The terms “hominin” and “hominid” frequently cause confusion because their definitions have changed as new scientific evidence emerged. This complexity stems from the shifting nature of biological classification, which maps the evolutionary relationships between species. Clarifying the current, standard classification system used in modern anthropology and biology is necessary to understand the difference between these two groups of primates. The distinction lies in which members of the ape family tree are included in each specific taxonomic group.
Defining Hominid
The term hominid is the common name for the taxonomic family Hominidae, which represents the broadest grouping in this discussion. Under the modern classification system, Hominidae includes all of the great apes, both living and extinct. This family encompasses four living genera: Pongo (orangutans), Gorilla (gorillas), Pan (chimpanzees and bonobos), and Homo (modern humans).
The hominid classification is a very inclusive category, covering species across Africa and Southeast Asia. Members of the Hominidae family share a relatively large body size, the absence of a tail, and a complex brain structure. Orangutans are more distantly related, having split from the other great apes approximately 14 million years ago. The remaining African apes, including humans, belong to the subfamily Homininae, a narrower group within the Hominidae family.
Defining Hominin
The term hominin is the common name for the taxonomic tribe Hominini, a significantly more restrictive group within the larger Hominidae family. Hominins include modern humans and all of our extinct ancestors that arose after the evolutionary split from the chimpanzee lineage. This tribe is defined by its relationship to the Homo genus, representing the human evolutionary line.
The defining criterion for inclusion in Hominini is being more closely related to modern humans than to any other living primate, such as the chimpanzee. This tribe includes the extinct genera Australopithecus (like “Lucy,” A. afarensis), the robust forms of Paranthropus, and earlier genera such as Ardipithecus and Sahelanthropus.
Fossil evidence suggests that the earliest hominins, such as Sahelanthropus, appeared between six and seven million years ago from the chimpanzee lineage. The Hominin tribe is the specific branch of the great ape family tree that leads directly to our own species, Homo sapiens.
The Shift in Classification
The confusion between “hominid” and “hominin” largely stems from a shift in classification that occurred in the late 20th century. Historically, the Family Hominidae was defined narrowly, including only humans and their direct ancestors, while the great apes were placed in a separate family called Pongidae. This older system relied primarily on physical characteristics, such as upright posture and large brain size.
The advent of molecular biology, specifically genetic analysis, revealed a closer relationship between humans and the African great apes. Genetic studies showed that chimpanzees and humans share over 98% of their DNA and are more closely related to each other than either species is to the orangutan. This finding invalidated the traditional classification because it did not accurately reflect evolutionary history.
To correct this, taxonomists expanded the Family Hominidae (hominid) to include all great apes, making it a more inclusive group that better represents shared ancestry. This required the creation of the new, more specific term, Hominin (Tribe Hominini), to designate the former, more restrictive group of humans and their extinct ancestors. The term hominin now refers to the lineage that used to be called hominid in older literature.
Distinguishing Physical Characteristics
Hominins are primarily distinguished from other hominids by a specific set of physical and behavioral characteristics that evolved along the human lineage. The defining trait is habitual bipedalism, the ability to walk upright on two legs. Skeletal markers of bipedalism include a forward-positioned foramen magnum, modifications to the pelvis, leg bones, and feet.
While early hominins like Australopithecus retained some tree-climbing features, their anatomy was adapted for terrestrial bipedal movement. Another characteristic is the trend toward encephalization, the evolutionary expansion of brain size relative to body size, particularly noticeable in the later Homo species. This brain expansion corresponds with the development of increasingly complex tool use.
Hominins also show a reduction in the size of the canine teeth and a decrease in prognathism, the jutting forward of the lower face. These dental and facial changes reflect a shift in diet and social behavior compared to the larger canines seen in male gorillas and chimpanzees.