Hominins are the group of species that includes modern humans and all our ancestors after the evolutionary split from chimpanzees, roughly 6 million years ago. The term covers every species on the human side of that divide, from the earliest upright-walking apes in Africa to Neanderthals to you. If a fossil species is more closely related to humans than to chimpanzees, it’s a hominin.
Why “Hominin” Instead of “Hominid”
The terminology has shifted over the past few decades, which causes a lot of confusion. Older textbooks used “hominid” to mean the human lineage specifically. But as genetic evidence accumulated, scientists reorganized the family tree. Great apes (gorillas, chimpanzees, orangutans) and humans are now all placed in a single family called Hominidae, making them all “hominids.”
To distinguish the specifically human branch, scientists use a lower classification level: the tribe Hominini. Members of this tribe are “hominins.” So when you see “hominin” in a news article or documentary, it refers to humans and our direct ancestors. “Hominid” now casts a wider net that includes all the great apes too.
What Makes a Hominin a Hominin
Two traits set the earliest hominins apart from other apes: walking upright on two legs and having smaller canine teeth. These features appear together in the oldest known candidates for hominin status and persist throughout the entire lineage.
Bipedalism left a clear signature in the skeleton. The foramen magnum, the opening at the base of the skull where the spinal cord exits, shifted to the underside of the skull rather than sitting toward the back as it does in four-legged animals. This repositioning means the head balances on top of a vertical spine. Over time, hominins also developed an S-shaped spinal curve, an angled thighbone that keeps the knees under the body’s center of gravity, and a big toe aligned with the other toes for push-off during walking rather than grasping branches.
The pelvis tells an equally clear story. Early apes had long, narrow hip bones suited for climbing. Hominins evolved a short, wide, bowl-shaped pelvis that wraps around the sides of the body. This shape stabilizes the trunk during upright walking, supports the internal organs from below, and eventually had to accommodate the birth of increasingly large-brained babies.
Canine tooth reduction is the other signature change. In most apes, the upper canines are large, projecting, and honed to a sharp edge against the lower teeth. In hominins, canines shrank in height and became more symmetrical early on. This shift is considered a signal of major behavioral changes, possibly related to reduced aggression between males or a transition to different ways of processing food.
The Earliest Known Hominins
The oldest species currently placed in the hominin group is Sahelanthropus tchadensis, which lived between 7 and 6 million years ago in what is now Chad, in west-central Africa. Its skull has a foramen magnum positioned on the underside, suggesting an upright posture, and its canine teeth are small. No complete limb bones have been found, so the evidence for bipedalism comes mainly from the skull.
Another early candidate, Orrorin tugenensis, dates to about 6 million years ago in Kenya and preserves thighbone fragments that suggest upright walking. By around 4.4 million years ago, a better-known species called Ardipithecus ramidus shows a mix of tree-climbing and bipedal features, including a short, wide hip bone. These species sit near the base of the hominin family tree, close to the point where human and chimpanzee lineages diverged. Genetic analysis of modern human and chimpanzee genomes places that split at less than 6.3 million years ago, possibly as recently as 5.4 million years ago.
Australopithecus: The Middle Chapter
By about 4 million years ago, a group of hominins collectively known as Australopithecus had become well established across eastern and southern Africa. These species were fully bipedal but still relatively small-brained, with brain volumes ranging from about 387 to 560 cubic centimeters depending on the species. For comparison, a chimpanzee brain averages around 363 cc. So Australopithecus brains were roughly a third larger than a chimp’s, but still far smaller than a modern human’s.
Australopithecus species had large, broad cheek teeth with thick enamel suited for chewing tough plant foods, and their jaws were more heavily built than those of later hominins. The famous fossil “Lucy,” an Australopithecus afarensis who lived about 3.2 million years ago in Ethiopia, is one of the best-known examples. Her skeleton confirmed that upright walking preceded the major expansion of the brain by millions of years.
The Rise of the Genus Homo
The genus Homo, which includes our own species, appears in the fossil record by about 2.8 million years ago. The earliest members are distinguished from Australopithecus by lighter skull construction, narrower cheek teeth, changes in jaw shape, and brains that averaged roughly 40% larger than those of Australopithecus. Early Homo also had hands capable of both powerful gripping and precise manipulation, a combination linked to the production of stone tools.
Recent fossil discoveries from the Ledi-Geraru area in Ethiopia have confirmed that early Homo and Australopithecus lived alongside each other in the same region before 2.5 million years ago. In fact, eastern Africa between 3 and 2.5 million years ago may have hosted as many as four different hominin lineages simultaneously: early Homo, Paranthropus (a group with massive jaws specialized for hard foods), and at least two Australopithecus species. The hominin family tree was not a single line but a branching bush, with multiple species coexisting and eventually going extinct while only one lineage continued.
Brain Size Over Time
The most dramatic trend in hominin evolution is the expansion of the brain. Australopithecus species clustered in the 400 to 550 cc range. Homo erectus, which appeared around 1.9 million years ago and spread from Africa into Asia, had brains ranging from 780 to 1,225 cc. Pleistocene Homo sapiens, the anatomically modern humans who lived during the Ice Ages, ranged from 1,285 to 1,775 cc. Contemporary humans fall in a similar range of 1,250 to 1,730 cc.
That represents roughly a tripling of brain volume from Australopithecus to modern humans, accomplished over about 3 million years. This expansion came with costs: larger brains require far more energy, and bigger-headed babies made childbirth more difficult, which is part of why the pelvis continued to reshape throughout hominin evolution.
How Hominins Differ From Living Apes
Beyond bipedalism and brain size, hominins diverged from other great apes in several visible ways. Humans have a much shorter face and jaw compared to chimpanzees and gorillas, which is why our wisdom teeth frequently become impacted: there simply isn’t enough jaw space left. Humans also have a larger, more opposable thumb relative to the fingers, enabling the fine motor control needed for toolmaking and writing. Our eyes have a large visible white area (the sclera), which may have evolved to help others follow our gaze during social communication.
Living great apes, on the other hand, retain considerably stronger skeletal muscles than humans. They also have longer arms relative to their legs, larger canine teeth, and feet with a divergent big toe that works like a thumb for gripping branches. Humans traded that grasping ability for a rigid, arch-supported foot optimized for walking and running over long distances. We also lost most of our body hair, a trait unique among the primates and still debated in terms of its evolutionary drivers.
How Many Hominin Species Have Existed
Depending on how researchers draw the lines between species, somewhere around 20 to 25 hominin species have been named. These span from Sahelanthropus at roughly 7 million years ago to Homo sapiens today. Many of these species are known from only a handful of fossils, and their exact relationships to each other remain actively debated. New discoveries continue to reshape the picture. The Ledi-Geraru fossils published in 2025, for instance, revealed an Australopithecus population in Ethiopia whose teeth don’t match any previously known species, hinting that still more diversity existed than the current fossil record captures.
What’s clear is that for most of hominin history, multiple species shared the planet at the same time. Homo sapiens has been the only surviving hominin for roughly 30,000 to 50,000 years, since the disappearance of Neanderthals and the small-bodied Homo floresiensis. Being the last hominin standing is the exception, not the rule.