Human Conservation status: Secure
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Primates Superfamily: Hominoidea Family: Hominidae Subfamily: Homininae Tribe: Hominini Genus: Hom Species: H. sapiens Subspecies: H. s. sapiens
Trinomial name
Homo sapiens sapiens Linnaeus, 1758

Humans, or human beings, are biologically classified as bipedal primates belonging to the mammalian species Homo sapiens (Latin for "wise man" or "thinking man") under the family Hominidae (the great apes).^[1] Humans have a highly developed brain capable of abstract reasoning, language, and introspection. This, combined with an erect body carriage that frees their upper limbs for manipulating objects, has allowed humans to make greater use of tools than any other species.

Like most primates, humans are by nature social. However, humans are particularly adept at utilizing systems of communication such as language for self-expression and the exchange of ideas. Humans create complex social structures composed of co-operating and competing groups, ranging in scale from nations to individual families, and social interaction between humans has established a variety of social norms, rituals, traditions, values, laws, and ethics which form the basis of human society. Humans also have an innate appreciation for enjoyment, beauty and aesthetics which, combined with the human desire for self-expression, has led to cultural innovations such as art, literature and music.

Humans are also noted for their desire to understand and influence the world around them, seeking to explain and manipulate natural phenomena through science, religion, philosophy and mythology. Humans are also one of the few species that have sex just for pleasure. This natural curiosity has led to the development of advanced tools and skills; humans are the only species to build fires, cook their food, clothe themselves, and use numerous other technologies. It has also led to the exploration of spiritual concepts such as the soul and deities.

Terminology

Humans are commonly referred to as human beings, people or persons, and collectively as humankind, man, mankind, humanity, or the human race. Until the 20th century, human was only used adjectivally ("pertaining to mankind"). As an adjective, "human" is used neutrally (as in "human race"), but "human" and especially "humane" may also emphasize positive aspects of human nature, and can be synonymous with "benevolent" (as opposed to "inhumane"; cf. humanitarian).

In general, the word people is a collective noun used to define a specific group of humans. However, when used to refer to a group of humans possessing a common ethnic, cultural or national unitary characteristic or identity, "people" is a singular noun, and as such takes an "s" in the plural; (examples: "the English-speaking peoples of the world", "the indigenous peoples of Brazil"). A distinction is also maintained in philosophy and law between the notions "human being", or "man", and "person". The former refers to the species, while the latter refers to a rational agent.

Juvenile male humans are called boys, adult males men (not to be confused with the gender-neutral man or mankind), juvenile females girls, and adult females women.

Physiology and genetics

Main articles: Human anatomy, Human physical appearance and Genetics of humans

Human body types vary substantially. Although body size is largely determined by genes, it is also significantly influenced by environmental factors such as diet and exercise. The average height of a North American adult female is 162 centimetres (5 feet 4 inches), and the average weight is 62 kilograms (137 pounds). Human males are typically larger than females: the average height and weight of a North American adult male is 175 centimeters (5 feet 9 inches) and 78 kilograms (172 pounds).

Humans are capable of fully bipedal locomotion, thus leaving their arms available for manipulating objects using their hands, aided especially by opposable thumbs. Because human physiology has not fully adapted to bipedalism, the pelvic region and vertebral column tend to become worn, creating locomotion difficulties in old age.

Although humans appear relatively hairless compared to other primates, with notable hair growth occurring chiefly on the top of the head, underarms and pubic area, the average human has more hair on his or her body than the average chimpanzee. The main distinction is that human hairs are shorter, finer, and less colored than the average chimpanzee's, thus making them harder to see.^[2]

The color of human hair and skin is determined by the presence of pigments called melanins. Human skin color can range from very dark brown to very pale pink, while human hair ranges from blond to brown to red.^[3] Most researchers believe that skin darkening was an adaptation that evolved as a defense against ultraviolet solar radiation: melanin is an effective sun-block.^[4] The skin color of contemporary humans is geographically stratified, and in general correlates with the level of ultraviolet radiation. Human skin also has a capacity to darken (sun tanning) in response to exposure to ultraviolet radiation.^[5][6]

The average sleep requirement is between seven and eight hours a day for an adult and nine to ten hours for a child; elderly people usually sleep for six to seven hours. Negative effects result from restriction of sleep. For instance, a sustained restriction of adult sleep to four hours per day has been shown to correlate with changes in physiology and mental state, including fatigue, aggression, and bodily discomfort. It is common in modern societies for people to get less sleep than they need, leading to a state of sleep deprivation.

