Animalia

Human[1] Temporal range: 0.195–0 Ma PreЄ Є O S D C P T J K Pg N ↓ Pleistocene – Recent
Humans from Southeast Asia
Conservation status
Least Concern (IUCN 3.1)[2]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Primates
Family:	Hominidae
Tribe:	Hominini
Genus:	Homo
Species:	H. sapiens
Binomial name
Homo sapiens Linnaeus, 1758
Subspecies
†Homo sapiens idaltu White et al., 2003 Homo sapiens sapiens
Range of Homo sapiens (green)
Synonyms
Species synonymy[1] aethiopicus Bory de St. Vincent, 1825 americanus Bory de St. Vincent, 1825 arabicus Bory de St. Vincent, 1825 aurignacensis Klaatsch & Hauser, 1910 australasicus Bory de St. Vincent, 1825 cafer Bory de St. Vincent, 1825 capensis Broom, 1917 columbicus Bory de St. Vincent, 1825 cro-magnonensis Gregory, 1921 drennani Kleinschmidt, 1931 eurafricanus (Sergi, 1911) grimaldiensis Gregory, 1921 grimaldii Lapouge, 1906 hottentotus Bory de St. Vincent, 1825 hyperboreus Bory de St. Vincent, 1825 indicus Bory de St. Vincent, 1825 japeticus Bory de St. Vincent, 1825 melaninus Bory de St. Vincent, 1825 monstrosus Linnaeus, 1758 neptunianus Bory de St. Vincent, 1825 palestinus McCown & Keith, 1932 patagonus Bory de St. Vincent, 1825 priscus Lapouge, 1899 proto-aethiopicus Giuffrida-Ruggeri, 1915 scythicus Bory de St. Vincent, 1825 sinicus Bory de St. Vincent, 1825 spelaeus Lapouge, 1899 troglodytes Linnaeus, 1758 wadjakensis Dubois, 1921

Humans (known taxonomically as Homo sapiens,^[3][4] Latin for "wise man" or "knowing man")^[5] are the only living species in the Homo genus. Anatomically modern humans originated in Africa about 200,000 years ago, reaching full behavioral modernity around 50,000 years ago.^[6]

Humans have a highly developed brain and are capable of abstract reasoning, language, introspection, and problem solving. This mental capability, combined with an erect body carriage that frees the hands for manipulating objects, has allowed humans to make far greater use of tools than any other living species on Earth. Other higher-level thought processes of humans, such as self-awareness, rationality, and sapience,^[7][8][9] are considered to be defining features of what constitutes a "person".^[10]

Humans are uniquely adept at utilizing systems of communication for self-expression, the exchange of ideas, and organization. Humans create complex social structures composed of many cooperating and competing groups, from families and kinship networks, to nations. Social interactions between humans have established an extremely wide variety of values, social norms, and rituals, which together form the basis of human society. With individuals widespread in every continent except Antarctica, humans are a cosmopolitan species. As of November 2011^[update], the human population was estimated by the United Nations Population Division to be about 7 billion,^[11] and by the United States Census Bureau to be about 6.97 billion.^[12]

Humans are noted for their desire to understand and influence their environment, seeking to explain and manipulate phenomena through science, philosophy, mythology, and religion. This natural curiosity has led to the development of advanced tools and skills, which are passed down culturally; humans are the only extant species known to build fires and cook their food, as well as the only known species to clothe themselves, and create and use numerous other technologies and arts. The study of humans is the scientific discipline of anthropology.

Etymology[link]

The English adjective human is a Middle English loanword from Old French humain, ultimately from Latin hūmānus, the adjective form of homō "man". The word's use as a noun (with a plural: humans) dates to the 16th century.^[13] The native English term man can refer to the species generally (a synonym for mankind), and could formerly refer to specific individuals of either sex. The latter use is now obsolete.^[14] Generic uses of the term "man" are declining, in favor of reserving it for referring specifically to adult males. The word is from Proto-Germanic *mannaz, from a Proto-Indo-European (PIE) root *man-, a cognate to Sanskrit manu-.^{[citation needed]}

The species binomial Homo sapiens was coined by Carl Linnaeus in his 18th century work Systema Naturae, and he himself is the lectotype specimen.^[15] The generic name Homo is a learned 18th century derivation from Latin homō "man", ultimately "earthly being" (Old Latin hemō, a cognate to Old English guma "man", from PIE *dʰǵʰ_emon-, meaning 'earth' or 'ground').^[16] The species-name sapiens means "wise" or "sapient".

