Table of Contents
Humans as One Primate Species Among Many
In modern biology, humans are not placed in a special, separate category outside the rest of life. Instead, we are classified within the same hierarchical system as all other organisms. Our position is defined by shared ancestry and shared traits, especially with other primates.
Here we focus on where humans sit in the biological classification system and what distinguishes our branch within the primates.
Taxonomic Position of Humans
In the biological classification (systematics), humans are placed in a nested series of groups. The main levels relevant here are:
- Domain: Eukarya (organisms with eukaryotic cells)
- Kingdom: Animalia (animals)
- Phylum: Chordata (chordates, with a notochord at some stage)
- Class: Mammalia (mammals)
- Order: Primates
- Suborder: Haplorhini
- Infraorder: Simiiformes (Anthropoidea)
- Parvorder: Catarrhini
- Superfamily: Hominoidea (apes)
- Family: Hominidae (great apes)
- Subfamily: Homininae
- Tribe: Hominini
- Genus: Homo
- Species: Homo sapiens
Two points are especially important:
- Humans are primates.
- Humans are great apes.
Both statements are central to understanding our position in the natural system.
Humans Within the Primates
Primates include lemurs, lorises, tarsiers, monkeys, and apes (including humans). Within this order, a number of traits are especially characteristic. These have already been introduced when discussing primate relationships in general; here we focus on which of these link us most strongly to particular primate groups.
Strepsirrhines and Haplorhines
Primates are first divided into:
- Strepsirrhini: lemurs and lorises
- Haplorhini: tarsiers, monkeys, and apes (including humans)
Humans belong to the Haplorhini, which are characterized by features such as:
- A dry, simple nose (instead of the moist, complex nose of many strepsirrhines)
- Greater reliance on vision over smell
- A more complete bony eye socket
These shared derived traits (synapomorphies) show that humans are more closely related to monkeys and apes than to lemurs or lorises.
New World Monkeys and Old World Monkeys/Apes
Within the haplorhines, the “higher” primates (Simiiformes) divide into:
- Platyrrhini (New World monkeys): living in the Americas
- Catarrhini: Old World monkeys and apes (including humans), in Africa and Eurasia
Humans are catarrhines. Shared traits include:
- Nostrils that are close together and point downward
- Typically 2 premolars per quadrant of the jaw (a 2.1.2.3 dental formula)
- Certain aspects of ear and palate structure
This indicates that humans share a more recent common ancestor with Old World monkeys and apes than with New World monkeys.
Humans as Apes (Hominoidea)
The next important level is the superfamily Hominoidea, the apes. This group includes:
- “Lesser apes”: gibbons (family Hylobatidae)
- “Great apes” and humans: orangutans, gorillas, chimpanzees/bonobos, and humans (family Hominidae)
Humans are hominoids. Traits shared by hominoids include, among others:
- Absence of a tail
- More mobile shoulder joint and shoulder girdle suited for arm-swinging or overhead use
- A generally larger brain relative to body size than most monkeys
- A more upright posture of the trunk (to varying extents)
These characteristics show that humans are evolutionarily closer to apes than to monkeys. In taxonomic terms, any classification that keeps humans outside “the apes” would be artificial and would cut across real evolutionary relationships.
Humans Among the Great Apes (Hominidae)
Within Hominoidea, the great apes and humans form the family Hominidae. Modern systematics generally includes in Hominidae:
- Genus Pongo (orangutans)
- Genus Gorilla (gorillas)
- Genus Pan (chimpanzees and bonobos)
- Genus Homo (humans, and extinct human species)
Within Hominidae, humans belong to the subfamily Homininae (African apes and humans). Within that, the tribe Hominini includes:
- Genus Pan: chimpanzees (Pan troglodytes) and bonobos (Pan paniscus)
- Genus Homo: humans (Homo sapiens) and extinct relatives
This means:
- Our closest living relatives are chimpanzees and bonobos, not orangutans or gorillas.
- The last common ancestor of humans and chimpanzees/bonobos lived more recently than the last common ancestor we share with gorillas, and even more recently than the one shared with orangutans.
Molecular studies (especially DNA comparisons) support this arrangement and have been crucial in refining it.