Humans are a eukaryotic species. Each diploid cell has two sets of 23 chromosomes, each set received from one parent. There are 22 pairs of autosomes and one pair of sex chromosomes. By present estimates, humans have approximately 20,000–25,000 genes and share 98.4% of their DNA with their closest living evolutionary relatives, the two species of chimpanzees.^[7]

Like other mammals, humans have an XY sex determination system, so that females have the sex chromosomes XX and males have XY. The X chromosome is larger and carries many genes not on the Y chromosome, which means that recessive diseases associated with X-linked genes, such as hemophilia, affect men more often than women.

Life cycle

The human life cycle is similar to that of other placental mammals. New humans develop viviparously from conception. An egg is usually fertilized inside the female by sperm from the male through sexual intercourse, though the recent technology of in vitro fertilization is also occasionally used. The fertilized egg, called a zygote, divides inside the female's uterus to become an embryo, which over a period of thirty-eight weeks becomes a human fetus. At birth, the fully-grown fetus is expelled from the female's body and breathes independently as an infant for the first time. At this point, most modern cultures recognize the baby as a person entitled to the full protection of the law, though some jurisdictions extend personhood to human fetuses while they remain in the uterus.

Compared with that of other species, human childbirth is fairly complicated. Painful labors lasting twenty-four hours or more are not uncommon, and may result in injury, or even death, to the child or mother, although the chances of a successful labour increased significantly during the 20th century in wealthier countries with the advent of new medical technologies. Natural childbirth remains a common, and relatively dangerous, ordeal in remote, underdeveloped regions of the world.

Human children are born after a nine-month gestation period, and are typically 3–4 kilograms (6–9 pounds) in weight and 50–60 centimeters (20–24 inches) in height in developed countries.^[8] Helpless at birth, they continue to grow for some years, typically reaching sexual maturity at twelve to fifteen years of age. Human girls continue to grow physically until around the age of eighteen, while human boys continue growing until around the age of twenty-one.

The human life span can be split into a number of stages: infancy, childhood, adolescence, young adulthood, maturity and old age. The lengths of these stages, however — particularly the later ones — are not fixed.

There are striking differences in life expectancy around the world. The developed world is quickly getting older, with the median age around 40 years (highest in Monaco at 45.1 years), while in the developing world, the median age is 15–20 years (lowest in Uganda at 14.8 years). Life expectancy at birth is 77.2 years in the U.S. as of 2001.^[9] The expected life span at birth in Singapore is 84.29 years for a female and 78.96 years for a male, while in Botswana, due largely to AIDS, it is 30.99 years for a male and 30.53 years for a female. One in five Europeans, but one in twenty Africans, is 60 years or older, according to The World Factbook.^[10]

The number of centenarians (humans 100 years or older) in the world was estimated by the United Nations at 210,000 in 2002.^[11] The current maximum life span of humans is about 120 years (Jeanne Calment lived for 122 years and 164 days). Worldwide, there are 81 men aged 60 or over for every 100 women of the same age, and among the oldest, there are 53 men for every 100 women.

The philosophical questions of when human personhood begins and whether it persists after death are the subject of considerable debate. The prospect of death causes unease or fear for most humans. Burial ceremonies are characteristic of human societies, often inspired by beliefs in an afterlife or immortality.

Race and ethnicity

Main articles: Race, Ethnic group

Humans often categorize themselves in terms of race or ethnicity, although the scientific validity of human races as categories is disputed. Human racial categories are based on both ancestry and visible traits, especially skin color and facial features. Ethnic groups, on the other hand, are more often linked by linguistic, cultural, and national or regional ties. Self-identification with an ethnic group is based on kinship and descent. Race and ethnicity can lead to variant treatment and impact social identity, giving rise to the theory of identity politics.

Although most humans recognize that variances occur within a species, it is often a point of dispute as to what these differences entail, their importance, and whether discrimination based on race (racism) is acceptable. Some societies have placed a great deal of emphasis on race, while others have not. Three disparate historical examples include the "melting pot" of Ancient Egypt; slavery, Jim Crow laws and the later establishment of the Civil Rights Acts in the United States; and the racial policy of Nazi Germany.

Evolution

Main article: Human evolution

The study of human evolution encompasses the development of the genus Homo, but usually involves studying other hominids and hominines as well, such as the australopithecines. "Modern humans" are defined as the Homo sapiens species, of which the only extant subspecies is Homo sapiens sapiens; Homo sapiens idaltu (roughly translated as "elder wise man"), the other known subspecies, is extinct.^[12]

The closest living relatives of Homo sapiens are the Common Chimpanzee and the Bonobo. Full genome sequencing resulted in the conclusion that "After 6.5 [million] years of separate evolution, the differences between chimpanzee and human are just 10 times greater than those between two unrelated people and 10 times less than those between rats and mice." In fact, chimpanzee and human DNA is 96% identical.^[13] It has been estimated that the human lineage diverged from that of chimpanzees about five million years ago, and from gorillas about eight million years ago. However, a hominid skull discovered in Chad in 2001, classified as Sahelanthropus tchadensis, is approximately seven million years old, which may indicate an earlier divergence.