History[link]

Evolution[link]

Plesiadapis, the earliest known primate

Craniums
1. Gorilla 2. Australopithecus 3. Homo erectus 4. Neanderthal (La Chapelle aux Saints) 5. Steinheim Skull 6. Euhominid

Scientific study of human evolution is concerned, primarily, with the development of the genus Homo, but usually involves studying other hominids and hominines as well, such as Australopithecus. "Modern humans" are defined as the Homo sapiens species, of which the only extant subspecies is known as Homo sapiens sapiens. Homo sapiens idaltu (roughly translated as "elder wise human"), the other known subspecies, is now extinct.^[17] Homo neanderthalensis, which became extinct 30,000 years ago, has sometimes been classified as a subspecies, "Homo sapiens neanderthalensis"; genetic studies now suggest that the functional DNA of modern humans and Neanderthals diverged 500,000 years ago.^[18] More recent genetics suggest that modern humans mated with "at least two groups" of ancient humans: Neanderthals and Denisovans.^[19] Nonetheless, the discovered specimens of the Homo rhodesiensis species have been classified by some as a subspecies, but this classification is not widely accepted.

Anatomically modern humans first appear in the fossil record in Africa about 195,000 years ago, and studies of molecular biology give evidence that the approximate time of divergence from the common ancestor of all modern human populations was 200,000 years ago.^{[20][21][22][23][24]} The broad study of African genetic diversity headed by Dr. Sarah Tishkoff found the San people to express the greatest genetic diversity among the 113 distinct populations sampled, making them one of 14 "ancestral population clusters". The research also located the origin of modern human migration in south-western Africa, near the coastal border of Namibia and Angola.^[25]

The evolutionary history of primates can be traced back 65 million years. Primates are one of the oldest of all surviving placental mammal groups. The oldest known primate-like mammal species (those of the genus Plesiadapis) come from North America, but inhabited Eurasia and Africa on a wide scale during the tropical conditions of the Paleocene and Eocene. Molecular evidence suggests that the last common ancestor between humans and the remaining great apes diverged 4–8 million years ago.^{[citation needed]}

The orangutans were the first group to split from the line leading to the humans, then gorillas followed by chimpanzees (genus Pan). The functional portion of human DNA is approximately 98.4% identical to that of chimpanzees when comparing single-nucleotide polymorphisms (see human evolutionary genetics). Some studies put that as low as 94%. Therefore, the closest living relatives of humans are gorillas and chimpanzees, as they share a relatively recent common ancestor.^[26]

Humans are probably most closely related to two chimpanzee species: the common chimpanzee and the bonobo.^[26] Full genome sequencing has resulted in the conclusion that "after 6.5 [million] years of separate evolution, the differences between chimpanzee and human are ten times greater than those between two unrelated people and ten times less than those between rats and mice".^{[attribution needed]} Current estimates of suggested concurrence between functional human and chimpanzee DNA sequences range between 95% and 99%.^{[27][28][29][30]} Early estimates indicated that the human lineage may have diverged from that of chimpanzees about five million years ago, and from that of 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, and may be evidence of an earlier divergence.^[31]

Human evolution is characterized by a number of important changes—morphological, developmental, physiological, and behavioral—that have taken place since the split between the last common ancestor of humans and chimpanzees. The first major morphological change was the evolution of a bipedal locomotor adaptation from an arboreal or semi-arboreal one,^[32] with all its attendant adaptations (a valgus knee, low intermembral index (long legs relative to the arms), reduced upper-body strength).

