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Anatomy and physiology

Main articles: Human anatomy and Human physiology

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.

Humans, like most of the other apes, lack external tail, have several blood type systems and 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. Humans' thinner body hair and more productive sweat glands help avoid heat exhaustion while running for long distances.

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 that 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 in which 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.

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 in Earth.

The average human male is about 1.7-1.8 m, the average human female is about 1.6-1.7 m height. Shrinkage of stature may begin in middle age in some individuals but tends to be universal in the extremely aged. Through history human populations universally became taller, probably as a consequence of better nutrition, healthcare, and living conditions. 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. Like many other conditions, body weight and body type is influence by both genetic susceptibility and environment and varies greatly among individuals. (see obesity).

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. 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 in the palm of the hand and the soles of the feet.

The dental formula of humans is: 2.1.2.32.1.2.3. 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.

Biological variation

Main article: Human genetic variation

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.

Most current genetic and archaeological evidence supports a recent single origin of modern humans in East Africa 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. 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 genome.

Human body’s ability to adapt to different environmental stresses is remarkable. Humans are a cosmopolitan species found in almost all regions of the world. Humans live in tropical rainforests, arid desert, extremely cold arctic regions, and heavily polluted cities. Humans can acclimatize to a wide variety of temperatures, humidity, and altitudes. Most other species are confined to few geographical areas by their limited adaptability.

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 v aries significantly depending on sex and ethnic origin. 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 gives 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 population 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 from areas in which Malaria has been endemic throughout history — it is believed that the same gene that causes increased resistance to Malaria among those who are unaffected carriers of the gene. Similarly, populations that have inhabited specific climates 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 again ultraviolet radiation and lighter skin tones closer to the poles.

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 pinkish-white, or even nearly white or colorless in cases of Albinism. Human hair ranges from white to brown to red to most commonly black. This 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, and vitamin D, which requires sunlight to form. The skin pigmentation of contemporary humans is clinally distributed across the planet, 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.