A human being is a living member of the species Homo sapiens, a primate that walks upright, has an unusually large brain, and communicates through complex language. We are one of more than 200 species in the order of Primates, nested within the great ape family, and our closest extinct relatives are Neanderthals. What sets us apart from every other animal on Earth is not a single trait but a combination of physical, cognitive, and social features that together make humans unique.
Where Humans Fit in the Animal Kingdom
Biologically, humans are animals. More specifically, we are mammals, primates, and great apes. Our species most likely evolved from Homo heidelbergensis, a common ancestor we share with Neanderthals. The earliest anatomically modern humans appeared in Africa roughly 300,000 years ago, and from there spread across every continent except Antarctica.
Our genome contains about 3 billion nucleotide base pairs per copy (around 6 billion total, since you carry two copies). Despite sharing roughly 98.7% of our DNA with chimpanzees, the differences in that remaining fraction have profound effects on brain development, skeletal structure, and behavior. Small genetic changes, amplified over millions of years, produced a species that looks and acts dramatically different from its closest living relatives.
The Body Built for Walking
Humans are obligate bipeds, meaning we walk on two legs as our primary and default form of movement. This required a complete redesign of the skeleton compared to other primates. The most important change is in the pelvis: human hip bones are short, wide, and bowl-shaped, curving along the sides of the body to stabilize each step and support our internal organs. A bony projection on the pelvis called the anterior inferior iliac spine, found only in human ancestors, provides attachment points for the ligament and muscle that make upright hip flexion possible.
The gluteal muscles on the side of the hip also shifted position during our evolution. In four-legged primates, these muscles mainly push the leg backward. In humans, they prevent the pelvis from dropping to one side every time you lift a foot off the ground. This is why you can walk and run efficiently over long distances, something no other great ape can do. The human foot also features a permanent arch that acts as a spring, and a big toe aligned with the other toes rather than splayed out for gripping branches.
The Human Brain
The average human brain is about 1,300 cubic centimeters in volume, roughly three times larger than a chimpanzee’s. But size alone doesn’t explain human intelligence. The outer layer of the brain, responsible for reasoning, planning, and language, develops differently in humans than in any other primate. During fetal development, brain stem cells in humans proliferate for a longer period and at a higher rate than in chimpanzees or gorillas. The transition from simple, multiplying cells to specialized nerve cells happens more slowly in humans, which gives the brain more time to produce a larger pool of raw material before wiring begins.
Specific genes play a role in this expansion. One, called ARHGAP11B, increases the production of a particular type of brain progenitor cell that is especially abundant in species with folded, complex brains. Another set of genes found only in humans boosts the number of cells that eventually become neurons. The net result is a brain with a thin-walled, high-vaulted skull, a flat and nearly vertical forehead, and the dense neural circuitry that supports abstract thought, language, and long-term planning.
Language and Symbolic Thinking
Many animals communicate, but no other species uses language the way humans do. Human language is recursive, meaning you can embed ideas within ideas to create sentences of unlimited complexity. It is also symbolic: the word “tree” has no physical resemblance to an actual tree, yet every speaker of English understands it instantly.
The genetic basis for this ability is still being unraveled, but one gene, FOXP2, was the first linked to an inherited speech and language disorder. When FOXP2 is mutated, it disrupts the motor circuits connecting the front of the brain to deeper structures involved in movement, making coordinated speech difficult or impossible. Interestingly, FOXP2 is not unique to humans. Versions of it exist in many species, from songbirds to mice. What differs is how the human version interacts with other genes to build the specific neural circuits that support speech.
Language enabled something no other adaptation could: the ability to teach, record, and accumulate knowledge across generations. This is the foundation of culture, technology, and science.
Social Life and Culture
Humans are intensely social. We form pair bonds, families, tribes, cities, and nations. The size of our social networks has been a subject of debate. A widely cited estimate suggests humans can maintain meaningful relationships with roughly 150 people, a figure derived from the relationship between brain size and group size in primates. However, more recent cross-cultural analysis suggests this number is too simplistic. Human social organization depends not just on brain capacity but on cultural tools like kinship systems, shared rituals, written records, and now digital communication, all of which extend our ability to coordinate far beyond what raw neural hardware would predict.
Culture itself is a defining human trait. Other animals have limited traditions (certain chimp groups use sticks to fish for termites, for example), but only humans build on previous generations’ innovations in a cumulative way. The wheel led to the cart, which led to the engine, which led to the airplane. This ratchet effect of culture is why humans have transformed the planet in ways no other species has.
The Human Body by the Numbers
A few basic measurements help define what a typical human body looks like. The classic “normal” body temperature of 37.0°C (98.6°F) is outdated. A large study found the actual average across participants was 36.1°C (97.0°F), with individual readings ranging from 33.8°C to 37.7°C. Body temperature varies by person, time of day, age, and activity level, so there is no single number that applies to everyone.
Global life expectancy reached 73.1 years in 2019, up more than six years from 66.8 in 2000. The COVID-19 pandemic reversed some of that progress, pushing life expectancy back to around 71.4 years by 2021. These averages mask enormous variation: people in high-income countries with good healthcare routinely live into their 80s, while life expectancy in some regions remains below 60. The primary drivers of longer life over the past century have been improved sanitation, vaccination, antibiotics, and better nutrition rather than any change in the human body itself.
What Makes Humans Physically Distinct
Compared to earlier human species and other great apes, modern humans have lighter, more graceful skeletons. Our faces are flatter, lacking the heavy brow ridges and protruding jaws common in our ancestors. Our teeth are smaller. These changes are partly related to diet: cooking food, which humans have practiced for at least 400,000 years, softens it and reduces the need for powerful chewing muscles and thick jaw bones.
Humans are also nearly hairless compared to other primates, with millions of sweat glands that allow efficient cooling during prolonged physical activity. This thermoregulation system, combined with bipedalism and the ability to carry water, made early humans effective endurance hunters and long-distance travelers in hot African environments.
The Legal Definition
In law, a human being is called a “natural person,” a term that distinguishes a living human from an “artificial person” like a corporation or partnership. An artificial person can own property, enter contracts, and be sued, but it is a legal fiction created by statute. A natural person holds rights simply by being born. This distinction matters in business law, bankruptcy, and constitutional law, where the rights of actual humans sometimes conflict with the legal privileges granted to organizations.