The human species developed a much larger brain than that of other primates – typically 1,400 cm³ (85 cu in) in modern humans, over twice the size of that of a chimpanzee or gorilla. The pattern of human postnatal brain growth differs from that of other apes (heterochrony), and allows for extended periods of social learning and language acquisition in juvenile humans. Physical anthropologists argue that the differences between the structure of human brains and those of other apes are even more significant than their differences in size.^[33][34]

Other significant morphological changes included the evolution of a power and precision grip,^[35] a reduced masticatory system, a reduction of the canine tooth, and the descent of the larynx and hyoid bone, making speech possible. An important physiological change in humans was the evolution of hidden estrus, or concealed ovulation, which may have coincided with the evolution of important behavioral changes, such as pair bonding. Another significant behavioral change was the development of material culture, with human-made objects becoming increasingly common and diversified over time. The relationship between all these changes is the subject of ongoing debate.^[36][37]

The forces of natural selection have continued to operate on human populations, with evidence that certain regions of the genome display directional selection in the past 15,000 years.^[38]

The hominoids are descendants of a common ancestor.

Paleolithic[link]

This section's factual accuracy may be compromised due to out-of-date information. Please help improve the article by updating it. There may be additional information on the talk page. (May 2012)

Artistic expression appeared in the Upper Paleolithic: The Venus of Dolní Věstonice figurine, one of the earliest known depictions of the human body, dates to approximately 29,000–25,000 BP (Gravettian).

Anatomically modern humans evolved from archaic Homo sapiens in Africa in the Middle Paleolithic, about 200,000 years ago. By the beginning of the Upper Paleolithic period (50,000 BP [Before Present]), full behavioral modernity, including language, music and other cultural universals had developed.

The out of Africa migration is estimated to have occurred about 70,000 years BP. Modern humans subsequently spread globally, replacing earlier hominids: they inhabited Eurasia and Oceania by 40,000 years BP, and the Americas at least 14,500 years BP.^[39] A popular theory is that they displaced Homo neanderthalensis and other species descended from Homo erectus^[40] (which had inhabited Eurasia as early as 2 million years ago) through more successful reproduction and competition for resources.^[41] The exact manner or extent of the coexistence and interaction of these species is unknown and continues to be a controversial subject.^[42]

Evidence from archaeogenetics accumulating since the 1990s has lent strong support to the "out-of-Africa" scenario, and has marginalized the competing multiregional hypothesis, which proposed that modern humans evolved, at least in part, from independent hominid populations.^[43]

Geneticists Lynn Jorde and Henry Harpending of the University of Utah propose that the variation in human DNA is minute compared to that of other species.^[44] They also propose that during the Late Pleistocene, the human population was reduced to a small number of breeding pairs – no more than 10,000, and possibly as few as 1,000 – resulting in a very small residual gene pool.^[45] Various reasons for this hypothetical bottleneck have been postulated, one being the Toba catastrophe theory.^[46]

Transition to civilization[link]

The rise of agriculture, and domestication of animals, led to stable human settlements.

The path followed by humans in the course of history

Until c. 10,000 years ago, most humans lived as hunter-gatherers. They generally lived in small nomadic groups known as band societies. The advent of agriculture prompted the Neolithic Revolution, when access to food surplus led to the formation of permanent human settlements, the domestication of animals and the use of metal tools for the first time in history. Agriculture encouraged trade and cooperation, and led to complex society. Because of the significance of this date for human society, it is the epoch of the Holocene calendar or Human Era.

About 6,000 years ago, the first proto-states developed in Mesopotamia, Egypt's Nile Valley and the Indus Valleys. Military forces were formed for protection, and government bureaucracies for administration. States cooperated and competed for resources, in some cases waging wars. Around 2,000–3,000 years ago, some states, such as Persia, India, China, Rome, and Greece, developed through conquest into the first expansive empires. Ancient Greece was the seminal civilization that laid the foundations of Western culture, being the birthplace of Western philosophy, democracy, major scientific and mathematical advances, the Olympic Games, Western literature and historiography, as well as Western drama, including both tragedy and comedy.^[47] Influential religions, such as Judaism, originating in West Asia, and Hinduism, a religious tradition that originated in South Asia, also rose to prominence at this time.

