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=== Physiology and genetics ===
=== Physiology and genetics ===
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.<ref>[http://www.freerepublic.com/focus/f-news/966532/posts ''Why Humans and Their Fur Parted Way''] by Nicholas Wade, ''New York Times'', [[August 19]] [[2003]], retrieved [[March 17]], [[2006]].</ref>
Human body types vary substantially. Although body size is largely determined by [[gene]]s, it is also significantly influenced by environmental factors such as [[diet (nutrition)|diet]] and [[exercise]]. The average [[human height|height]] of an adult human is about 5 to 6 [[foot (length)|feet]] (1.5 to 1.8 m) tall, although this varies significantly from place to place. Britannica Concise Encyclopedia. Encyclopædia Britannica, Inc., 2006. [http://www.answers.com/topic/pygmy] Humans are capable of fully bipedal [[terrestrial locomotion in animals|locomotion]], thus leaving their arms available for manipulating objects using their [[hand]]s, aided especially by opposable [[thumb]]s.
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.<ref>[http://www.freerepublic.com/focus/f-news/966532/posts ''Why Humans and Their Fur Parted Way'' by Nicholas Wade, ''New York Times'',
The hue of human hair and skin is determined by the presence of [[pigment]]s called [[melanin]]s. Human skin hues can range from very dark brown to very pale pink, while human hair ranges from [[blond]] to [[brown hair|brown]] to [[red hair|red]] to, most commonly, [[black hair|black]]., 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.<ref>Jablonski, N.G. & Chaplin, G. (2000). ''[http://www.bgsu.edu/departments/chem/faculty/leontis/chem447/PDF_files/Jablonski_skin_color_2000.pdf]The evolution of human skin coloration]'' (pdf), 'Journal of Human Evolution 39: 57-106. 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.
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.
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 [[eukaryote|eukaryotic]] species. Each [[ploidy|diploid]] [[cell (biology)|cell]] has two sets of 23 [[chromosome]]s, each set received from one parent. There are 22 pairs of [[autosome]]s and one pair of [[sex-determination system|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 [[female]]s have the sex chromosomes XX and [[male]]s have XY. The X chromosome is larger and carries many genes not on the Y chromosome, which means that [[recessive gene|recessive]] diseases associated with X-linked genes, such as [[hemophilia]], affect men more often than women.
Humans are an [[eukaryote|eukaryotic]] species. Each [[ploidy|diploid]] [[cell (biology)|cell]] has two sets of 23 [[chromosome]]s, each set received from one parent. There are 22 pairs of [[autosome]]s and one pair of [[sex-determination system|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 [[female]]s have the sex chromosomes XX and [[male]]s have XY. The X chromosome is larger and carries many genes not on the Y chromosome, which means that [[recessive gene|recessive]] diseases associated with X-linked genes, such as [[hemophilia]], affect men more often than women.
=== Life cycle ===
=== Life cycle ===
