Human: Encyclopedia II - Human - Biology

Human - Biology

Human - Anatomy and physiology

Human body types varies substantially, with many individuals diverging significantly from the mean height and weight. Some of this variation is caused by locality and historical factors. Although body size is largely determined by genes, it is also significantly influenced by diet and exercise. The mean height of a North American adult female is 162 centimetres (5 feet 4 inches), and the mean weight is 62 kilograms (137 pounds). Human males are typically larger than females: the mean height and weight of a North American adult male is 175 centimetres (5 feet 9 inches) and 78 kilograms (172 pounds).

Although human skin appears relatively hairless compared to that of other primates, with notable hair growth occurring chiefly on the top of the head, the average human has a larger number of hairs on his or her body than the average chimpanzee. The main distinction is that human hairs are shorter, finer, and less coloured than the average chimpanzee's, thus rendering them harder to see. The colour of human hair and skin is determined by the presence of coloured pigments called melanins. Human skin colour can range from very dark brown to very pale pink, while human hair ranges from blond to brown to red. Most researchers believe that skin darkening was an adaptation that evolved as a defence against ultraviolet solar radiation; melanin is an effective sunblock. The skin colour of contemporary humans is geographically stratified, and in general correlates with the environmental level of ultraviolet radiation. Human skin and hair colour is controlled in part by the Mc1r gene. For example, the red hair and pale skin of some Europeans is the result of mutations in Mc1r. Human skin has a capacity to darken (sun tanning) in response to exposure to ultraviolet radiation; this is also controlled in part by Mc1r.

Humans are capable of fully bipedal locomotion, walking upright on two legs. This leaves the forelimbs available for manipulating objects using their hands, aided especially by opposable thumbs. Because humans are bipedal, the pelvic region and spinal column tend to become worn, creating locomotion difficulties in old age.

The individual need for regular intake of food and drink is prominently reflected in human culture, and has led to the development of food science. Failure to obtain food leads to hunger and eventually starvation, while failure to obtain water leads to dehydration and thirst. Both starvation and dehydration cause death if not alleviated. In modern times, obesity amongst some human populations has increased to almost epidemic proportions, leading to health complications and increased mortality in some developed countries, and is becoming problematic elsewhere.

It is sometimes said that the average sleep requirement is between seven and eight hours a day for an adult and nine to ten hours for a child. More accurately, 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 variables, including tiredness-fatigue, anger-aggression, and bodily discomfort. Normative data suggests elderly people usually sleep for six to seven hours, although such normative data varies depending upon the sociocultural characteristics of the population. It is common in modern societies for people to get less sleep than they need to avoid adverse effects, leading to a state of sleep deprivation.

Human - Life cycle

The human life cycle is similar to that of other placental mammals. New human life develops viviparously from conception. An egg is usually fertilised inside the female by sperm from the male through sexual intercourse, though in vitro fertilisation methods are also used. The fertilised egg is called a zygote. The zygote divides inside the female's uterus to become an embryo which over a period of thirty-eight weeks becomes the fetus. At birth, the fully grown fetus is expelled from the female's body and breathes independently as a baby for the first time. At this point, most modern cultures recognise 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 relatively complicated. Painful labours lasting twenty-four hours or more are not uncommon, and may result in injury to the child or the death of the mother, although the chances of a successful labour increased significantly during the twentieth century in wealthier countries. Natural childbirth remains an arguably more dangerous ordeal in remote, underdeveloped regions of the world, though the women who live in these regions have argued that their natural childbirth methods are safer and less traumatic for mother and child.

Human children are born after a nine-month gestation period, with typically 3–4 kilograms (6–9 pounds) in weight and 50–60 centimetres (20–24 inches) in height in developed countries. [1] Helpless at birth, they continue to grow for some years, typically reaching sexual maturity at twelve to fifteen years of age. Boys continue growing for some time after this, reaching their maximum height around the age of eighteen. These values vary too, depending on genes and environment.

The human lifespan can be split into a number of stages: infancy, childhood, adolescence, young adulthood, maturity and old age, though the lengths of these stages, especially 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 (the lowest in Uganda at 14.8 years). Life expectancy at birth is 77.2 years in the U.S. as of 2001. [2] The expected life span at birth in Singapore is 84.29 years for a female and 78.96 years for a male, while in Botswana, due largely to AIDS, it is 30.99 years for a male and 30.53 years for a female. One in five Europeans, but one in twenty Africans, is 60 years or older, according to The World Factbook. [3]

The number of centenarians in the world was estimated by the United Nations [4] at 210,000 in 2002. The maximum life span for humans is thought to be over 120 years. Worldwide, there are 81 men aged 60 or over for every 100 women, 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 may cause unease or fear. People who are near death sometimes report having a near-death experience, in which they have visions. Burial ceremonies are characteristic of human societies, often inspired by beliefs in an afterlife. Institutions of inheritance or ancestor worship may extend an individual's presence beyond his physical lifespan (see immortality).