The late Middle Ages saw the rise of revolutionary ideas and technologies. In China, an advanced and urbanized society promoted innovations and sciences, such as printing and seed drilling. In India, major advancements were made in mathematics, philosophy, religion and metallurgy. The Islamic Golden Age saw major scientific advancements in Muslim empires. In Europe, the rediscovery of classical learning and inventions such as the printing press led to the Renaissance in the 14th and 15th centuries. Over the next 500 years, exploration and colonialism brought great parts of the world under European control, leading to later struggles for independence. The Scientific Revolution in the 17th century and the Industrial Revolution in the 18th–19th centuries promoted major innovations in transport, such as the railway and automobile; energy development, such as coal and electricity; and government, such as representative democracy and Communism.

With the advent of the Information Age at the end of the 20th century, modern humans live in a world that has become increasingly globalized and interconnected. As of 2010, almost 2 billion humans are able to communicate with each other via the Internet,^[48] and 3.3 billion by mobile phone subscriptions.^[49]

Although interconnection between humans has encouraged the growth of science, art, discussion, and technology, it has also led to culture clashes and the development and use of weapons of mass destruction. Human civilization has led to environmental destruction and pollution significantly contributing to the ongoing mass extinction of other forms of life called the holocene extinction event,^[50] that may be further accelerated by global warming in the future.^[51]

Habitat and population[link]

Humans often live in family-based social structures and create artificial shelter.

Early human settlements were dependent on proximity to water and, depending on the lifestyle, other natural resources used for subsistence, such as populations of animal prey for hunting and arable land for growing crops and grazing livestock. But humans have a great capacity for altering their habitats by means of technology, through irrigation, urban planning, construction, transport, manufacturing goods, deforestation and desertification. Deliberate habitat alteration is often done with the goals of increasing material wealth, increasing thermal comfort, improving the amount of food available, improving aesthetics, or improving ease of access to resources or other human settlements. 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 virtually all climates. Within the last century, humans have explored Antarctica, the ocean depths, and outer space, although large-scale colonization of these environments is not yet feasible. With a population of over six billion, humans are among the most numerous of the large mammals. Most humans (61%) live in Asia. The remainder live in the Americas (14%), Africa (14%), Europe (11%), and Oceania (0.5%).^{[citation needed]}

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.^[52] Between 1969 and 1972, two humans at a time spent brief intervals on the Moon. As of June 2012, no other celestial body has been visited by humans, 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.^[53] However, other celestial bodies have been visited by human-made objects.

Since 1800, the human population has increased from one billion to over six billion.^[54] In 2004, some 2.5 billion out of 6.3 billion people (39.7%) lived in urban areas, and this percentage is expected to continue to rise throughout the 21st century. In February 2008, the U.N. estimated that half the world's population will live in urban areas by the end of the year.^[55] Problems for humans living in cities include various forms of pollution and crime,^[56] especially in inner city and suburban slums.

Humans have had a dramatic effect on the environment. As humans are rarely preyed upon, they have been described as superpredators.^[57] Currently, through land development, combustion of fossil fuels, and pollution, humans are thought to be the main contributor to global climate change.^[58] If this continues at its current rate it is predicted that it will wipe out half of all species over the next century.^[59][60]

Biology[link]

Anatomy[link]

Basic anatomical features of female and male humans. These models have had body hair and male facial hair removed and head hair trimmed.

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 an adult human is about 1.5 to 1.8 m (5 to 6 feet) tall, although this varies significantly from place to place and depending on ethnic origin.^[61] The average mass of an adult human is 54–64 kg (120–140 lbs) for females and 76–83 kg (168–183 lbs) for males.^[62] Weight can also vary greatly (e.g. obesity). Unlike most other primates, humans are capable of fully bipedal locomotion, thus leaving their arms available for manipulating objects using their hands, aided especially by opposable thumbs.

Vitruvian Man, Leonardo da Vinci's image is often used as an implied symbol of the essential symmetry of the human body, and by extension, of the universe as a whole.

