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Human

Common name of Homo sapiens, unique extant species of the genus Homo

Top 10 Human related articles

  (Redirected from Homo sapiens)
Human[1]
Temporal range: 0.35–0 Ma
Middle PleistocenePresent
An adult human male (left) and female (right) from the Akha tribe in Northern Thailand
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Suborder: Haplorhini
Infraorder: Simiiformes
Family: Hominidae
Subfamily: Homininae
Tribe: Hominini
Genus: Homo
Species:
H. sapiens
Binomial name
Homo sapiens
Linnaeus, 1758
Subspecies
Homo sapiens population density
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 (Homo sapiens) are a species of highly intelligent primates. They are the only extant members of the subtribe Hominina and—together with chimpanzees, gorillas, and orangutans—are part of the family Hominidae (the great apes, or hominids). Humans are terrestrial animals, characterized by their erect posture and bipedal locomotion; high manual dexterity and heavy tool use compared to other animals; open-ended and complex language use compared to other animal communications; larger, more complex brains than other primates; and highly advanced and organized societies.[3][4]

Several early hominins used fire and occupied much of Eurasia. Early modern humans are thought to have diverged in Africa from an earlier hominin around 300,000 years ago, with the earliest fossil evidence of Homo sapiens also appearing around 300,000 years ago in Africa.[5] Humans began to exhibit evidence of behavioral modernity at least by about 150,000–75,000 years ago and possibly earlier.[6][7][8][9][10] In several waves of migration, H. sapiens ventured out of Africa and populated most of the world.[11][12] The spread of the large and increasing population of humans has profoundly affected the biosphere and millions of species worldwide. Among the key advantages that explain this evolutionary success is the presence of a larger, well-developed brain, which enables advanced abstract reasoning, language, problem solving, sociality, and culture through social learning. Humans use tools more frequently and effectively than any other animal: they are the only extant species to build fires, cook food, clothe themselves, and create and use numerous other technologies and arts.

Humans uniquely use systems of symbolic communication such as language and art to express themselves and exchange ideas, as well as to organize themselves into purposeful groups. Humans create complex social structures composed of many cooperating and competing groups, from families and kinship networks to political states. Social interactions between humans have established an extremely wide variety of values,[13] social norms, and rituals, which together undergird human society. Curiosity and the human desire to understand and influence the environment and to explain and manipulate phenomena have motivated humanity's development of science, philosophy, mythology, religion, and other fields of knowledge.

Though most of human existence has been sustained by hunting and gathering in band societies,[14] many human societies transitioned to sedentary agriculture approximately 10,000 years ago,[15] domesticating plants and animals, thus enabling the growth of civilization. These human societies subsequently expanded, establishing various forms of government and culture around the world, and unifying people within regions to form states and empires. The rapid advancement of scientific and medical understanding in the 19th and 20th centuries permitted the development of more efficient medical tools and healthier lifestyles, resulting in increased lifespans and causing the human population to rise exponentially.[16] The global human population is about 7.8 billion in 2021.[17][18]

Human Intro articles: 43

Etymology and definition

Although it can be applied to other members of the genus Homo, in common usage the word "human" generally refers to the only extant species—Homo sapiens. The definition of H. sapiens itself is debated. Some paleoanthropologists include fossils that others have allocated to different species, while the majority assign only fossils that align anatomically with the species as it exists today.[19]

The English word "human" is a Middle English loanword from Old French humain, ultimately from Latin hūmānus, the adjectival form of homō ("man" - in the sense of humankind).[20] The native English term man can refer to the species generally (a synonym for humanity) as well as to human males. It may also refer to individuals of either sex, though this latter form is less common in contemporary English.[21]

The species binomial "Homo sapiens" was coined by Carl Linnaeus in his 18th-century work Systema Naturae.[22] The generic name "Homo" is a learned 18th-century derivation from Latin homō, which refers to humans of either sex.[23] The species name "sapiens" means "wise", "sapient", "knowledgeable" (Latin sapiens is the singular form, plural is sapientes).[24]