Two prominent scientific theories of the origins of contemporary humans exist. They concern the relationship between modern humans and other hominids. The single-origin, or "out of Africa", hypothesis proposes that modern humans evolved in Africa and later migrated outwards to replace hominids in other parts of the world. The multiregional hypothesis, on the other hand, proposes that modern humans evolved, at least in part, from independent hominid populations.^[14]

Geneticists Lynn Jorde and Henry Harpending of the University of Utah proposed that the variation in human DNA is minute compared to that of other species, and that during the Late Pleistocene, the human population was reduced to a small number of breeding pairs — no more than 10,000—resulting in a very small residual gene pool. Various reasons for this hypothetical bottleneck have been postulated, the most popular being the Toba catastrophe theory.

Human evolution is characterized by a number of important physiological trends, including the expansion of the brain cavity and brain itself, which is typically 1,400 cm³ in volume, over twice that of a chimpanzee or gorilla (compare capacities). The pattern of human postnatal brain growth differs from that of other apes (heterochrony), allowing for an extended period of social learning in juvenile humans. Physical anthropologists argue that a reorganization of the structure of the brain is more important than cranial expansion itself. Other significant evolutionary changes included a reduction of the canine tooth, development of bipedal locomotion, and the descent of the larynx and hyoid bone, making speech possible. How these trends are related and what their role is in the evolution of complex social organization and culture are matters of ongoing debate in the field of physical anthropology.^[15][16]

Habitat and population

Main articles: Demography, World population

The most widely accepted view among current anthropologists is that Homo sapiens originated in the African savanna between 200 and 250 thousand years ago, descending from Homo erectus, and colonized Eurasia and Oceania by 40,000 years ago, and finally colonized the Americas by 10,000 years ago.^[17] They displaced Homo neanderthalensis, Homo floresiensis and other species descended from Homo erectus (which had colonized Eurasia as early as 2 million years ago) through more successful reproduction and competition for resources.

The earliest humans were hunter-gatherers, a lifestyle well-suited to the savanna. Some later groups of humans were nomads, often to facilitate animal herding, and still later humans established permanent settlements, which was made possible by the development of agriculture. Permanent human settlements were dependent on proximity to water and, depending on the lifestyle, other natural resources, such as fertile land for growing crops and grazing livestock, or seasonally by hunting populations of prey. However, humans have a great capacity for altering their habitats by various methods, such as through irrigation, urban planning, construction, transport, and manufacturing goods, and with the advent of large-scale trade and transport infrastructure, proximity to these resources has become unnecessary, and in many places these factors are no longer a driving force behind the growth and decline of a population. Nonetheless, the manner in which a habitat is altered is often a major determinant in population change.

Technology has allowed humans to colonize all of the continents and adapt to all climates. Within the last few decades, humans have explored Antarctica, the ocean depths, and space, although long-term habitation of these environments is not yet possible. With a population of over six billion, humans are among of the most numerous of the large mammals. Most humans (61%) live in Asia. The vast majority of the remainder live in the Americas (14%), Africa (13%) and Europe (12%), with 0.5% in Oceania. (See list of countries by population and list of countries by population density.)

Human habitation within closed ecological systems in hostile environments, such as Antarctica and outer space, is expensive, typically limited in duration, and restricted to scientific, military, or industrial expeditions. Life in space has been very sporadic, with no more than thirteen humans in space at any given time. Between 1969 and 1972, two humans at a time spent brief intervals on the Moon. As of 2006, no other celestial body has been visited by human beings, although there has been a continuous human presence in space since the launch of the initial crew to inhabit the International Space Station on October 31, 2000.

From AD 1800 to 2000, the human population increased from one to six billion. In 2004, around 2.5 billion out of 6.3 billion people (39.7%) lived in urban areas, and this percentage is expected to rise throughout the 21st century. Problems for humans living in cities include various forms of pollution, crime, and poverty, especially in inner city and suburban slums.

Food and drink

The need for regular intake of food and drink is prominently reflected in human culture, and has led to the development of food science. Failure to obtain food leads to hunger and eventually starvation, while failure to obtain water leads to thirst and dehydration. Both starvation and dehydration cause death if not alleviated — generally, most humans can survive for over two months without food, but at most between ten to fourteen days without water. In modern times, obesity amongst some human populations has increased to almost epidemic proportions, leading to health complications and increased mortality in some developed countries. In the United States nearly 50% of the population is now obese, and over 80% is overweight. Obesity is usually caused by over eating.