Human - Genetics

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. At present estimate, 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. [5] 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 affect men more often than women. For example, genes that control the clotting of blood reside on the X chromosome. Women have a blood-clotting gene on each X chromosome so that one normal blood-clotting gene can compensate for a flaw in the gene on the other X chromosome. But men are hemizygous for the blood-clotting gene, since there is no gene on the Y chromosome to control blood clotting. As a result, men will suffer from haemophilia more often than women.

Human - Race and ethnicity

Humans often categorise themselves and others in terms of race or ethnicity, although the validity of human races is disputed. The most widely used human racial categories are based on visible traits (especially skin color and facial features), genes, and self-identification. Language and ethnicity, blood type, ancestry, and other factors are also sometimes considered. Self identification with an ethnic group is based on kinship and descent, as well as presumed advantage. When race and ethnicity often lead to variant treatment and impact social identity, giving rise to the theory of identity politics.

An ethnic group is a culture or subculture whose members are readily distinguishable by outsiders based on traits originating from a common racial, national, linguistic, regional or religious source. Members of an ethnic group are often presumed to be culturally or genetically similar, although this is not in fact necessarily the case.

Although most humans recognise that variances occur within a species, it is often a point of dispute as to what these differences entail, their import, and if discrimination based on race (racism) is acceptable. Race and intelligence, scientific racism, xenophobia and ethnocentrism are just a few of the many bases for such practices.

Some societies have placed a great deal of emphasis on race, others have not. Two extremes include ancient egypt, and the racial policy of Nazi Germany.

Human - Habitat

The view most widely accepted by the anthropological community is that the human species originated in the African savanna between 100 and 200 thousand years ago, had colonised the rest of the Old World and Oceania by 40,000 years ago, and finally colonised the Americas by 10,000 years ago. Homo sapiens displaced groups such as Neanderthals and Homo floresiensis through more successful reproduction and competition for resources, and/or extermination. (See Human evolution, Vagina gentium, and Environment of Evolutionary Adaptedness.) Technology has allowed humans to colonise all of the continents and adapt to all climates. Within the last few decades, humans have been able to explore Antarctica, the ocean depths, and space, although long-term habitation of these environments are not yet possible. Humans, with a population of over six billion, are one of the most numerous mammals on Earth.

Most humans (61%) live in the Asian region. The vast majority of the remainder live in the Americas (14%), Africa (13%) and Europe (12%), with 5% in Oceania. (See list of countries by population and list of countries by population density.)

The original human lifestyle is hunting-gathering, which is adapted to the savanna. Other human lifestyles are nomadism (often linked to animal herding) and permanent settlements made possible by the development of agriculture. Humans have a great capacity for altering their habitats by various methods, such as agriculture, irrigation, urban planning, construction, transport, and manufacturing goods.

Permanent human settlements are 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 populations of prey. With the advent of large-scale trade and transport infrastructure, immediate proximity to these resources has become unnecessary, and in many places these factors are no longer the driving force behind growth and decline of population.

Human habitation within closed ecological systems in hostile environments (Antarctica, outer space) is expensive, typically limited in duration, and restricted to scientific, military, or industrial expeditions. Life in space has been very sporadic, with a maximum of thirteen humans in space at any given time, starting with Yuri Gagarin's space flight in 1961. Between 1969 and 1972, up to two humans at a time spent brief intervals on the Moon. As of 2005, 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.

Human - Food and drink

Cuisine | Kitchens
Wikibooks: Cookbook

Humans are commonly believed to be omnivorous animals that can consume both plant and animal products. Evidence suggests that early Homo Sapiens employed Hunter-gatherer as their primary means of food collection. This involves 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. Additionally, it is believed that humans have used fire to prepare food prior to eating since their divergence from Homo erectus, possibly even earlier.

At least ten thousand years ago, humans developed agriculture, which has altered substantially the kind of food people eat. This has led to a variety of important historical consequences, such as increased population, the development of cities, and 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 also to many other world cuisines, as well. New cuisines are constantly evolving, as certain aesthetics rise and fall in popularity among professional chefs and their clientele.