Human Etymology and definition articles: 11

Evolution

The genus Homo evolved and diverged from other hominins in Africa several million years ago, after the human clade split from the chimpanzee lineage of the hominids (great apes) branch of the primates.[25] Modern humans, specifically the subspecies Homo sapiens sapiens, proceeded to colonize all the continents and larger islands, arriving in Eurasia 125,000–60,000 years ago,[26][27] Australia around 65,000 years ago,[28] the Americas around 15,000 years ago, and remote islands such as Hawaii, Easter Island, Madagascar, and New Zealand between the years 300 and 1280.[29][30]

Family tree showing the extant hominoids: humans (genus Homo), chimpanzees and bonobos (genus Pan), gorillas (genus Gorilla), orangutans (genus Pongo), and gibbons (four genera of the family Hylobatidae: Hylobates, Hoolock, Nomascus, and Symphalangus). All except gibbons are hominids.

The closest living relatives of humans are chimpanzees and bonobos (genus Pan),[31][32] as well as gorillas (genus Gorilla).[33] The gibbons (family Hylobatidae) and orangutans (genus Pongo) were the first groups to split from the lineage leading to humans, then gorillas, and finally, chimpanzees. The splitting date between human and chimpanzee lineages is placed 4–8 million years ago, during the late Miocene epoch.[34][35] During this split, chromosome 2 was formed from the joining of two other chromosomes, leaving humans with only 23 pairs of chromosomes, compared to 24 for the other apes.[36]

The earliest fossils that have been proposed as members of the hominin lineage are Sahelanthropus tchadensis, dating from 7 million years ago; Orrorin tugenensis, dating from 5.7 million years ago; and Ardipithecus kadabba, dating to 5.6 million years ago. From these early species, the australopithecines arose around 4 million years ago, diverging into robust (Paranthropus) and gracile (Australopithecus) branches,[37] possibly one of which—such as A. garhi, dating to 2.5 million years ago—is a direct ancestor of the genus Homo.[38]

The earliest members of Homo evolved around 2.8 million years ago.[39] H. habilis has been considered the first species for which there is clear evidence of the use of stone tools.[40] Nonetheless, the brains of H. habilis were about the same size as that of a chimpanzee, and their main adaptation was bipedalism. During the next million years a process of encephalization began, and with the arrival of Homo erectus in the fossil record, cranial capacity had doubled. H. erectus were the first of the hominina to leave Africa, between 1.3 to 1.8 million years ago. One population, also sometimes classified as a separate species Homo ergaster, stayed in Africa and evolved into Homo sapiens. It is believed that these species were the first to use fire and complex tools.

Although the narratives of human evolution are often contentious, several discoveries since 2010 show that human evolution should not be seen as a simple linear or branched progression, but a mix of related species.[41][42][43] In fact, genomic research has shown that hybridization between substantially diverged lineages is the rule, not the exception, in human evolution.[44] Furthermore, it is argued that hybridization was an essential creative force in the emergence of modern humans.[44]

The earliest transitional fossils between H. ergaster/erectus and archaic humans are from Africa, such as Homo rhodesiensis, but seemingly transitional forms have also been found in Dmanisi, Georgia. These descendants of H. erectus spread through Eurasia c. 500,000 years ago, evolving into H. antecessor, H. heidelbergensis and H. neanderthalensis. Fossils of anatomically modern humans that date from the Middle Paleolithic (about 200,000 years ago) include the Omo-Kibish I remains of Ethiopia[45][46][47] and the fossils of Herto Bouri, Ethiopia. Earlier remains now classified as early Homo sapiens, such as the Jebel Irhoud remains from Morocco and the Florisbad Skull from South Africa, have been dated to about 300,000 and 259,000 years old respectively.[48][5][49][50][51][52] Fossil records of archaic Homo sapiens from Skhul in Israel and Southern Europe begin around 90,000 years ago.[53]

Anatomical adaptations

Human evolution is characterized by a number of morphological, developmental, physiological, and behavioral changes that have taken place since the split between the last common ancestor of humans and chimpanzees. The most significant of these adaptations are 1. bipedalism, 2. increased brain size, 3. lengthened ontogeny (gestation and infancy), 4. decreased sexual dimorphism (neoteny). The relationship between all these changes is the subject of ongoing debate.[54] Other significant morphological changes included the evolution of a power and precision grip, a change first occurring in H. erectus.[55]

