Encyclopedia
Humans, or
human beings, are
bipedal
primates belonging to the mammalian species
Homo sapiens under the family
Hominidae . Humans have a highly developed
brain capable of abstract reasoning,
language and introspection. This, combined with an erect body carriage that frees their upper limbs for manipulating objects, has allowed humans to make greater use of
tools than any other species.
Like most primates, humans are by nature social. However, humans are particularly adept at utilizing systems of communication for self-expression and the exchange of ideas. Humans create complex social structures composed of co-operating and
competing groups, ranging in scale from individual
families to nations, and social interaction between humans has established a variety of traditions, rituals, ethics, values, social norms, and laws which form the basis of human
society. Humans also have a marked appreciation for
beauty and
aesthetics which, combined with the human desire for self-expression, has led to
cultural innovations such as
art,
literature and
music.
Humans are also noted for their desire to understand and influence the world around them, seeking to explain and manipulate natural phenomena through
science,
religion,
philosophy and mythology. This natural curiosity has led to the development of advanced tools and skills; humans are the only known species to build fires, cook their food,
clothe themselves, and use numerous other
technologies.
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 a North American adult female is 162
centimetres , and the average weight is 62
kilograms . Human males are typically larger than females: the average height and weight of a North American adult male is 175 centimeters and 78 kilograms .
Humans are capable of fully
bipedal
locomotion, thus leaving their arms available for manipulating objects using their
hands, aided especially by opposable thumbs. Because human physiology has not fully adapted to bipedalism, the pelvic region and
vertebral column tend to become worn, creating locomotion difficulties in old age.
Although humans appear relatively hairless compared to other primates, with notable
hair growth occurring chiefly on the top of the head, underarms and pubic area, the average human has more hair on his or her body than the average
chimpanzee. The main distinction is that human hairs are shorter, finer, and less colored than the average chimpanzee's, thus making them harder to see.
The color of human hair and skin is determined by the presence of
pigments called
melanins. Human skin color can range from very dark brown to very pale pink, while human hair ranges from
blond to
brown to
red, but most commonly,
black. Most researchers believe that skin darkening was an adaptation that evolved as a defense against
ultraviolet solar radiation: melanin is an effective sun-block. The skin color of contemporary humans is geographically stratified, and in general correlates with the level of ultraviolet radiation. Human skin also has a capacity to darken in response to exposure to ultraviolet radiation.
The average
sleep requirement is between seven and eight hours a day for an adult and nine to ten hours for a child; elderly people usually sleep for six to seven hours. Negative effects result from restriction of sleep. For instance, a sustained restriction of adult sleep to four hours per day has been shown to correlate with changes in physiology and mental state, including fatigue, aggression, and bodily discomfort. It is common in modern societies for people to get less sleep than they need, leading to a state of sleep deprivation.
Humans are a
eukaryotic species. Each diploid cell has two sets of 23
chromosomes, each set received from one parent. There are 22 pairs of autosomes and one pair of sex chromosomes. By present estimates, humans have approximately 20,000–25,000 genes and share 95% of their
DNA with their closest living evolutionary relatives, the two species of
chimpanzees.
Like other mammals, humans have an XY sex-determination system, so that
females have the sex chromosomes XX and
males have XY. The X chromosome is larger and carries many genes not on the Y chromosome, which means that
recessive diseases associated with X-linked genes, such as
hemophilia, affect men more often than women.
Life cycle
The human
life cycle is similar to that of other
placental mammals. New humans develop viviparously from
conception. An
egg is usually fertilized inside the female by
sperm from the male through
sexual intercourse, though the recent technology of
in vitro fertilization is also occasionally used. The fertilized egg, called a zygote, divides inside the female's uterus to become an
embryo, which over a period of thirty-eight weeks becomes a human
fetus. At birth, the fully-grown fetus is expelled from the female's body and breathes independently as an
infant for the first time. At this point, most modern cultures recognize the baby as a person entitled to the full protection of the law, though some jurisdictions extend personhood to human fetuses while they remain in the uterus.