Although humans appear 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 follicles on his or her body than the average chimpanzee. The main distinction is that human hairs are shorter, finer, and less heavily pigmented than the average chimpanzee's, thus making them harder to see.^[63]

The hue of human skin and hair is determined by the presence of pigments called melanins. Human skin hues can range from dark brown to pale pink. Human hair ranges from white to brown to red to most commonly black.^[64] This depends on the amount of melanin (an effective sun blocking pigment) in the skin and hair, with hair melanin concentrations in hair fading with increased age, leading to grey or even white hair. Most researchers believe that skin darkening was an adaptation that evolved as a protection against ultraviolet solar radiation. However, more recently it has been argued that particular skin colors are an adaptation to balance folate, which is destroyed by ultraviolet radiation, and vitamin D, which requires sunlight to form.^[65] The skin pigmentation 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.^[66][67] Humans tend to be physically weaker than other similarly sized primates, with young, conditioned male humans having been shown to be unable to match the strength of female orangutans, which are at least three times stronger.^[68]

The construction of the human pelvis differs from other primates, as do the toes. As a result, humans are slower for short distances than most other animals, but are among the best long-distance runners in the animal kingdom.^[69] Humans' thinner body hair and more productive sweat glands also help avoid heat exhaustion while running for long distances. For this reason persistence hunting was most likely a very successful strategy for early humans – in this method, prey is chased until it is literally exhausted. This may have also helped the early human Cro-Magnon population out-compete the Neanderthal population for food. The otherwise physically stronger Neanderthal would have much greater difficulty hunting in this way, and much more likely hunted larger game in close quarters. A trade-off for these advantages of the modern human pelvis is that childbirth is more difficult and dangerous.

The construction of modern human shoulders enabled throwing weapons, which also were much more difficult or even impossible for Neanderthal competitors to use effectively.^[70]

The dental formula of Humans is the following: Failed to parse (Missing texvc executable; please see math/README to configure.): \tfrac{ 2.1.2.3}{ 2.1.2.3} . Humans have proportionately shorter palates and much smaller teeth than other primates. They are the only primates to have short, relatively flush canine teeth. Humans have characteristically crowded teeth, with gaps from lost teeth usually closing up quickly in young individuals. Humans are gradually losing their wisdom teeth, with some individuals having them congenitally absent.^[72]

Physiology[link]

Human physiology is the science of the mechanical, physical, and biochemical functions of humans in good health, their organs, and the cells of which they are composed. The principal level of focus of physiology is at the level of organs and systems. Most aspects of human physiology are closely homologous to corresponding aspects of animal physiology, and animal experimentation has provided much of the foundation of physiological knowledge. Anatomy and physiology are closely related fields of study: anatomy, the study of form, and physiology, the study of function, are intrinsically tied and are studied in tandem as part of a medical curriculum.

Genetics[link]

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 22,000 genes.^[73] 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 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[link]

A 10 mm human embryo at 5 weeks

As with other mammals, human reproduction takes place as internal fertilization by sexual intercourse. During this process, the erect penis of the male is inserted into the female's vagina until the male ejaculates semen, which contains sperm. The sperm travels through the vagina and cervix into the uterus or Fallopian tubes for fertilization of the ovum. Upon fertilization and implantation, gestation then occurs within the female's uterus.

The zygote divides inside the female's uterus to become an embryo, which over a period of thirty-eight weeks (9 months) of gestation becomes a fetus. After this span of time, the fully grown fetus is birthed from the woman'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 various levels of personhood earlier to human fetuses while they remain in the uterus.

Compared with other species, human childbirth is dangerous. Painful labors lasting twenty-four hours or more are not uncommon and sometimes lead to the death of the mother, or the child.^[74] This is because of both the relatively large fetal head circumference (for housing the brain) and the mother's relatively narrow pelvis (a trait required for successful bipedalism, by way of natural selection).^[75][76] The chances of a successful labor increased significantly during the 20th century in wealthier countries with the advent of new medical technologies. In contrast, pregnancy and natural childbirth remain hazardous ordeals in developing regions of the world, with maternal death rates approximately 100 times greater than in developed countries.^[77]