In addition to food, a cuisine is also often held to include beverages, including wine, liquor, tea, coffee and other drinks.

There are also different cultural attitudes to food, for example:

• In India, consumption of food is regarded as an offering, a Yajna. Thus the stomach is considered to be a homagunda (holy fire) and all the food consumed is an offering to the holy fire.

• In Japan, Tea drinking is a fine-art and there is an elaborate ceremony about it. Not drinking tea in the right way is considered to be an act of barbarism.

• Some persons are vegetarian, refusing to consume animal meat, and sometimes animal products (this varies). Many others consume fast food at rates alarming to dieticians.

Human - Population

From 1800 to 2000, the human population increased from one to six billion. It is expected to crest at around ten billion during the 21st century. In 2004, around 2.5 billion out of 6.3 billion people lived in urban areas, and this is expected to rise during the 21st century. Problems for humans living in cities include various forms of pollution, crime, and poverty, especially in inner city and suburban slums.

Geneticists Lynn Jorde and Henry Harpending of the University of Utah have concluded that the variation in the total stock of human DNA is minute compared to that of other species; and that around 74,000 years ago, human population was reduced to a small number of breeding pairs, possibly as small as 1000, resulting in a very small residual gene pool. Various reasons for this bottleneck have been postulated, the most popular, called the Toba catastrophe theory, being the eruption of a volcano at Lake Toba.

Human - Evolution

The study of human evolution encompasses many scientific disciplines, but most notably physical anthropology and genetics. The term "human", in the context of human evolution, refers to the genus Homo, but studies of human evolution usually include other hominids and hominines, such as the australopithecines.

Biologically, humans are defined as hominids of the species Homo sapiens, of which the only extant subspecies is Homo sapiens sapiens (Latin for "very wise man"); Homo sapiens idaltu (roughly translated as "elderly wise man") is the extinct subspecies. Modern humans are usually considered the only surviving species in the genus Homo, although some argue that the two species of chimpanzees should be reclassified from Pan troglodytes (Common Chimpanzee) and Pan paniscus (Bonobo/Pygmy Chimpanzee) to Homo troglodytes and Homo paniscus respectively, given that they share a recent ancestor with man. [6]

Full genome sequencing resulted in these conclusions: "After 6 [million] years of separate evolution, the differences between chimp and human are just 10 times greater than those between two unrelated people and 10 times less than those between rats and mice." Chimp 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, in 2001 a hominine skull approximately seven million years old, classified as Sahelanthropus tchadensis, was discovered in Chad and seems to indicate an earlier divergence.

Two prominent scientific theories of the origins of contemporary humans exist. 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 replaced hominids in other parts of the world.

The multiregional hypothesis proposes that modern humans evolved at least in part from independent hominid populations.

Human evolution is characterised by a number of important physiological trends:

• 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 (heterochrony), allowing for an extended period of social learning in juvenile humans. Physical anthropologists argue that a reorganisation of the structure of the brain is more important than cranial expansion itself;
• canine tooth reduction;
• bipedal locomotion;
• descent of the larynx, which makes speech possible.

Humans are classified as Homo sapiens sapiens. A camp of physical anthropologists see neanderthalensis as a subspecies and classify the neanderthals as Homo sapiens neanderthalensis. A second camp of physical anthropologists see the neanderthals as a distinct species diverging from the modern human lineage over 500,000 years ago. Under this classification, neaderthals are Homo neanderthalensis. Recent DNA analysis suggests that neanderthalensis were not a subspecies.

How these trends are related and what their role is in the evolution of complex social organisation and culture are matters of ongoing debate.

Human - Intelligence

Human beings are the most intelligent living being, animal or otherwise on Earth. While other animals are capable of creating structures (mostly as a result of instinct) and using tools, human technology is in a class by itself, constantly evolving and improving with time. Indeed, even the most ancient human tools, and structures are far more advanced than any structure or tool created by another animal. In terms of brain to body mass ratio, humans are the second highest ratio. While some consider this a good measure of intelligence, the tree shrew which is ranked with the highest ratio, directly contradicts this notion, because it is far less intelligent than even animals with lower brain to body mass ratios than humans.

The human ability to abstract may be unparalleled in the animal kingdom. Human beings are one of five species to pass the mirror test — which tests whether an animal recognises its reflection as an image of itself — along with chimpanzees or bonobos, orangutans, and dolphins. Human beings under the age of four usually fail the test.

Adapted from the Wikipedia article "Biology", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki