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Further information: Human physical appearance, Anatomically modern humans, and Sex differences in humans Most aspects of human physiology are closely homologous to corresponding aspects of animal physiology. The human body consists of the legs, the torso, the arms, the neck, and the head. An adult human body consists of approximately 100 trillion cells. Most commonly defined body systems in humans are the nervous, the cardiovascular, the circulatory, the digestive, the endocrine, the immune, the integumentary, the lympathic, the muscoskeletal, the reproductive, the respiratory, and the urinary system.[65][66] Humans, like most of the other apes, lack external tail, have several blood type systems, opposable thumbs, and are sexually dimorphic. The comparatively minor anatomical differences between humans and chimpanzees are a result of human bipedalism. As a result, humans are slower over short distances, but are among the best long-distance runners in the animal kingdom.[67][68] Humans' thinner body hair and more productive sweat glands help avoid heat exhaustion while running for long distances.[69] As a consequence of bipedalism human females have narrower birth canals. The construction of the human pelvis differs from other primates, as do the toes. A trade-off for these advantages of the modern human pelvis is that childbirth is more difficult and dangerous than in most mammals, especially given the larger head size of human babies compared to other primates. This means that human babies must turn around as they pass through the birth canal which other primates do not do, and it makes humans the only species where females require help from their conspecifics to reduce the risks of birthing. As a partial evolutionary solution, human fetuses are born less developed and more vulnerable. Chimpanzee babies are cognitively more developed than human babies until the age of six months when the rapid development of human brains surpasses chimpanzees’. Another difference between women and chimpanzee females is that women go through menopause and become unfertile decades before the end of their lives. All non-human apes are capable of giving birth until death. Menopause has probably developed among aged women as it has provided an evolutionary advantage (more caring time) to young relatives.[68] Other than bipedalism, humans differ from chimpanzees mostly in smelling, hearing, digesting proteins, brain size, and the ability of language. Humans have about three times bigger brain than chimpanzees. More importantly, the brain to body ratio is much higher in humans than in chimpanzees and humans have a significantly more developed cerebral cortex with a larger number of neurons. The mental abilities of humans are remarkable compared to other apes. Humans’ ability of speech is unique among primates. Humans are able to create new and complex ideas, and to develop technology, which is unprecedented among other organisms on Earth.[68] The average human male is about 1.7–1.8 metres (5.6–5.9 ft), the average human female is about 1.6–1.7 metres (5.2–5.6 ft) in height.[70] Shrinkage of stature may begin in middle age in some individuals but tends to be universal in the extremely aged.[71] Through history human populations universally became taller, probably as a consequence of better nutrition, healthcare, and living conditions.[72] The average mass of an adult human is 54–64 kg (120–140 lbs) for females and 76–83 kg (168–183 lbs) for males.[73] Like many other conditions, body weight and body type is influenced by both genetic susceptibility and environment and varies greatly among individuals. (see obesity)[74][75] Although humans appear 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.[76] Humans have about 2 million sweat glands spread over their entire bodies, much more than that of the chimpanzees whose sweat glands are scarce and are mainly located on the palm of the hand and on the soles of the feet.[77] The dental formula of humans is: 2.1.2.3 2.1.2.3 . Humans have proportionately shorter palates and much smaller teeth than other primates. They are the only primates to have short, relatively flush canine teeth. Humans have characteristically crowded teeth, with gaps from lost teeth usually closing up quickly in young individuals. Humans are gradually losing their wisdom teeth, with some individuals having them congenitally absent.[78] Genetics Main article: Human genetics A graphical representation of the ideal human karyotype, including both the male and female variant of the sex chromosome (number 23). Like all mammals, humans are a diploid eukaryotic species. Each somatic cell has two sets of 23 chromosomes, each set received from one parent, gametes have only one set of chromosomes which is a mixture of the two parental sets. Among the 23 chromosomes there are 22 pairs of autosomes and one pair of sex chromosomes. Like other mammals, humans have an XY sex-determination system, so that females have the sex chromosomes XX and males have XY. One human genome was sequenced in full in 2003, and currently efforts are being made to achieve a sample of the genetic diversity of the species (see International HapMap Project). By present estimates, humans have approximately 22,000 genes.[79] The variation in human DNA is minute compared to that of other species, possibly suggesting a population bottleneck during the Late Pleistocene (ca. 100,000 years ago), in which the human population was reduced to a small number of breeding pairs.[80][81] Nucleotide diversity is based on single mutations called single nucleotide polymorphisms (SNPs). The nucleotide diversity between humans is about 0.1%, which is 1 difference per 1,000 base pairs.[82][83] A difference of 1 in 1,000 nucleotides between two humans chosen at random amounts to approximately 3 million nucleotide differences since the human genome has about 3 billion nucleotides. Most of these SNPs are neutral but some (about 3 to 5%) are functional and influence phenotypic differences between humans through alleles. By comparing the parts of the genome that are not under natural selection and which therefore accumulate mutations at a fairly steady rate, it is possible to reconstruct a genetic tree incorporating the entire human species since the last shared ancestor. Each time a certain mutation (Single nucleotide polymorphism) appears in an individual and is passed on to his or her descendants a haplogroup is formed including all of the descendants of the individual who will also carry that mutation. By comparing mitochondrial DNA which is inherited only from the mother, geneticists have concluded that the last female common ancestor whose genetic marker is found in all modern humans, the so-called mitochondrial Eve, must have lived around 200,000 years ago. The forces of natural selection have continued to operate on human populations, with evidence that certain regions of the genome display directional selection in the past 15,000 years.[84] Life cycle See also: Childbirth and Life expectancy A 10 mm human embryo at 5 weeks Boy and girl before puberty Adult man and woman in the reproductive age Elderly man and woman As with other mammals, human reproduction takes place as internal fertilization by sexual intercourse. During this process, the erect penis of the male is inserted into the female's vagina until the male ejaculates semen, which contains sperm. The sperm travels through the vagina and cervix into the uterus or Fallopian tubes for fertilization of the ovum. Upon fertilization and implantation, gestation then occurs within the female's uterus. The zygote divides inside the female's uterus to become an embryo, which over a period of 38 weeks (9 months) of gestation becomes a 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 various levels of personhood earlier to human fetuses while they remain in the uterus. Compared with other species, human childbirth is dangerous. Painful labors lasting 24 hours or more are not uncommon and sometimes lead to the death of the mother, the child or both.[85] This is because of both the relatively large fetal head circumference and the mother's relatively narrow pelvis.[86][87] The chances of a successful labor increased significantly during the 20th century in wealthier countries with the advent of new medical technologies. In contrast, pregnancy and natural childbirth remain hazardous ordeals in developing regions of the world, with maternal death rates approximately 100 times greater than in developed countries.[88] 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.[89][not in citation given] However, low birth weight is common in developing countries, and contributes to the high levels of infant mortality in these regions.[90] Helpless at birth, humans continue to grow for some years, typically reaching sexual maturity at 12 to 15 years of age. Females continue to develop physically until around the age of 18, whereas male development continues until around age 21. The human life span can be split into a number of stages: infancy, childhood, adolescence, young adulthood, adulthood and old age. The lengths of these stages, however, have varied across cultures and time periods. Compared to other primates, humans experience an unusually rapid growth spurt during adolescence, where the body grows 25% in size. Chimpanzees, for example, grow only 14%, with no pronounced spurt.[91] The presence of the growth spurt is probably necessary to keep children physically small until they are psychologically mature. Humans are one of the few species in which females undergo menopause. It has been proposed that menopause increases a woman's overall reproductive success by allowing her to invest more time and resources in her existing offspring and/or their children (the grandmother hypothesis), rather than by continuing to bear children into old age.[92][93] For various reasons, including biological/genetic causes,[94] women live on average about four years longer than men — as of 2013 the global average life expectancy at birth of a girl is estimated at 70.2 years compared to 66.1 for a boy.[95] There are significant geographical variations in human life expectancy, mostly correlated with economic development — for example life expectancy at birth in Hong Kong is 84.8 years for girls and 78.9 for boys, while in Swaziland, primarily because of AIDS, it is 31.3 years for both sexes.[96] The developed world is generally aging, with the median age around 40 years. In the developing world the median age is between 15 and 20 years. While one in five Europeans is 60 years of age or older, only one in twenty Africans is 60 years of age or older.[97] 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.