Compared with that of other species, human
childbirth is fairly complicated. Painful labors lasting twenty-four hours or more are not uncommon, and may result in injury, or even death, to the child or mother. This is because of both the relatively large fetal head circumference and the mother's relatively narrow
pelvis , by way of natural selection. The chances of a successful labour increased significantly during the 20th century in wealthier countries with the advent of new medical technologies. Natural childbirth remains a common, and relatively dangerous ordeal in unindustrialized regions of the world and for poor people in industrialized regions.
Human children are born after a nine-month gestation period, and are typically 3–4 kilograms in weight and 50–60 centimeters in height in developed countries. Helpless at birth, they continue to grow for some years, typically reaching sexual maturity at 12 to 15 years of age. Human girls continue to grow physically until around the age of 18, and human boys until around age 21.
The human life span can be split into a number of stages:
infancy, childhood,
adolescence, young adulthood, maturity and
old age. The lengths of these stages, however—particularly the later ones—are not fixed.
There are striking differences in
life expectancy around the world. The developed world is quickly getting older, with the median age around 40 years , while in the
developing world, the median age is 15–20 years . Life expectancy at birth is 77.2 years in the U.S. as of 2001. The expected life span at birth in
Singapore is 84.29 years for a female and 78.96 for a male, while in
Botswana, mostly because of
AIDS, it is 30.99 years for a male and 30.53 for a female. One in five Europeans, but one in 20 Africans, is 60 years or older.
The number of centenarians in the world was estimated by the
United Nations at 210,000 in 2002. At least one person 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.
The philosophical questions of when human personhood begins and whether it persists after death are the subject of considerable debate. The prospect of death causes unease or fear for most humans.
Burial ceremonies are characteristic of human societies, often inspired by beliefs in an afterlife or
immortality.
Race and ethnicity
Humans often categorize themselves in terms of
race or ethnicity, although the scientific validity of human races as categories is disputed. Human racial categories are based on both ancestry and visible traits, especially
skin color and facial features. Ethnic groups, on the other hand, are more often linked by linguistic, cultural, and national or regional ties. Self-identification with an ethnic group is based on kinship and descent. Race and ethnicity can lead to variant treatment and impact social identity, giving rise to the theory of
identity politics.
Evolution
The study of human evolution encompasses the development of the genus
Homo, but usually involves studying other
hominids and
hominines as well, such as the australopithecines. "Modern humans" are defined as the
Homo sapiens species, of which the only extant subspecies is
Homo sapiens sapiens;
Homo sapiens idaltu , the other known subspecies, is extinct.
The closest living relatives of
Homo sapiens are the
Common Chimpanzee and the
Bonobo. Full genome sequencing resulted in the conclusion that "After 6.5 [million] years of separate evolution, the differences between chimpanzee and human are just 10 times greater than those between two unrelated people and 10 times less than those between rats and mice." In fact, chimpanzee and human DNA is 96% identical. It has been estimated that the human lineage diverged from that of chimpanzees about five million years ago, and from
gorillas about eight million years ago. However, a hominid skull discovered in
Chad in 2001, classified as
Sahelanthropus tchadensis is a fossil [i] ape [i] originally classified as the oldest possible membe...
, is approximately seven million years old, which may indicate an earlier divergence.
There are two prominent scientific theories of the origins of contemporary humans. They concern the relationship between modern humans and other hominids. The
single-origin, or "out of Africa", hypothesis proposes that modern humans evolved in Africa and later
migrated outwards to replace hominids in other parts of the world. The
multiregional hypothesis, on the other hand, proposes that modern humans evolved, at least in part, from independent hominid populations.
Geneticists Lynn Jorde and
Henry Harpending of the
University of Utah proposed that the variation in human DNA is minute compared to that of other species, and that during the Late Pleistocene, the human population was reduced to a small number of breeding pairs—no more than 10,000—resulting in a very small residual gene pool. Various reasons for this hypothetical bottleneck have been postulated, the most popular being the
Toba catastrophe theory.
