The primate order includes hundreds of living species, and to understand their relationships, scientists use a phylogenetic tree. This is a visual map of evolutionary connections, tracing how different groups are related through shared ancestry. Much like a family tree, its branches represent lineages splitting over millions of years. The root of this tree signifies a common ancestor from which all primates descended.
The First Major Split in the Primate Order
The first major split in the primate tree separates the order into two suborders: Strepsirrhini and Haplorhini. The distinguishing feature between these groups is the nature of their noses. Strepsirrhines, or “wet-nosed” primates, possess a rhinarium, which is the moist, naked skin surrounding their nostrils, similar to that of a dog. This feature is linked to a more developed sense of smell compared to their dry-nosed relatives.
This ancient lineage includes the lemurs of Madagascar, as well as the lorises and galagos of Africa and Asia. These animals often exhibit traits considered more ancestral among primates, such as a reliance on olfaction and, in many cases, a nocturnal lifestyle. The presence of a grooming claw on the second toe and a dental structure called a toothcomb are other characteristics that help define the Strepsirrhini suborder.
In contrast, the Haplorhini, or “dry-nosed” primates, lack a wet rhinarium and have a more continuous, furry upper lip. This group has a reduced reliance on the sense of smell and a greater emphasis on vision. The Haplorhini suborder encompasses a wide array of primates, including tarsiers, monkeys, apes, and humans.
The Haplorhine Branches
The first Haplorhine group to branch off was the tarsiers. These small, nocturnal primates from Southeast Asia have unique features, including enormous eyes, placing them in a distinct category. After the tarsier lineage diverged, the remaining haplorhines, known as the anthropoids, split into two major infraorders: Platyrrhini and Catarrhini. This division occurred around 40 million years ago and corresponds to a geographic separation.
The Platyrrhini, or “flat-nosed” primates, are found exclusively in Central and South America and are commonly called New World monkeys. They are defined by broad, flat noses with nostrils that face outwards. Many species within this group, such as spider monkeys and howler monkeys, possess prehensile tails, which are capable of grasping and can function like an extra limb. Their dental formula is also distinct, consisting of three premolars in each quadrant of the jaw.
The Catarrhini, or “hook-nosed” primates, are native to Africa and Asia and include Old World monkeys, apes, and humans. They have narrow noses with downward-facing nostrils. Unlike their New World counterparts, no catarrhine has a prehensile tail, and some, particularly apes, have no tail at all. This group is distinguished by a dental formula that includes only two premolars in each jaw quadrant. Examples of Old World monkeys include baboons and macaques.
The Ape Lineage and Human Placement
The Catarrhini branch splits again, separating Old World monkeys from the superfamily Hominoidea, the apes. Apes are distinguished from monkeys by a lack of a tail, a broader chest, and a different shoulder structure that allows for greater rotational mobility. This lineage first divided about 15 million years ago, creating the lesser apes and the great apes. The lesser apes are the gibbons of Southeast Asia.
The great apes, or the family Hominidae, include orangutans, gorillas, chimpanzees, bonobos, and humans. The ancestors of modern orangutans were the first to diverge from this group, between 12 and 13 million years ago. The gorilla lineage branched off next, between eight and ten million years ago. This left a single line that would eventually lead to both the human and chimpanzee lineages.
The final split in the great ape family occurred around six million years ago, separating human ancestors from the ancestors of chimpanzees and bonobos. This makes chimpanzees and bonobos our closest living relatives in the animal kingdom. The classification group that includes modern humans and all our extinct ancestors after this split is called Hominini.
Understanding Common Ancestry
A frequent misunderstanding of the primate tree is the idea that it represents a linear ladder of progress, with humans at the top. This view incorrectly implies that humans evolved from the monkeys and apes we see today. The branching structure of the phylogenetic tree shows that all living primates are the modern-day tips of ancient, radiating lineages.
Humans did not evolve from chimpanzees any more than you evolved from your cousin. Instead, humans and chimpanzees share a common ancestor that lived millions of years ago. That ancestral species was neither a modern human nor a modern chimpanzee; it was a unique organism whose population diverged into two separate evolutionary paths.
One of those paths led to the genus Pan (chimpanzees and bonobos), and the other led to the genus Homo (humans). Both lineages have been evolving independently for the same amount of time since that split.