Bipedalism is the basic adaption of the hominin line, and it is considered the main cause behind a suite of skeletal changes shared by all bipedal hominins. The earliest bipedal hominin is considered to be either Sahelanthropus[56] or Orrorin, with Ardipithecus, a full bipedal,[57] coming somewhat later. The knuckle walkers, the gorilla and chimpanzee, diverged around the same time, and either Sahelanthropus or Orrorin may be humans' last shared ancestor with those animals. [58] The early bipedals eventually evolved into the australopithecines and later the genus Homo. [59] There are several theories of the adaptational value of bipedalism. It is possible that bipedalism was favored because it freed up the hands for reaching and carrying food, because it saved energy during locomotion, because it enabled long-distance running and hunting, or as a strategy for avoiding hyperthermia by reducing the surface exposed to direct sun. [60][61]

The human species developed a much larger brain than that of other primates—typically 1,330 cm3 (81 cu in) in modern humans, over twice the size of that of a chimpanzee or gorilla.[62] The pattern of encephalization started with Homo habilis which at approximately 600 cm3 (37 cu in) had a brain slightly larger than chimpanzees, and continued with Homo erectus (800–1,100 cm3 (49–67 cu in)), and reached a maximum in Neanderthals with an average size of 1,200–1,900 cm3 (73–116 cu in), larger even than Homo sapiens (but less encephalized).[63] 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. However, the differences between the structure of human brains and those of other apes may be even more significant than differences in size.[64][65][66][67] The increase in volume over time has affected different areas within the brain unequally—the temporal lobes, which contain centers for language processing have increased disproportionately, as has the prefrontal cortex which has been related to complex decision making and moderating social behavior.[62] Encephalization has been tied to an increasing emphasis on meat in the diet,[68][69] or with the development of cooking,[70] and it has been proposed [71] that intelligence increased as a response to an increased necessity for solving social problems as human society became more complex.

The reduced degree of sexual dimorphism is primarily visible in the reduction of the male canine tooth relative to other ape species (except gibbons). Another important physiological change related to sexuality in humans was the evolution of hidden estrus. Humans are the only ape in which the female is intermittently fertile year round, and in which no special signals of fertility are produced by the body (such as genital swelling during estrus). Nonetheless humans retain a degree of sexual dimorphism in the distribution of body hair and subcutaneous fat, and in the overall size, males being around 25% larger than females.

Human Evolution articles: 67

History

World map of early human migrations according to mitochondrial population genetics (numbers are millennia before present, the North Pole is at the center).

As early Homo sapiens dispersed, it encountered varieties of archaic humans both in Africa and in Eurasia, in Eurasia notably Homo neanderthalensis. Since 2010, evidence for gene flow between archaic and modern humans during the period of roughly 100,000 to 30,000 years ago has been discovered. This includes modern human admixture in Neanderthals, Neanderthal admixture in all modern humans outside Africa,[72][73] Denisova hominin admixture in Melanesians[74] as well as admixture from unnamed archaic humans to some Sub-Saharan African populations.[75]

The "out of Africa" migration of Homo sapiens took place in at least two waves, the first around 130,000 to 100,000 years ago, the second (Southern Dispersal) around 70,000 to 50,000 years ago,[76][77] resulting in the colonization of Australia around 65–50,000 years ago,[78][79][80] This recent out of Africa migration derived from East African populations, which had become separated from populations migrating to Southern, Central and Western Africa at least 100,000 years earlier.[81] Modern humans subsequently spread globally, replacing archaic humans (either through competition or hybridization). By the beginning of the Upper Paleolithic period (50,000 BP), and likely significantly earlier[8][9][6][7][10][82][83][84] behavioral modernity, including language, music and other cultural universals had developed.[85][86] They inhabited Eurasia and Oceania by 40,000 years ago, and the Americas at least 14,500 years ago.[87][88]