[98] At least one person, Jeanne Calment, is known to have reached the age of 122 years;[99] higher ages have been claimed but they are not well substantiated. Diet Main article: Human nutrition Humans preparing a meal in Bali, Indonesia Venus of Willensdorf statuette from the Upper Palaeolithic period Two starved boys during the Russian famine of 1921 Humans are omnivorous, capable of consuming a wide variety of plant and animal material.[100][101] Varying with available food sources in regions of habitation, and also varying with cultural and religious norms, human groups have adopted a range of diets, from purely vegetarian to primarily carnivorous. In some cases, dietary restrictions in humans can lead to deficiency diseases; however, stable human groups have adapted to many dietary patterns through both genetic specialization and cultural conventions to use nutritionally balanced food sources.[102] The human diet is prominently reflected in human culture, and has led to the development of food science. Until the development of agriculture approximately 10,000 years ago, Homo sapiens employed a hunter-gatherer method as their sole means of food collection. This involved combining stationary food sources (such as fruits, grains, tubers, and mushrooms, insect larvae and aquatic mollusks) with wild game, which must be hunted and killed in order to be consumed.[103] It has been proposed that humans have used fire to prepare and cook food since the time of Homo erectus.[104] Around ten thousand years ago, humans developed agriculture,[105] which substantially altered their diet. This change in diet may also have altered human biology; with the spread of dairy farming providing a new and rich source of food, leading to the evolution of the ability to digest lactose in some adults.[106][107] Agriculture led to increased populations, the development of cities, and because of increased population density, the wider spread of infectious diseases. The types of food consumed, and the way in which they are prepared, has varied widely by time, location, and culture. In general, humans can survive for two to eight weeks without food, depending on stored body fat. Survival without water is usually limited to three or four days. About 36 million humans die every year from causes directly or indirectly related to hunger.[108] Childhood malnutrition is also common and contributes to the global burden of disease.[109] However global food distribution is not even, and obesity among some human populations has increased rapidly, leading to health complications and increased mortality in some developed, and a few developing countries. Worldwide over one billion people are obese,[110] while in the United States 35% of people are obese, leading to this being described as an "obesity epidemic".[111] Obesity is caused by consuming more calories than are expended, so excessive weight gain is usually caused by a combination of an energy-dense high fat diet and insufficient exercise.[110] Biological variation Main article: Human genetic variation People in warm climates are often relatively slender, tall and dark skinned, such as these Maasai men from Kenya. People in cold climates tend to be relatively short, heavily built and fair skinned such as these Inuit women from Canada. No two humans – not even monozygotic twins – are genetically identical. Genes and environment influence human biological variation from visible characteristics to physiology to disease susceptibly to mental abilities. The exact influence of genes and environment on certain traits is not well understood.[112][113] Most current genetic and archaeological evidence supports a recent single origin of modern humans in East Africa[114] with first migrations placed at 60,000 years ago. Compared to the other great apes, human gene sequences – even among African populations – are remarkably homogeneous.[115] On average, genetic similarity between any two humans is 99.9%.[116][117] There is about 2–3 times more genetic diversity within the wild chimpanzee populations on a single hillside in Gombe, than in the entire human gene pool.[118][119][120][121] The human body’s ability to adapt to different environmental stresses is remarkable, allowing humans to acclimatize to a wide variety of temperatures, humidity, and altitudes. As a result, humans are a cosmopolitan species found in almost all regions of the world, including tropical rainforests, arid desert, extremely cold arctic regions, and heavily polluted cities. Most other species are confined to a few geographical areas by their limited adaptability.[122] There is biological variation in the human species — with traits such as blood type, cranial features, eye color, hair color and type, height and build, and skin color varying across the globe. Human body types vary substantially. The average height of an adult human is between 1.4 m (4 ft 7 in) to 1.9 m (6 ft 3 in) tall and this varies significantly depending on sex and ethnic origin.