Human evolution is characterized by a number of important physiological trends, including the expansion of the brain cavity and brain itself, which is typically 1,400 cm³ in volume, over twice that of a chimpanzee or gorilla. The pattern of human postnatal brain growth differs from that of other apes , allowing for an extended period of social learning in juvenile humans. Physical anthropologists argue that a reorganization of the structure of the brain is more important than cranial expansion itself. Other significant evolutionary changes included a reduction of the canine tooth, development of
bipedal locomotion, and the descent of the
larynx and hyoid bone, making speech possible. How these trends are related and what their role is in the evolution of complex social organization and culture are matters of ongoing debate in the field of physical anthropology.
Habitat and population
The most widely accepted view among current
anthropologists is that
Homo sapiens originated in the
African
savanna between 200,000 and 250,000 years ago, descending from
Homo erectus is an extinct species of genus
Homo [i]. ...
, and colonized
Eurasia and
Oceania by 40,000 years ago, and finally colonized the
Americas by 10,000 years ago. They displaced
Homo neanderthalensis,
Homo floresiensis is the proposed name for a possible species [i] in the genus [i]
Homo [i] ...
and other species descended from
Homo erectus through more successful reproduction and competition for resources.
The earliest humans were hunter-gatherers, a lifestyle well-suited to the savanna. Some later groups of humans were
nomads, often to facilitate animal
herding, and still later humans established permanent settlements, which was made possible by the development of
agriculture. Permanent human settlements were dependent on proximity to
water and, depending on the lifestyle, other natural resources, such as fertile land for growing crops and grazing
livestock, or seasonally by
hunting populations of prey. However, humans have a great capacity for altering their habitats by various methods, such as through
irrigation,
urban planning,
construction,
transport, and manufacturing goods, and with the advent of large-scale trade and transport infrastructure, proximity to these resources has become unnecessary, and in many places these factors are no longer a driving force behind the growth and decline of a population. Nonetheless, the manner in which a habitat is altered is often a major determinant in population change.
Technology has allowed humans to colonize all of the continents and adapt to all climates. Within the last few decades, humans have explored
Antarctica, the ocean depths, and
space, although long-term habitation of these environments is not yet possible. With a population of over six billion, humans are among the most numerous of the large mammals. Most humans live in
Asia. The vast majority of the remainder live in the Americas , Africa and
Europe , 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 2006, no other celestial body has been visited by human beings, although there has been a continuous human presence in space since the launch of the initial crew to inhabit the
International Space Station on October 31, 2000.
From AD 1800 to 2000, the human population increased from one to six billion. In 2004, around 2.5 billion out of 6.3 billion people lived in
urban areas, and this percentage is expected to rise throughout the 21st century. Problems for humans living in
cities include various forms of
pollution, crime, and
poverty, especially in inner city and suburban slums. Benefits of urban living include increased literacy, access to the global canon of human knowledge and decreased susceptibility to rural
famines.
Diet
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. In modern times,
obesity among some human populations has increased to almost epidemic proportions, leading to health complications and increased mortality in some
developed, and, increasingly, in
developing countries. The United States Center for Disease Control states that 32% of adults over the age of 20 are obese, while 66.5% are obese or overweight. Obesity is believed to be caused by a number of factors, with many attributing excessive weight gain to overeating.
Humans are animals who can consume both plant and animal products. Most biologists agree humans are
omnivorous. A minority believes they are an anatomically
carnivorous species, and have started using agriculture foodstuffs only recently. Another minority believes that anatomically, they are primarily
herbivorous, many members of which have begun consuming food of animal origin.
Early
Homo sapiens employed a "hunter-gatherer" methodology as their primary means of food collection, involving combining stationary plant and fungal food sources with wild game which must be hunted and killed in order to be consumed. Some humans choose to be
vegans or
vegetarians, abstaining from eating meat for religious, ethical, ecological, or health reasons. It is believed that humans have used fire to prepare and cook food prior to eating since the time of their divergence from
Homo erectus is an extinct species of genus
Homo [i]. ...
—possibly even earlier. However, a small number of individuals choose a raw foodist approach, consuming little to no cooked food; the raw diet may be
fruitarian,
vegetarian, or omnivorous.
At least ten thousand years ago, humans developed agriculture, which has substantially altered the kind of food people eat. This has led to increased populations, the development of cities, and 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.
The last century or so has produced enormous improvements in food production, preservation, storage and shipping. Today almost every locale in the world has access to not only its traditional cuisine, but many other world cuisines.
Brain, mind, and consciousness
The
human brain is the center of the
central nervous system in humans, as well as the primary control center for the peripheral nervous system. The brain controls "lower", or involuntary,
autonomic activities such as
heart rate, respiration, and
digestion. The brain also controls "higher" order, conscious activities, such as
thought, reasoning, and
abstraction. The human brain is generally regarded as more capable of these higher order activities, and more "intelligent" in general, than any other species. While other animals are capable of creating structures and using simple tools—mostly as a result of
instinct and learning through mimicry—human technology is vastly more complex, constantly evolving and improving with time. Even the most ancient human tools and structures are far more advanced than any structure or tool created by any other animal.
The human ability to think abstractly may be unparalleled in the animal kingdom. Human beings are one of only six species to pass the mirror test—which tests whether an animal recognizes its reflection as an image of itself—along with
chimpanzees,
bonobos,
orangutans,
dolphins and possibly
pigeons. Human beings under the age of 2 typically fail this test. However, this may be a matter of degree rather than a sharp divide. Monkeys have been trained to apply abstract rules in tasks.
The brain
perceives the external world through the senses, and each individual human is influenced greatly by his or her experiences, leading to subjective views of existence and the passage of
time.
Humans are variously said to possess
consciousness, self-awareness, and a mind, which correspond roughly to the mental processes of thought. These are said to possess qualities such as self-awareness, sentience, sapience, and the ability to
perceive the relationship between oneself and one's
environment. The extent to which the mind constructs or experiences the outer world is a matter of debate, as are the definitions and validity of many of the terms used above.
Cognitive scientist Daniel Dennett, for example, argues that there is no such thing as a narrative centre called the "mind", but that instead there is simply a collection of sensory inputs and outputs: different kinds of "software" running in parallel.
Humans study the more physical aspects of the mind and brain, and by extension of the nervous system, in the field of neurology, the more behavioral in the field of
psychology, and a sometimes loosely-defined area between in the field of psychiatry, which treats mental illness and behavioral disorders. Psychology does not necessarily refer to the brain or nervous system, and can be framed purely in terms of phenomenological or information processing theories of the mind. Increasingly, however, an understanding of brain functions is being included in psychological theory and practice, particularly in areas such as
artificial intelligence, neuropsychology, and cognitive neuroscience.
The nature of thought is central to psychology and related fields. Cognitive psychology studies
cognition, the mental processes underlying behavior. It uses information processing as a framework for understanding the mind.
Perception,
learning, problem solving,
memory, attention,
language and
emotion are all well-researched areas as well. Cognitive psychology is associated with a school of thought known as cognitivism, whose adherents argue for an information processing model of mental function, informed by positivism and experimental psychology. Techniques and models from cognitive psychology are widely applied and form the mainstay of psychological theories in many areas of both research and applied psychology. Largely focusing on the development of the human mind through the life span,
developmental psychology seeks to understand how people come to perceive, understand, and act within the world and how these processes change as they age. This may focus on intellectual, cognitive, neural, social, or
moral development.
Social psychology links
sociology with psychology in their shared study of the nature and causes of human social behavior, with an emphasis on how people think towards each other and how they relate to each other. The behavior and mental processes of animals, both human and non-human, can be described through animal cognition, ethology,
evolutionary psychology, and comparative psychology as well. Human ecology is an
academic discipline that investigates how humans and human
societies interact with both their natural environment and the human social environment.
Complete scientific classification
The complete scientific classification of the modern human consists of many sub- and super- sections to the basic chart:
Eukaryota /
Animalia /
Eumetazoa 
