Hear it pronouncedHuman
| Human[1] Fossil range: Pleistocene - Recent |
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| Homo sapiens sapiens Linnaeus, 1758 |
Humans, or human beings (Homo sapiens)(Latin: "wise human" or "knowing human"[2]) are bipedal primates in the family Hominidae.[3][4] DNA evidence indicates that modern humans originated in Africa about 200,000 years ago.[5] Humans have a highly developed brain, capable of abstract reasoning, language, introspection, and emotional suffering. This mental capability, combined with an erect body carriage that frees the forelimbs (arms) for manipulating objects, has allowed humans to make far greater use of tools than any other species. Humans now inhabit every continent on Earth, except Antarctica (although several governments maintain permanent research stations there, inhabited for short periods by scientists and other researchers). Humans also now have a continuous presence in low Earth orbit, occupying the International Space Station. The human population on Earth now amounts to over 6.6 billion, as of May 2008.[6]
Like most primates, humans are social by nature. However, they are particularly adept at utilizing systems of communication for self-expression, exchanging of ideas, and organization. Humans create complex social structures composed of many cooperating and competing groups, from families to nations. Social interactions between humans have established an extremely wide variety of traditions, rituals, ethics, values, social norms, and laws, which together form the basis of human society. Humans have a marked appreciation for beauty and aesthetics, which, combined with the desire for self-expression, has led to cultural innovations such as art, literature and music.
Humans are notable for their desire to understand and influence the world around them, seeking to explain and manipulate natural phenomena through science, philosophy, mythology and religion. This natural curiosity has led to the development of advanced tools and skills; humans are the only extant species known to build fires, cook their food, clothe themselves, and manipulate and develop numerous other technologies. Humans pass down their skills and knowledge to the next generations through education.
[ History
[ Origin
The scientific study of human evolution encompasses 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 - our own - was formerly known as Homo sapiens sapiens (now simply known as Homo sapiens). Homo sapiens idaltu (roughly translated as "elder wise human"), the other known subspecies, is now extinct.[7] Anatomically modern humans first appear in the fossil record in Africa about 130,000 years ago, although 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.[8][9]
The closest living relatives of Homo sapiens are the two chimpanzee species: the Common Chimpanzee and the Bonobo. Full genome sequencing has 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". Suggested concurrence between human and chimpanzee DNA sequences range between 95% and 99%.[10][11][12][13] It has been estimated that the human lineage 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, which may indicate an earlier divergence.[14]
The Recent African Origin (RAO), or "out-of-Africa", hypothesis proposes that modern humans evolved in Africa before later migrating outwards to replace hominids in other parts of the world. Evidence from archaeogenetics accumulating since the 1990s has lent strong support to RAO, and has marginalized the competing multiregional hypothesis, which proposed that modern humans evolved, at least in part, from independent hominid populations.[15] 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. 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. Various reasons for this hypothetical bottleneck have been postulated, one being the Toba catastrophe theory.
Human evolution is characterized by a number of important morphological, developmental, physiological and behavioural changes, which 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,[16] with all its attendant adaptations, such as a valgus knee, low intermembral index (long legs relative to the arms), and reduced upper-body strength.
Later, ancestral humans developed a much larger brain – typically 1,400 cm³ 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.
Other significant morphological changes included: the evolution of a power and precision grip;[17] 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 oestrus, or concealed ovulation, which may have coincided with the evolution of important behavioural changes, such as pair bonding. Another significant behavioural 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.[18][19]
[ Rise of civilization
The most widely accepted view among current anthropologists is that Homo sapiens originated in the African savanna around 200,000 BP (Before Present), descending from Homo erectus, had inhabited Eurasia and Oceania by 40,000 BP, and finally inhabited the Americas approximately 14,500 years ago.[20] They displaced Homo neanderthalensis and other species descended from Homo erectus (which had inhabited Eurasia as early as 2 million years ago) through more successful reproduction and competition for resources.
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. 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 and the Indus Valley. 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. Influential religions, such as Judaism, originating in the Middle East, 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 economy promoted innovations such as printing and the compass, while 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 century. Over the next 500 years, exploration and imperialistic conquest brought much of the Americas, Asia, and Africa 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.
As a result of such changes, modern humans live in a world that has become increasingly globalized and interconnected. Although this has encouraged the growth of science, art, and technology, it has also led to culture clashes, the development and use of weapons of mass destruction, and increased environmental destruction and pollution, affecting not only themselves but also most other life forms on the planet.
[ Habitat and population
- For more details on this topic, see Demography and World population.
Early 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, manufacturing goods, deforestation and desertification. 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 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 (14%) and Europe (11%), with 0.5% in Oceania.
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 early 2008, 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. Other celestial bodies have, however, been visited by human-made objects.
Since 1800, the human population increased from one billion to over six billion.[21] In 2004, some 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. In February 2008, the U.N. estimated that half the world's population will live in urban areas by the end of the year.[citation needed] Problems for humans living in cities include various forms of pollution and crime,[22] especially in inner city and suburban slums. Benefits of urban living include increased literacy, access to the global canon of human knowledge and decreased susceptibility to rural famines.
Humans have had a dramatic effect on the environment. It has been hypothesized that human predation has contributed to the extinction of numerous species. As humans stand at the top of the food chain and are not generally preyed upon, they have been described as superpredators.[23] Currently, through land development and pollution, humans are thought to be the main contributor to global climate change.[24] This is believed to be a major contributor to the ongoing Holocene extinction event, a mass extinction which, if it continues at its current rate, is predicted to wipe out half of all species over the next century.[25][26]
[ Biology
[ Physiology and genetics
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.[27][28] 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.