[123][124] Body size is determined by genes and is significantly influenced by environmental factors such as diet, exercise, and sleep patterns, especially as an influence in childhood. Adult height for one sex in a particular ethnic group follows more or less a normal distribution. Those aspects of genetic variation that give clue to human evolutionary history, or which are relevant for medical research have received particular attention. For example the genes that cause adult humans to be able to digest lactose are present in high frequencies in populations that have long histories of cattle domestication, suggesting natural selection having favored that gene in populations that depend on cow milk. Some hereditary diseases such as sickle cell anemia are frequent in populations where Malaria has been endemic throughout history — it is believed that the same gene causes increased resistance to Malaria among those who are unaffected carriers of the gene. Similarly, populations that have inhabited specific climates for a long time such as arctic or tropical regions or high altitudes, tend to have developed specific phenotypes that are beneficial for conserving energy in those environments — short stature and stocky build in cold regions, tall and lanky in hot regions, and with high lung capacities in high altitudes. Similarly, skin color varies clinally with darker skin around the equator — where the added protection from the sun is thought to give an evolutionary advantage against ultraviolet radiation — and lighter skin tones closer to the poles.[125][126][127][128] The hue of human skin and hair is determined by the presence of pigments called melanins. Human skin color can range from darkest brown to lightest peach, or even nearly white or colorless in cases of albinism.[121] Human hair ranges in color from white to red to blond to brown to the most commonly black.[129] Hair color depends on the amount of melanin (an effective sun blocking pigment) in the skin and hair, with hair melanin concentrations in hair fading with increased age, leading to grey or even white hair. Most researchers believe that skin darkening was an adaptation that evolved as a protection against ultraviolet solar radiation, which also helps balancing folate, which is destroyed by ultraviolet radiation. Light skin pigmentation provides advantages against vitamin D depletion, which requires sunlight to make.[130] Skin pigmentation of contemporary humans is clinally distributed across the planet, and in general, correlates with the level of ultraviolet radiation in a particular geographic area. Human skin also has a capacity to darken (tan) in response to exposure to ultraviolet radiation.[131][132][133] Structure of variation A Libyan, a Nubian, a Syrian, and an Egyptian, drawing by an unknown artist after a mural of the tomb of Seti I. The ancestors of Native Americans, such as this Yanomami woman, crossed into the Americas from Northeast Asia, and genetic and linguistic evidence links them to North Asian populations, particularly those of East Siberia.[134] Within the human species, the greatest degree of genetic variation exists between males and females. While the nucleotide genetic variation of individuals of the same sex across global populations is no greater than 0.1%, the genetic difference between males and females is between 1% and 2%. Although different in nature, this approaches the genetic differentiation between men and male chimpanzees or women and female chimpanzees. The genetic difference between sexes contributes to anatomical, hormonal, neural, and physiological differences between men and women, although the exact degree and nature of social and environmental influences on sexes are not completely understood. Males on average are 15% heavier and 15 cm taller than females. There is a difference between body types, body organs and systems, hormonal levels, sensory systems, and muscle mass between sexes. There is a difference of about 40–50% in upper body strength and 20–30% in lower body strength between men and women. Women generally have higher body fat percentage than men. Women of the same population have lighter skin than men; this has been explained by a higher need for vitamin D (which is synthesized by sunlight) in females during pregnancy and lactation. As there are chromosomal differences between females and males, some X and Y chromosome related conditions and disorders only affect either men or women. Other conditional differences between males and females are not related to sex chromosomes. Even after controlling for body weight and volume, male voice is usually an octave deeper than females’. Women have a longer life span in almost every population around the world.[135][136][137][138][139][140][141][142][143] Males typically have larger tracheae and branching bronchi, with about 30 percent greater lung volume per body mass. They have larger hearts, 10 percent higher red blood cell count, higher hemoglobin, hence greater oxygen-carrying capacity. They also have higher circulating clotting factors (vitamin K, prothrombin and platelets). These differences lead to faster healing of wounds and higher peripheral pain tolerance.[144] Females typically have more white blood cells (stored and circulating), more granulocytes and B and T lymphocytes. Additionally, they produce more antibodies at a faster rate than males. Hence they develop fewer infectious diseases and succumb for shorter periods.[144] Ethologists argue that females, interacting with other females and multiple offspring in social groups, have experienced such traits as a selective advantage.[145][146][147][148][149] According to Daly and Wilson, "The sexes differ more in human beings than in monogamous mammals, but much less than in extremely polygamous mammals."