In developed countries, infants are typically 3–4 kg (6–9 pounds) in weight and 50–60 cm (20–24 inches) in height at birth.^[78] However, low birth weight is common in developing countries, and contributes to the high levels of infant mortality in these regions.^[79] Helpless at birth, humans continue to grow for some years, typically reaching sexual maturity at 12 to 15 years of age. Females continue to develop physically until around the age of 18, whereas male development continues until around age 21. The human life span can be split into a number of stages: infancy, childhood, adolescence, young adulthood, adulthood and old age. The lengths of these stages, however, have varied across cultures and time periods. Compared to other primates, humans experience an unusually rapid growth spurt during adolescence, where the body grows 25% in size. Chimpanzees, for example, grow only 14%, with no pronounced spurt.^[80] The presence of the growth spurt is probably necessary to keep children physically small until they are psychologically mature. Humans are one of the few species in which females undergo menopause. It has been proposed that menopause increases a woman's overall reproductive success by allowing her to invest more time and resources in her existing offspring and/or their children (the grandmother hypothesis), rather than by continuing to bear children into old age.^[81][82]

There are significant differences in life expectancy around the world. The developed world is generally aging, with the median age around 40 years. In the developing world the median age is between 15 and 20 years. Life expectancy at birth in Hong Kong is 84.8 years for a female and 78.9 for a male, while in Swaziland, primarily because of AIDS, it is 31.3 years for both sexes.^[83] While one in five Europeans is 60 years of age or older, only one in twenty Africans is 60 years of age or older.^[84] The number of centenarians (humans of age 100 years or older) in the world was estimated by the United Nations at 210,000 in 2002.^[85] At least one person, Jeanne Calment, is known to have reached the age of 122 years;^[86] higher ages have been claimed but they are not well substantiated. Worldwide, there are 81 men aged 60 or older for every 100 women of that age group, and among the oldest, there are 53 men for every 100 women.

• A selection of different humans at various stages of the human life cycle
• 

  Girl (before puberty)

• 

  Woman of reproductive age

• 

  Older woman (after menopause)

• 

  Boy (before puberty)

• 

  Adult man

• 

  Elderly man

Diet[link]

Humans are omnivorous, capable of consuming a wide variety of plant and animal material.^[87][88] Varying with available food sources in regions of habitation, and also varying with cultural and religious norms, human groups have adopted a range of diets, from purely vegetarian to primarily carnivorous. In some cases, dietary restrictions in humans can lead to deficiency diseases; however, stable human groups have adapted to many dietary patterns through both genetic specialization and cultural conventions to use nutritionally balanced food sources.^[89] The human diet is prominently reflected in human culture, and has led to the development of food science.

Until the development of agriculture approximately 10,000 years ago, Homo sapiens employed a hunter-gatherer method as their sole means of food collection. This involved combining stationary food sources (such as fruits, grains, tubers, and mushrooms, insect larvae and aquatic mollusks) with wild game, which must be hunted and killed in order to be consumed.^[90] It has been proposed that members of H. sapiens have used fire to prepare and cook food since the time of their divergence from Homo rhodesiensis (which itself had previously speciated from Homo erectus).^[91] Around ten thousand years ago, humans developed agriculture,^[92] which substantially altered their diet. This change in diet may also have altered human biology; with the spread of dairy farming providing a new and rich source of food, leading to the evolution of the ability to digest lactose in some adults.^[93][94] Agriculture led to increased populations, the development of cities, and because of 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.

In general, humans can survive for two to eight weeks without food, depending on stored body fat. Survival without water is usually limited to three or four days. About 36 million humans die every year from causes directly or indirectly related to hunger.^[95] Childhood malnutrition is also common and contributes to the global burden of disease.^[96] However global food distribution is not even, and obesity among some human populations has increased rapidly, leading to health complications and increased mortality in some developed, and a few developing countries. Worldwide over one billion people are obese,^[97] while in the United States 35% of people are obese, leading to this being described as an "obesity epidemic".^[98] Obesity is caused by consuming more calories than are expended, so excessive weight gain is usually caused by a combination of an energy-dense high fat diet and insufficient exercise.^[97]

Sleep[link]

Humans are generally diurnal. The average sleep requirement is between seven and nine hours per day for an adult and nine to ten hours per day for a child; elderly people usually sleep for six to seven hours. Experiencing less sleep than this is common in modern societies; this sleep deprivation can have negative effects. 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.^[99]

Biological variation[link]

Most current genetic and archaeological evidence supports a recent single origin of modern humans in East Africa^[100] with first migrations placed at 60,000 years ago. Current genetic studies have demonstrated that humans on the African continent are the most genetically diverse.^[101] However, compared to the other great apes, human gene sequences are remarkably homogeneous.^[102]

Nonetheless, there is important biological variation in the human species - with traits such as skin color, eye color, hair color and texture, height and built, and cranial features varying clinally across the globe. Those aspects of genetic variation that gives clue to human evolutionary history, or which are relevant for medical research have received particular attention. For example the genes that cause adult humans to be able to digest lactose are present in high frequencies in population that have long histories of cattle domestication, suggesting natural selection having favored that gene in populations that depend on cow milk. Some hereditary diseases such as Sickle cell anemia are frequent in populations from areas in which Malaria has been endemic throughout history - it is believed that the same gene that causes increased resistance to Malaria among those who are unaffected carriers of the gene. Similarly populations that have inhabited specific climates such as arctic or tropical regions or high altitudes, tend to have developed specific phenotypes that are beneficial for conserving energy in those environments - short stature and stocky built in cold regions, tall and lanky in hot regions and with high lung capacities in high altitudes. Similarly variation in skincolor varies clinally with darker colors around the equator where the added protection from the sun is thought to gives an evolutionary advantage, and lighter skin tones closer to the poles where there is less sunlight and the lighter colored skin improves Vitamin D synthesis.

Today it is possible to determine, by genetic analysis, the geographic ancestry of a person and the degree of ancestry from each region. Such analyses can pinpoint the migrational history of a persons ancestors with a high degree of accuracy. Often, due to practices of group endogamy, allele frequencies cluster locally around kin groups and lineages, or by national, cultural or linguistic boundaries, giving a detailed degree of correlation between genetic clusters and population groups when considering many alleles simultaneously.

Race[link]

There is no scientific consensus on the biological relevance of race. Few anthropologists endorse the notion of human "race" as a basically biological concept. Most anthropologists also maintain that the term "race" tacitly assumes that races are clearly bounded groups with essential characteristics, often ordered hierarchically and used to justify social inequality. For these reasons Anthropologists tend to reject using the word race to describe biological variation. They tend to see race as a social construct superimposed on, but partly obscuring, underlying biological variation.^{[103][104][105]} An opposing view has it that it is possible to talk about "races" without making essentialist or hierarchical assumptions, and some biologists and many forensic scientists use the word race to describe biological variation associated with continental ancestry. It is generally agreed upon that certain genetic traits, including some common illnesses, correlate with genetic ancestry from specific regions, and genetic ancestry as determined by racial identification is becoming an increasingly common tool for risk assessment in medicine.^{[106][107][108][109][110][111][112][113]}

The use of the term "race" to mean something like "subspecies" among humans is obsolete; Homo sapiens has no existing subspecies. In its modern scientific connotation, the term is not applicable to a species as genetically homogeneous as the human one, as stated in the declaration on race (UNESCO 1950, re-ratified 1978^[114]).^[115] Genetic studies have substantiated the absence of clear biological borders; thus the term "race" is rarely used in scientific terminology, either in biological anthropology and in human genetics.^[116] What in the past had been defined as "races"—whites, blacks, or Asians—are now defined as "ethnic groups" or "populations", in correlation with the field (sociology, anthropology, genetics) in which they are considered.^[117][118]

Psychology[link]

The human brain, the focal point of the central nervous system in humans, controls the peripheral nervous system. In addition to controlling "lower", involuntary, or primarily autonomic activities such as respiration and digestion, it is also the locus of "higher" order functioning such as thought, reasoning, and abstraction.^[119] These cognitive processes constitute the mind, and, along with their behavioral consequences, are studied in the field of psychology.

