic and molecular studies show that humans and the Pan lineage share a more recent common ancestor than either does with gorillas or any other ape.
- The genetic distance between humans and chimpanzees/bonobos is small compared to the distance to other primates, making the human–chimpanzee–bonobo group the tightest cluster among great apes.
A simplified relationship tree (cladogram) of living apes would therefore look like this (from earliest split to closest relationship):
Gibbons – Orangutans – Gorillas – Chimpanzees/Bonobos – Humans
This ordering refers to branching sequence, not a linear “ladder” from “lower” to “higher” apes; each lineage has followed its own evolutionary path.
How Relationships Among Primates Are Determined
The relationships described above are based on multiple independent lines of evidence. For beginners, it is important to recognize that no single feature defines relatedness; instead, patterns across many features are used.
Comparative Anatomy and Behavior
Biologists compare:
- Skeletal features (skull shape, dental patterns, limb bones, pelvis, vertebral column)
- Soft-tissue structures (muscles, internal organs, brain regions)
- Reproductive and developmental traits (gestation length, age at maturity)
- Behavioral patterns (social systems, communication, tool use, parental care)
Shared derived traits (those that evolved in a common ancestor and are present in its descendants) are especially important for reconstructing evolutionary relationships.
Fossils
Fossil primates provide snapshots of past forms:
- Show combinations of traits that may be intermediate between modern primate groups
- Help date when major branches (strepsirrhines vs. haplorhines, New vs. Old World monkeys, apes vs. monkeys) diverged
- Provide context for when and how key adaptations (e.g., upright posture, changes in dentition, brain expansion) appeared
Fossils do not usually preserve DNA, but they complement genetic data by anchoring evolutionary changes in time and environment.
Molecular and Genetic Data
Modern primate relationships are strongly supported by molecular evidence:
- DNA sequence comparisons across many genes or whole genomes
- Similarities in non-coding regions of DNA
- Shared genetic “errors” or insertions (e.g., retroviral insertions, pseudogenes) that are unlikely to arise independently
The more similar two species’ DNA sequences are overall, the more recent their common ancestor is likely to be. Molecular clocks—calibrated against fossil dates—allow estimates of when major splits occurred.
Humans as One Primate Species Among Many
Within this framework, humans are:
- Members of the order Primates
- Within Haplorhini, the “dry-nosed” primates
- Part of the catarrhine branch (Old World monkeys and apes)
- Members of the Hominoidea (apes)
- Within the family Hominidae (great apes)
- Most closely related to the genus Pan (chimpanzees and bonobos)
This means that, in evolutionary terms, humans are one twig on a large primate tree. Many traits once thought to be unique to humans—such as tool use, complex social structures, and some forms of communication—are now known to occur, in different ways and degrees, in other primate lineages.
Understanding these relationships clarifies that human evolution did not occur in isolation but is one particular outcome of the broader diversification of primates. Subsequent chapters on human evolution and on the position of humans in the natural system build on this primate context.