[150] But given that sexual dimorphism in the closest relatives of humans is much greater than among humans, the human clade must be considered to be characterized by decreasing sexual dimorphism, probably due to less competitive mating patterns. One proposed explanation is that human sexuality has developed more in common with its close relative the bonobo, who have similar sexual dimorphism and which are polygynandrous and use recreational sex to reinforce social bonds and reduce aggression.[151] Humans of the same sex are 99.9% genetically identical. There is extremely little variation between human geographical populations and most of the variation that does occur is in the personal level within local areas, and not between populations.[121][152][153] Of the 0.1% of human genetic differentiation, 85% exists within any randomly chosen local population, be they Italians, Koreans, or Kurds. Two randomly chosen Koreans may be genetically as different as a Korean and an Italian. Any ethnic group contains 85% of the human genetic diversity of the world. Genetic data shows that no matter how population groups are defined, two people from the same population group are about as different from each other as two people from any two different population groups.[121][154][155][156] Most of the world's genetic diversity is represented in Africa. Current genetic research have demonstrated that humans on the African continent are the most genetically diverse.[157] There is more human genetic diversity in Africa than anywhere else on Earth. The genetic structure of Africans was traced to 14 ancestral population clusters. Human genetic diversity decreases in native populations with migratory distance from Africa and this is thought to be the result of bottlenecks during human migration.[158][159] Humans have lived in Africa for the longest time which allowed accumulation of a higher diversity of genetic mutations in these populations. Only part of Africa’s population migrated out of the continent, bringing just part of the original African genetic variety with them. African populations harbor genetic alleles that are not found in other places of the world. All the common alleles found in populations outside of Africa are found on the African continent.[121] Geographical distribution of human variation is complex and constantly shifts through time which reflects complicated human evolutionary history. Most human biological variation is clinally distributed and blends gradually from an area to the next. Groups of people around the world have different frequencies of polymorphic genes. Furthermore, different traits are non-concordant and each have different clinal distribution. Adaptability varies both from person to person and from population to population. The most efficient adaptive responses are found in geographical populations where the environmental stimuli are the strongest (e.g. Tibetans are highly adapted to high altitudes). The clinal geographic genetic variation is further complicated by the migration and mixing between human populations which has been occurring since prehistoric times.[121][160][161][162][163][164] Human variation is highly non-concordant: most of the genes do not cluster together and are not inherited together. Skin and hair color are not correlated to height, weight, or athletic ability. Human species do not share the same patterns of variation through geography. Skin color varies with latitude and certain people are tall or have brown hair. There is a statistical correlation between particular features in a population, but different features are not expressed or inherited together. Thus, genes which code for superficial physical traits – such as skin color, hair color, or height – represent a minuscule and insignificant portion of the human genome and do not correlate with genetic affinity. Dark-skinned populations that are found in Africa, Australia, and South Asia are not closely related to each other.[128][133][163][164][165][166] Even within the same region, physical phenotype is not related to genetic affinity: dark-skinned Ethiopians are more closely related to light-skinned Armenians than to dark-skinned Bantu populations.[167] Despite pygmy populations of South East Asia (Andamanese) having similar physical features with African pygmy populations such as short stature, dark skin, and curly hair, they are not genetically closely related to these populations.[168] Genetic variants affecting superficial anatomical features (such as skin color) – from a genetic perspective, are essentially meaningless – they involve a few hundred of the billions of nucleotides in a person's DNA.[169] Individuals with the same morphology do not necessarily cluster with each other by lineage, and a given lineage does not include only individuals with the same trait complex.[121][155][170] Due to practices of group endogamy, allele frequencies cluster locally around kin groups and lineages, or by national, ethnic, cultural and linguistic boundaries, giving a detailed degree of correlation between genetic clusters and population groups when considering many alleles simultaneously. Despite this, there are no genetic boundaries around local populations that biologically mark off any discrete groups of humans. Human variation is continuous, with no clear points of demarcation. There are no large clusters of relatively homogeneous people and almost every individual has genetic alleles from several ancestral groups |
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