Human

Human Fossil range: Pleistocene - Recent
Humans as depicted on the Pioneer plaque
Scientific classification

Humans, or human beings, are
bipedal
primates
belonging to the
mammalian species Homo sapiens (Latin:
"wise man" or "knowing man") in the family
Hominidae
(the great apes).^[1][2]
Compared to other living organisms on Earth, humans have a

highly developed
brain
capable of abstract
reasoning,
language,
and
introspection. This mental capability, combined with an erect body
carriage that frees their upper limbs for manipulating objects, has allowed
humans to make far greater use of
tools than any
other species. DNA
evidence indicates that modern humans originated in
Africa about
200,000 years ago,^[3]
and they now inhabit every
continent,

with a total population of over 6.6 billion
as of
2007.^[4]

Like most primates, humans are

social by nature; however, humans are particularly adept at utilizing
systems of

communication for self-expression, the exchange of ideas, and
organization. Humans create complex

social structures composed of

cooperating and

competing groups, ranging in scale from small
families and
partnerships to species-wide political, scientific and economic unions.

Social interactions between humans have also established an extremely wide
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,

philosophy,
mythology
and religion.
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.

Physiologically, humans are noted for their bipedal gait (and the big toe
which enables it), their highly dexterous hands (especially the opposable
thumb), and placement of the larynx to allow for speech. It is widely
speculated that evolutionary pressures in the Africa prompted the development
of bipedal gait so that individuals could see over long grass. This freed the
hands to become more dexterous, and this new tool prompted the development of
human intelligence.

 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 is Homo sapiens sapiens;

Homo sapiens idaltu (roughly translated as "elder wise human"), the
other known subspecies, is extinct.^[5]
Anatomically modern humans appear in the fossil record in Africa about 130,000
years ago.^[6][7]

The closest living relatives of Homo sapiens are two distinct
species of the genus Pan: the
Bonobo (Pan
paniscus) and the

Common Chimpanzee (Pan troglodytes). These species share the same
common ancestor. The main difference between them is the social organization:

matriarchal for the Bonobo and

patriarchal for the Common Chimpanzee. Full
genome
sequencing resulted in the conclusion that "after 6.5
million years
of separate evolution, the differences between bonobo/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, 95 per cent of the DNA
sequence is identical between the two Pan species and human.^{[8][9][10][11]}
It has been estimated that the human

lineage diverged from that of chimpanzees about five million years ago,
and from
gorillas about eight million years ago. However, a hominid skull
discovered in Chad
in 2001, classified as

Sahelanthropus tchadensis, is approximately seven million years old,
which may indicate an earlier divergence.^[12]

The

Recent African Origin (RAO), 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.
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.^[13]

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. 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 of those is the

Toba catastrophe theory.

Human evolution is characterized by a number of important morphological,
developmental, physiological and behavioral changes which have taken place
since the split between the last common ancestor of Homo and Pan.
The primary change, both in terms of chronology and in terms of it being the
trait that defines the human
subtribe
the Hominina,
was the evolution of a bipedal locomotor adaptation from an arboreal or
semi-arboreal locomotor adaptation, with all its attendant adaptations, such
as a valgus knee, low intermembral index (long legs relative to the arms) and
reduced upper body strength.^{[citation
needed]} Following this was the evolution of a larger

brain cavity and brain itself, which is typically 1,400 cm³ in modern
humans; over twice that of a chimpanzee or gorilla. Other significant
morphological changes included: the evolution of a power and precision grip;^{[citation
needed]} a reduced masticatory system; a reduction of the

canine tooth; and the descent of the
larynx and
hyoid
bone, making speech possible. With respect to development, the pattern of
human postnatal brain growth differs from that of other apes (heterochrony),
allowing for an extended period of social learning and

language acquisition in juvenile humans.

Physical anthropologists argue that a reorganization of the structure of
the brain is more important than cranial expansion itself. One important
physiological change in humans was the evolution of hidden estrus or

concealed ovulation in females, which may have coincided with the
evolution of important behavioral changes, such as
pair
bonding. Another significant behavioral change includes the development of

material culture, or the (over time) increasingly wide variety of
human-made objects which are used to manipulate humans' physical and social
environments. How all these changes are related and what their role is in the
evolution of complex social organization and culture are matters of ongoing
debate.^[14][15]

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 10,000 years ago.^[16]
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.

Up until only around 10,000 years ago, all humans lived as

hunter-gatherers (with some communities persisting until this day). They
generally lived in small,
nomadic groups.
The advent of

agriculture prompted the

Neolithic Revolution. Access to food surplus led to the formation of
permanent human settlements, the

domestication of animals, and the use of
metal
tools. Agriculture also encouraged
trade and
cooperation, leading to complex societies.
Villages
developed into thriving

civilizations in regions such as the
Middle
East's

Fertile Crescent.

Around 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, China,
and
Rome, 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.

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,
and
manufacturing goods. 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 (13%) and
Europe (12%),
with 0.5% in Oceania. (See

list of countries by population and

list of countries by population density.)

Human habitation within

closed ecological systems in hostile environments, such as Antarctica and
outer space, is expensive, typically limited in duration, and restricted to
scientific, military, or industrial expeditions. Life in space has been very
sporadic, with no more than thirteen humans in space at any given time.
Between 1969 and 1972, two humans at a time spent brief intervals on the

Moon.
As of 2007, no other celestial body has been visited by human beings,
although there has been a continuous human presence in outer space since the
launch of the initial crew to inhabit the

International Space Station on
October
31, 2000;
however, humans have made
robots that
have visited other celestial bodies.

From AD 1800 to 2000, the human population increased from one billion to
six billion. In 2004, around 2.5 billion out of 6.3 billion people (39.7%)
lived in
urban areas, and this percentage is expected to rise throughout the 21st
century. Problems for humans living in
cities include
various forms of
pollution
and crime,^[17]
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 in the past, human predation
has contributed to the extinction of a number of species; as humans are not
generally preyed on themselves, humans have been described as the ultimate

superpredators.^[18]
Currently, through land development and
pollution,
humans are thought to be the main contributor to global

climate change.^[19]
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.^[20][21]

 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 5 to 6 feet
(1.5 to 1.8 m) tall, although this varies significantly from place to place.^[22][23]
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.^[24]

An Inuit
woman, circa 1907.

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.^[25],
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.^[26]
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.^[27][28]

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

hemophilia, affect men more often than women.

Human
embryo at 5 weeks.

 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 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 (9 months) of
gestation
becomes a human
fetus. After this span of time, 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.

Two young human
girls.

Compared with that of other species, human

childbirth is dangerous. Painful labors lasting twenty-four hours or more
are not uncommon, and may result in injury, or even death, to the child and/or
mother. 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).^[29][30]
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.^[31]

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.^[32]
However, low
birth
weight is common in developing countries, and contributes to the high
levels of

infant mortality in these regions.^[33]
Helpless at birth, humans 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,
adulthood and
old age.
The lengths of these stages, however — particularly the later ones — are not
fixed.

There are striking differences in

life expectancy around the world. The developed world is quickly getting
older, with the median age around 40 years (highest in
Monaco at
45.1 years), while in the

developing world, the median age is 15 – 20 years (lowest in
Uganda at
14.8 years). Life expectancy at birth 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.^[34]
While one in five Europeans is 60 years of age or older, only one in twenty
Africans is 60 years of age or older.^[35]

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.^[36]
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.

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 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.

Humans are

omnivorous, capable of consuming both plant and animal products. The 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,
while a pure plant diet can lead to deficiency of a number of nutrients,
including
Vitamin
B12. Supplementation, particularly for vitamin B12, is highly recommended
for people living on a pure plant diet.^[37]
However, according to

American Dietetic Association and the

Dietitians of Canada, properly planned

vegetarian and
vegan diets are nutritionally adequate and healthy.^[38]

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.^[39]
Childhood malnutrition is also common and contributes to the global burden of
disease.^[40]
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,^[41]
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

A sketch of the human brain, imposed upon the profile of

Michelangelo's

David - sketch by artist Priyan Weerappuli.

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 the

respiration, and
digestion.
The brain also controls "higher" order, conscious activities, such as
thought,
reasoning,
and
abstraction.^[42]
These

cognitive processes constitute the
mind, and, along
with their
behavioral consequences, are studied in the field of

psychology.

The human brain is generally regarded as more capable of these higher order
activities, and more "intelligent"
in general, than that of 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.^[43]

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".^[44]