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

Until about 12,000 years ago (the beginning of the Holocene), all humans lived as hunter-gatherers, generally in small nomadic groups known as band societies, often in caves. The Neolithic Revolution (the invention of agriculture) took place beginning about 10,000 years ago, first in the Fertile Crescent, spreading through large parts of the Old World over the following millennia, and independently in Mesoamerica about 6,000 years ago. 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 and sedentary lifestyle led to the emergence of early civilizations (the development of urban development, complex society, social stratification and writing) from about 5,000 years ago (the Bronze Age), first beginning in Mesopotamia.[89] The Scientific Revolution, Technological Revolution and the Industrial Revolution brought such discoveries as imaging technology, major innovations in transport, such as the airplane and automobile; and energy development, such as coal and electricity.[90] 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. Human population growth and industrialisation has led to environmental destruction and pollution significantly contributing to the ongoing mass extinction of other forms of life called the Holocene extinction,[91] which may be further accelerated by global warming in the future.[92]

Human History articles: 41

Habitat and population

Population statistics
World population[18] 7.9 billion
Population density[18][93] 15/km2 (40/sq mi) by total area
53/km2 (137/sq mi) by land area
Largest cities[94] Tokyo, Delhi, Shanghai, Mumbai, São Paulo, Beijing, Mexico City, Osaka, Cairo, New York-Newark, Dhaka, Karachi, Buenos Aires, Kolkata, Istanbul, Chongqing, Lagos, Manila, Guangzhou, Rio de Janeiro, Los Angeles-Long Beach-Santa Ana, Moscow, Kinshasa, Tianjin, Paris, Shenzhen, Jakarta, Bangalore, London, Chennai, Lima
The Earth, as seen from space in 2016, showing the extent of human occupation of the planet. The bright lights signify both the most densely inhabited areas and ones financially capable of illuminating those areas.
Tokyo, the world's largest metropolitan area, is an example of a mass human settlement called a city

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. Modern humans, however, have a great capacity for altering their habitats by means of technology, irrigation, urban planning, construction, deforestation and desertification.[95] Human settlements continue to be vulnerable to natural disasters, especially those placed in hazardous locations and with low quality of construction.[96] Deliberate habitat alteration is often done with the goals of increasing comfort or material wealth, increasing the amount of available food, 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 success of a population. Nonetheless, the manner in which a habitat is altered is often a major determinant in population change.

The human body's ability to adapt to different environmental stresses allows humans to acclimatize to a wide variety of temperatures, humidity, and altitudes. As a result, humans are a cosmopolitan species found in almost all regions of the world, including tropical rainforest, arid desert, extremely cold arctic regions, and heavily polluted cities. Most other species are confined to a few geographical areas by their limited adaptability.[97] The human population is not, however, uniformly distributed on the Earth's surface, because the population density varies from one region to another and there are large areas almost completely uninhabited, like Antarctica.[98][99] Most humans (61%) live in Asia; the remainder live in the Americas (14%), Africa (14%), Europe (11%), and Oceania (0.5%).[100]

Within the last century, humans have explored challenging environments such as Antarctica, the deep sea, and outer space. Human habitation within these hostile environments is restrictive and expensive, typically limited in duration, and restricted to scientific, military, or industrial expeditions. Human presence on other celestial bodies has been the case mainly with human-made robotic spacecraft[101][102][103] and with humans solely on the Moon, two at a time for brief intervals between 1969 and 1972. Long-term continuous human presence in space has been the case in orbit around Earth, uninterrupted since the initial crew of the International Space Station, arriving on 31 October 2000,[104] with peaks of thirteen humans at the same time in space.[105]

Since 1800, the human population has increased from one billion[106] to over seven billion.[107] The combined biomass of the carbon of all the humans on Earth in 2018 was estimated at 60 million tons, about 10 times larger than that of all non-domesticated mammals.[108]

In 2004, some 2.5 billion out of 6.3 billion people (39.7%) lived in urban areas.[109] Problems for humans living in cities include various forms of pollution and crime,[110] especially in inner city and suburban slums. Both overall population numbers and the proportion residing in cities are expected to increase significantly in the coming decades.[111]

Humans have had a dramatic effect on the environment. They are apex predators, being rarely preyed upon by other species.[112] Currently, through land development, combustion of fossil fuels, and pollution, humans are thought to be the main contributor to global climate change.[113] If this continues at its current rate, it is predicted that climate change will wipe out half of all plant and animal species over the next century.[114][115]

Human Habitat and population articles: 35

Biology

Basic anatomical features of female and male humans. These models have had body hair and male facial hair removed and head hair trimmed. The female model is wearing red nail polish on her toenails and a ring.

Anatomy and physiology

Most aspects of human physiology are closely homologous to corresponding aspects of animal physiology. The human body consists of the legs, the torso, the arms, the neck, and the head. An adult human body consists of about 100 trillion (1014) cells. The most commonly defined body systems in humans are the nervous, the cardiovascular, the circulatory, the digestive, the endocrine, the immune, the integumentary, the lymphatic, the musculoskeletal, the reproductive, the respiratory, and the urinary system.[116][117]

Humans, like most of the other apes, lack external tails, have several blood type systems, have opposable thumbs, and are sexually dimorphic. The comparatively minor anatomical differences between humans and chimpanzees are largely a result of human bipedalism and larger brain size. One difference is that humans have a far faster and more accurate throw than other animals. Humans are also among the best long-distance runners in the animal kingdom, but slower over short distances.[118][119] Humans' thinner body hair and more productive sweat glands help avoid heat exhaustion while running for long distances.[120]

As a consequence of bipedalism, human females have narrower birth canals. The construction of the human pelvis differs from other primates, as do the toes. A trade-off for these advantages of the modern human pelvis is that childbirth is more difficult and dangerous than in most mammals, especially given the larger head size of human babies compared to other primates. Human babies must turn around as they pass through the birth canal while other primates do not, which makes humans the only species where females usually require help from their conspecifics (other members of their own species) to reduce the risks of birthing. As a partial evolutionary solution, human fetuses are born less developed and more vulnerable. Chimpanzee babies are cognitively more developed than human babies until the age of six months, when the rapid development of human brains surpasses chimpanzees.

Apart from bipedalism, humans differ from chimpanzees mostly in smelling, hearing, digesting proteins, brain size, and the ability of language. Humans' brains are about three times bigger than in chimpanzees. More importantly, the brain to body ratio is much higher in humans than in chimpanzees, and humans have a significantly more developed cerebral cortex, with a larger number of neurons. The mental abilities of humans are remarkable compared to other apes. Humans' ability of speech is unique among primates. Humans are able to create new and complex ideas, and to develop technology, which is unprecedented among other organisms on Earth.[119]

It is estimated that the worldwide average height for an adult human male is about 171 cm (5 ft 7 in), while the worldwide average height for adult human females is about 159 cm (5 ft 3 in).[121] Shrinkage of stature may begin in middle age in some individuals, but tends to be typical in the extremely aged.[122] Through history human populations have universally become taller, probably as a consequence of better nutrition, healthcare, and living conditions.[123] The average mass of an adult human is 59 kg (130 lb) for females and 77 kg (170 lb) for males.[124][125] Like many other conditions, body weight and body type is influenced by both genetic susceptibility and environment and varies greatly among individuals. (see obesity)[126][127]

Humans have a density of hair follicles comparable to other apes. However, human body hair is vellus hair, most of which is so short and wispy as to be practically invisible. In contrast (and unusually among species), a follicle of terminal hair on the human scalp can grow for many years before falling out.[128][129] Humans have about 2 million sweat glands spread over their entire bodies, many more than chimpanzees, whose sweat glands are scarce and are mainly located on the palm of the hand and on the soles of the feet.[130] Humans have the largest number of eccrine sweat glands among species.

The dental formula of humans is: 2.1.2.32.1.2.3. 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 third molars, with some individuals having them congenitally absent.[131]

Genetics

A graphical representation of the standard human karyotype, including both the male (XY) and female (XX) sex chromosomes.