Humans are omnivorous animals who can consume both plant and animal products. Although early Homo sapiens employed a "hunter-gatherer" methodology as their primary means of food collection, involving combining stationary plant and fungal food sources (such as fruits, grains, tubers, and mushrooms) with wild game which must be hunted and killed in order to be consumed, many modern humans choose to be vegans or vegetarians, abstaining from eating meat for religious, ethical or health reasons. It is believed that humans have used fire to prepare and cook food prior to eating since the time of their divergence from Homo erectus — possibly even earlier.

At least ten thousand years ago, humans developed agriculture, which has substantially altered the kind of food people eat. This has led to increased populations, the development of cities, and, due to increased population density, the wider spread of infectious diseases. The types of food consumed, and the way in which they are prepared, has varied widely by time, location, and culture.

The last century or so has produced enormous improvements in food production, preservation, storage and shipping. Today almost every locale in the world has access to not only its traditional cuisine, but many other world cuisines.

Brain, mind, and consciousness

Main articles: Human brain, Mind and Consciousness

The human brain is the center of the central nervous system in humans, as well as the primary control center for the peripheral nervous system. The brain controls "lower", or involuntary, autonomic activities such as heart rate, respiration, and digestion. The brain also controls "higher" order, conscious activities, such as thought, reasoning, and abstraction.^[18] The human brain is generally regarded as more capable of these higher order activities, and more "intelligent" in general, than any other species. While other animals are capable of creating structures and using simple tools — mostly as a result of instinct and learning through mimicry — human technology is vastly more complex, constantly evolving and improving with time. Even the most ancient human tools and structures are far more advanced than any structure or tool created by any other animal.^[19]

The human ability to think abstractly may be unparalleled in the animal kingdom. Human beings are one of only six species to pass the mirror test — which tests whether an animal recognizes its reflection as an image of itself — along with chimpanzees, bonobos, orangutans, dolphins and pigeons. Human beings under the age of 2 typically fail this test.^[20]

The brain perceives the external world through the senses, and each individual human is influenced greatly by his or her experiences, leading to subjective views of existence and the passage of time.

Humans are variously said to possess consciousness, self-awareness, and a mind, which correspond roughly to the mental processes of thought. These are said to possess qualities such as self-awareness, sentience, sapience, and the ability to perceive the relationship between oneself and one's environment. The extent to which the mind constructs or experiences the outer world is a matter of debate, as are the definitions and validity of many of the terms used above. Cognitive scientist Daniel Dennett, for example, argues that there is no such thing as a narrative centre called the "mind", but that instead there is simply a collection of sensory inputs and outputs: different kinds of "software" running in parallel.^[21]

Humans study the more physical aspects of the mind and brain, and by extension of the nervous system, in the field of neurology, the more behavioral in the field of psychology, and a sometimes loosely-defined area between in the field of psychiatry, which treats mental illness and behavioral disorders. Psychology does not necessarily refer to the brain or nervous system, and can be framed purely in terms of phenomenological or information processing theories of the mind. Increasingly, however, an understanding of brain functions is being included in psychological theory and practice, particularly in areas such as artificial intelligence, neuropsychology, and cognitive neuroscience.

The nature of thought is central to psychology and related fields. Cognitive psychology studies cognition, the mental processes underlying behavior. It uses information processing as a framework for understanding the mind. Perception, learning, problem solving, memory, attention, language and emotion are all well-researched areas as well. Cognitive psychology is associated with a school of thought known as cognitivism, whose adherents argue for an information processing model of mental function, informed by positivism and experimental psychology. Techniques and models from cognitive psychology are widely applied and form the mainstay of psychological theories in many areas of both research and applied psychology. Largely focusing on the development of the human mind through the life span, developmental psychology seeks to understand how people come to perceive, understand, and act within the world and how these processes change as they age. This may focus on intellectual, cognitive, neural, social, or moral development.

Social psychology links sociology with psychology in their shared study of the nature and causes of human social behavior, with an emphasis on how people think towards each other and how they relate to each other. The behavior and mental processes of animals, both human and non-human, can be described through animal cognition, ethology, evolutionary psychology, and comparative psychology as well. Human ecology is an academic discipline that investigates how humans and human societies interact with both their natural environment and the human social environment.

 

Part of the series on Human Evolution
Sahelanthropus tchadensis - Orrorin tugenensis
Ardipithecus: A. kadabba - A. ramidus
Australopithecines
Australopithecus : A. afarensis - A. africanus - A. anamensis - A. bahrelghazali - A. garhi
Paranthropus: P. aethiopicus - P. boisei - P. robustus
Humans and Proto-humans
Kenyanthropus platyops
Homo: H. antecessor - H. habilis - H. rudolfensis - H. rhodesiensis - H. cepranensis - H. georgicus - H. erectus - H. ergaster - H. heidelbergensis - H. neanderthalensis - H. floresiensis - H. sapiens idaltu - H. sapiens sapiens