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 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.[29]
The hue of human hair and skin is determined by the presence of pigments called melanins. Human skin hues can range from very dark brown to very pale pink, while human hair ranges from blond to brown to red to, most commonly, black,[30] depending on the amount of melanin (an effective sun blocking pigment) in the skin. Most researchers believe that skin darkening was an adaptation that evolved as a protection against ultraviolet solar radiation. More recently, however, 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.[31] 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.[32][33] 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.[34]
Humans have proportionately shorter palates and much smaller teeth than other primates. They are the only primates to have short 'flush' canine teeth. Humans have characteristically crowded teeth, with gaps from lost teeth usually closing up quickly in young specimens. Humans are gradually losing their wisdom teeth, with some individuals having them congenitally absent.[35]
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. Experiencing less sleep than this is common in modern societies; this sleep deprivation can lead to 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.
Humans are an 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. 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 haemophilia, affect men more often than women.
[ Life cycle
The human life cycle is similar to that of other placental mammals. The fertilized egg divides inside the female's uterus to become an embryo, which over a period of thirty-eight weeks (9 months) of gestation becomes a human 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 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 often leads to the death of the mother, or the child.[36] 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).[37][38] 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 relatively hazardous ordeals in developing regions of the world, with maternal death rates approximately 100 times more common than in developed countries.[39]
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.[41] However, low birth weight is common in developing countries, and contributes to the high levels of infant mortality in these regions.[42] 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%.[43]
There are significant differences in life expectancy around the world. The developed world generally aging, with the median age around 40 years (highest in Monaco at 45.1 years). In the developing world the median age is between 15 and 20 years. Life expectancy at birth in Hong Kong, China 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.[44] While one in five Europeans is 60 years of age or older, only one in twenty Africans is 60 years of age or older.[45] 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.[46] At least one person, Jeanne Calment, is known to have reached the age of 122 years; 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.
Humans are unique in the widespread onset of female menopause during the latter stage of life. Menopause is believed to have arisen due to the Grandmother hypothesis, in which it is in the mother's reproductive interest to forgo the risks of death from childbirth at older ages in exchange for investing in the viability of her already living offspring.[47]
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, distinct from the immediate awareness of a threat. Burial ceremonies are characteristic of human societies, often accompanied by beliefs in an afterlife or immortality.
[ Diet
Early Homo sapiens employed a hunter-gatherer method 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. 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.[citation needed]
Humans are omnivorous, capable of consuming both plant and animal products. A view of humans as omnivores is supported by the evidence that both a pure animal and a pure vegetable diet can lead to deficiency diseases in humans. A pure animal diet can, for instance, lead to scurvy, a vitamin C deficiency, while a pure plant diet may lead to vitamin B12 deficiency.[48] The biggest problem posed by a vitamin B12 deficiency is that it severely limits the body's ability to synthesize folic acid, a main source of B group carriage. In order to counter the constant folic acid deficiency, one must regularly consume large amounts of folic acid, as may be found in green, leafy vegetables. Properly planned vegetarian and vegan diets, however, have been found to completely satisfy nutritional needs in every stage of life. [49]
The human diet is prominently reflected in human culture, and has led to the development of food science. 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. Lack of food remains a serious problem, with about 300,000 people starving to death every year.[50] Childhood malnutrition is also common and contributes to the global burden of disease.[51] However global food distribution is not even, and obesity among some human populations has increased to almost epidemic proportions, leading to health complications and increased mortality in some developed, and a few developing countries. The United States Centers for Disease Control (CDC) state that 32% of American adults over the age of 20 are obese, while 66.5% are obese or overweight. Obesity is caused by consuming more calories than are expended, with many attributing excessive weight gain to a combination of overeating and insufficient exercise.
At least ten thousand years ago, humans developed agriculture,[52] which has substantially altered the kind of food people eat. This has 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.
[ Psychology
The human brain is the center of the central nervous system in humans, and acts as the primary control center for the peripheral nervous system. The brain controls "lower", or involuntary, autonomic activities such as the respiration, and digestion. The brain also controls "higher" order, conscious activities, such as thought, reasoning, and abstraction.[53] 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 many animals 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. Even the most ancient human tools and structures are far more advanced than any structure or tool created by any other animal.[54] 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".[55]
[ Consciousness and thought
The human ability to think abstractly may be unparalleled in the animal kingdom. Humans 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, orangutans, dolphins, and pigeons.[56] In October 2006, three elephants at the Bronx Zoo also passed this test.[57] Most human children will pass the mirror test at 18 months old.[58] However, the usefulness of this test as a true test of consciousness has been disputed (see mirror test), and this may be a matter of degree rather than a sharp divide. Monkeys have been trained to apply abstract rules in tasks.[59] 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 centre called the "mind", but that instead there is simply a collection of sensory inputs and outputs: different kinds of "software" running in parallel.[60] Psychologist B.F. Skinner has argued that the mind is an explanatory fiction that diverts attention from environmental causes of behavior,[61] and that what are commonly seen as mental processes may be better conceived of as forms of covert verbal behavior.[62]
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 thee mind. Increasingly, however, an understanding of brain functions is being included in psychological theory and practice, particularly in area
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