Like most animals, humans are a diploid eukaryotic species. Each somatic cell has two sets of 23 chromosomes, each set received from one parent; gametes have only one set of chromosomes, which is a mixture of the two parental sets. Among the 23 pairs of chromosomes there are 22 pairs of autosomes and one pair of sex chromosomes. Like other mammals, humans have an XY sex-determination system, so that females have the sex chromosomes XX and males have XY.[132]

No two humans—not even monozygotic twins—are genetically identical. Genes and environment influence human biological variation in visible characteristics, physiology, disease susceptibility and mental abilities. The exact influence of genes and environment on certain traits is not well understood.[133][134] Compared to the great apes, human gene sequences—even among African populations—are remarkably homogeneous.[135] On average, genetic similarity between any two humans is 99.5%-99.9%.[136][137][138][139][140][141] There is about 2–3 times more genetic diversity within the wild chimpanzee population than in the entire human gene pool.[142][143][144]

A rough and incomplete human genome was assembled as an average of a number of humans in 2003, and currently efforts are being made to achieve a sample of the genetic diversity of the species (see International HapMap Project). By present estimates, humans have approximately 22,000 genes.[145] The variation in human DNA is very small compared to other species, possibly suggesting a population bottleneck during the Late Pleistocene (around 100,000 years ago), in which the human population was reduced to a small number of breeding pairs.[146][147] By comparing mitochondrial DNA, which is inherited only from the mother, geneticists have concluded that the last female common ancestor whose genetic marker is found in all modern humans, the so-called mitochondrial Eve, must have lived around 90,000 to 200,000 years ago.[148][149][150]

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.[151]

Life cycle

A 10 mm human embryo at 5 weeks
Boy and girl before puberty (children)
Adolescent male and female
Adult man and woman
Elderly man and woman

As with other mammals, human reproduction takes place by internal fertilization via sexual intercourse. Typically the gestation period is 38 weeks (9 months). 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 24 hours or more are not uncommon and sometimes lead to the death of the mother, the child or both.[152] This is because of both the relatively large fetal head circumference and the mother's relatively narrow pelvis.[153][154] 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.[155]

In developed countries, infants are typically 3–4 kg (7–9 lb) in weight and 50–60 cm (20–24 in) in height at birth.[156] However, low birth weight is common in developing countries, and contributes to the high levels of infant mortality in these regions.[157] Both the mother and the father provide care for human offspring, in contrast to other primates, where parental care is mostly restricted to mothers.[158] 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.[159] 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 and become infertile decades before the end of their lives. All species of non-human apes are capable of giving birth until death. 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 in turn their children (the grandmother hypothesis), rather than by continuing to bear children into old age.[160][161]

Evidence-based studies indicate that the life span of an individual depends on two major factors, genetics and lifestyle choices.[162] For various reasons, including biological/genetic causes,[163] women live on average about four years longer than men. As of 2018, the global average life expectancy at birth of a girl is estimated to be 74.9 years compared to 70.4 for a boy.[164][165] There are significant geographical variations in human life expectancy, mostly correlated with economic development—for example life expectancy at birth in Hong Kong is 87.6 years for girls and 81.8 for boys, while in Central African Republic, it is 55.0 years for girls and 50.6 for boys.[166][167] 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. While one in five Europeans is 60 years of age or older, only one in twenty Africans is 60 years of age or older.[168] 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.[169]

Diet

Humans living in Bali, Indonesia preparing a meal.

Humans are omnivorous, capable of consuming a wide variety of plant and animal material.[170][171] Human groups have adopted a range of diets from purely vegan 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.[172] 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.[173] It has been proposed that humans have used fire to prepare and cook food since the time of Homo erectus.[174] Around ten thousand years ago, humans developed agriculture,[175] 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.[176][177] 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, have 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 starvation.[178] Childhood malnutrition is also common and contributes to the global burden of disease.[179] 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,[180] while in the United States 35% of people are obese, leading to this being described as an "obesity epidemic."[181] Obesity is caused by consuming more calories than are expended, so excessive weight gain is usually caused by an energy-dense diet.[180]

Biological variation

People in hot climates are often slender, lanky, and dark skinned, such as these Maasai men from Kenya.