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 tallest human population are the Dutch people, with the average height of a Dutch adult female being 170 centimetres (5 feet 7 inches), while the average height for a male is 185 centimeters (6 feet 1 inch). The shortest people are the Mbuti and Baka tribes in Congo, Cameroon and Gabon who have an average male height of less than 150 centimeters (4 feet 11 inches).

Humans are capable of fully bipedal locomotion, thus leaving their arms available for manipulating objects using their hands, aided especially by opposable thumbs. Because human physiology has not fully adapted to bipedalism, the pelvic region and vertebral column tend to become worn, creating locomotion difficulties in old age.

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 on his or her body than the average chimpanzee. The main distinction is that human hairs are shorter, finer, and less colored than the average chimpanzee's, thus making them harder to see.

An Inuit woman, circa 1907.The color of human hair and skin is determined by the presence of pigments called melanins. Human skin color can range from very dark brown to very pale pink, while human hair ranges from blond to brown to red, but most commonly, black. Most researchers believe that skin darkening was an adaptation that evolved as a defense against ultraviolet solar radiation: melanin is an effective sun-block. The skin color 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. 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 physiology and mental state, including fatigue, aggression, and bodily discomfort. It is common in modern societies for people to get less sleep than they need, leading to a state of sleep deprivation.

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. By present estimates, 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. Genetic studies indicate that humans are more closely related to chimpanzees, while gorillas diverged earlier from the chimpanzee/human line of descent. Consequently, use of the term 'apes' to mean chimpanzees and gorillas is incorrect, with humans and chimpanzees forming one group and gorillas a second, more distantly-related group.

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.