Generally regarded as more capable of these higher order activities, the human brain is believed to be more "intelligent" in general than that of any other known species. While some non-human species are capable of creating structures and using simple tools—mostly through instinct and mimicry—human technology is vastly more complex, and is constantly evolving and improving through time.

Although humans are vastly more advanced than many species in cognitive abilities, most of these abilities are known in primitive form among other species. Modern anthropology has tended to bear out Darwin's proposition that "the difference in mind between man and the higher animals, great as it is, certainly is one of degree and not of kind".^[120]

Consciousness and thought[link]

Humans are one of only several species known to pass the mirror test.^[121] Most human children will pass the mirror test at 18 months old.^[122] However, the usefulness of this test as a true test of consciousness has been disputed, and this may be a matter of degree rather than a sharp divide. Monkeys have been trained to apply abstract rules in tasks.^[123]

The human 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. The philosopher of cognitive science Daniel Dennett, for example, argues that there is no such thing as a narrative center called the "mind", but that instead there is simply a collection of sensory inputs and outputs: different kinds of "software" running in parallel.^[124] Psychologist B.F. Skinner argued that the mind is an explanatory fiction that diverts attention from environmental causes of behavior,^[125] and that what are commonly seen as mental processes may be better conceived of as forms of covert verbal behavior.^[126][127]

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.

Some philosophers divide consciousness into phenomenal consciousness, which is experience itself, and access consciousness, which is the processing of the things in experience.^[128] Phenomenal consciousness is the state of being conscious, such as when they say "I am conscious." Access consciousness is being conscious of something in relation to abstract concepts, such as when one says "I am conscious of these words." Various forms of access consciousness include awareness, self-awareness, conscience, stream of consciousness, Husserl's phenomenology, and intentionality. The concept of phenomenal consciousness, in modern history, according to some, is closely related to the concept of qualia. Social psychology links sociology with psychology in their shared study of the nature and causes of human social interaction, with an emphasis on how people think towards each other and how they relate to each other. The behavior and mental processes, 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.

Motivation and emotion[link]

Motivation is the driving force of desire behind all deliberate actions of humans. Motivation is based on emotion—specifically, on the search for satisfaction (positive emotional experiences), and the avoidance of conflict. Positive and negative is defined by the individual brain state, which may be influenced by social norms: a person may be driven to self-injury or violence because his brain is conditioned to create a positive response to these actions. Motivation is important because it is involved in the performance of all learned responses. Within psychology, conflict avoidance and the libido are seen to be primary motivators. Within economics, motivation is often seen to be based on incentives; these may be financial, moral, or coercive. Religions generally posit divine or demonic influences.

Happiness, or the state of being happy, is a human emotional condition. The definition of happiness is a common philosophical topic. Some people might define it as the best condition that a human can have—a condition of mental and physical health. Others define it as freedom from want and distress; consciousness of the good order of things; assurance of one's place in the universe or society.

Emotion has a significant influence on, or can even be said to control, human behavior, though historically many cultures and philosophers have for various reasons discouraged allowing this influence to go unchecked. Emotional experiences perceived as pleasant, such as love, admiration, or joy, contrast with those perceived as unpleasant, like hate, envy, or sorrow. There is often a distinction made between refined emotions that are socially learned and survival oriented emotions, which are thought to be innate. Human exploration of emotions as separate from other neurological phenomena is worthy of note, particularly in cultures where emotion is considered separate from physiological state. In some cultural medical theories emotion is considered so synonymous with certain forms of physical health that no difference is thought to exist. The Stoics believed excessive emotion was harmful, while some Sufi teachers felt certain extreme emotions could yield a conceptual perfection, what is often translated as ecstasy.

In modern scientific thought, certain refined emotions are considered a complex neural trait innate in a variety of domesticated and non-domesticated mammals. These were commonly developed in reaction to superior survival mechanisms and intelligent interaction with each other and the environment; as such, refined emotion is not in all cases as discrete and separate from natural neural function as was once assumed. However, when humans function in civilized tandem, it has been noted that uninhibited acting on extreme emotion can lead to social disorder and crime.

Society and culture[link]