Morphological and Genetic Evidence for Human Placement
Our placement within the primates and apes is based on both anatomical (morphological) and genetic evidence. Only the aspects most relevant to our taxonomic position are highlighted here.
Shared Anatomical Traits
Across the classification steps, humans share a number of derived features with successively smaller groups:
- With other primates:
- Forward-facing eyes with overlapping visual fields
- Grasping hands with nails instead of claws
- Large, complex brains relative to body size
- With other catarrhines:
- Downward-facing nostrils
- Similar tooth arrangement and structure
- With other apes:
- Absence of a tail
- Broad chest, shortened trunk, and modified shoulder girdle
- Relatively large brain and prolonged period of juvenile development
- With African apes in particular:
- Detailed similarities in skull shape and dentition
- Specific aspects of pelvic and limb anatomy
- Complex social behavior and communication patterns (to be explored elsewhere)
These shared traits reflect our common ancestry and justify our placement at each taxonomic level.
Genetic Similarities
Modern classification is strongly supported by molecular data:
- DNA sequence comparisons show that:
- Humans and chimpanzees/bonobos share a very high proportion of their DNA.
- Humans are slightly more distant genetically from gorillas, and more distant still from orangutans.
- All apes, including humans, form a clear cluster distinct from Old World monkeys, which in turn are distinct from New World monkeys.
- Certain genetic changes are shared exclusively by humans and African apes, supporting the grouping Homininae.
Molecular clocks (using approximate mutation rates) are used to estimate when lineages split, but the key point here is that genetic evidence aligns with morphological and fossil evidence in placing humans firmly within the African apes.
Distinctive Features of the Human Lineage
Although humans are one species within the ape clade, our lineage shows several distinctive traits. These are central to human evolution, but here we mention them only insofar as they clarify our taxonomic position as a specialized ape.
Important derived features of humans include:
- Habitual bipedalism:
- The human skeleton shows many adaptations for upright walking on two legs (e.g., shape of pelvis, alignment of lower limbs, structure of the foot, S-shaped spinal column).
- This contrasts with the locomotion patterns of other apes, which are not habitually bipedal.
- Very large brain relative to body size:
- Humans have a particularly enlarged cerebral cortex.
- This supports complex cognitive abilities, which will be considered in the context of cognitive and cultural evolution.
- Highly developed language and symbolic behavior:
- Human vocal tract anatomy and brain organization enable articulate speech and complex symbolic communication.
- This is linked to cultural transmission of knowledge and technologies.
- Reduced body hair and other physiological specializations:
- Humans are relatively hairless compared to other apes and possess specialized sweat glands; these may relate to thermoregulation and lifestyle changes during evolution.
These specializations explain why humans are placed in their own genus, Homo, and species, Homo sapiens, within the ape family. However, they do not justify placing humans outside the great apes group as a whole.
The Concept of “Special Status” Revisited
Historically, humans were often separated from “animals” in classification schemes, sometimes placed in their own category apart from apes. Modern evolutionary biology rejects this separation because it conflicts with evidence of common descent.
From a biological standpoint:
- Humans are animals.
- Humans are mammals.
- Humans are primates.
- Humans are apes.
- Within apes, humans form one branch (the genus Homo) of the African ape clade.
What sets humans apart—such as culture, technology, and symbolic thought—is real and important, but these are differences within the animal kingdom, not outside it. Our taxonomic position is based on shared ancestry and shared traits; it shows continuity, not absolute separation.
Summary of the Human Position
- Humans are classified in the order Primates, suborder Haplorhini, infraorder Simiiformes, parvorder Catarrhini.
- Within Catarrhini, humans belong to the superfamily Hominoidea (apes), family Hominidae (great apes), subfamily Homininae, tribe Hominini.
- Our closest living relatives are chimpanzees and bonobos (Pan), with whom we share a recent common ancestor.
- Anatomical and genetic evidence consistently support our placement as one specialized ape lineage, not an isolated or fundamentally separate form of life.
- Human-specific traits (such as habitual bipedalism and exceptional cognitive abilities) justify our classification at the genus and species level, but do not remove us from the primate and ape clades.
In the broader context of evolutionary biology, this classification underscores that humans are part of the same natural system as all other organisms, connected to them by descent with modification.