Human

"Anatomically, you Humans are somewhat simplistic. But what you lack biologically, you make up for your charming optimism. Not to mention your egg drop soup."

- Doctor Phlox

Humans (Homo sapiens) are a species of bipedal sapient mammals native to planet Earth. They are omnivorous and generally believed to have descended from arboreal ancestors.

Biology
Humans have an erect posture, with two legs, two arms and a head which comprises the brain and most of the sense organs. Extraterrestrial species showing this same body plan, or a very similar one, are collectively known as humanoids. Although some humanoid races look almost exactly like humans, as is the case with Vulcans, they are often very different internally. Vulcans, for example, have green blood containing copper, while the Human blood is red and contains iron.

Humans are carbon-based lifeforms, and have an internal skeleton containing calcium, which gives the bones strength and durability. More than 60% of the Human body is composed of liquid water. Their circulatory system is closed and they have a heart and two lungs (though they are able to survive with only one lung), located in the chest area. They have five fingers on their hands and five toes on their feet.

Despite having evolved as a predatory species, Humans are generally considered weak and not well adapted to hunt without the aid of tools. Their canine teeth, for example, are not sharp like those of their evolutionary relatives, the other Earth primates, and their fingers and toes have nails, rather than claws.

Anatomy
Main article: Human anatomyFurther information: Human physical appearance and Anatomically modern humansBasic anatomical features of female and male humans. Note that these models have had body hair and male facial hair removed and head hair trimmed.Human body types vary substantially. Although body size is largely determined by genes, it is also significantly influenced by environmental factors such as diet andexercise. The average height of an adult human is about 1.5 to 1.8 m (5 to 6 feet) tall, although this varies significantly from place to place and depending on ethnic origin.[51] [52] 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.[53] Weight can also vary greatly (e.g.obesity). Unlike most other primates, humans are capable of fully bipedal locomotion, thus leaving their arms available for manipulating objects using their hands, aided especially by opposable thumbs. Vitruvian Man, Leonardo da Vinci's image is often used as an implied symbol of the essential symmetry of the human body, and by extension, of the universe as a whole.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.[54]

The hue of human skin and hair is determined by the presence of pigments called melanins. Human skin hues can range from very dark brown to very pale pink. Human hair ranges from white to brown to red to most commonly black.[55] 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. However, more recently 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.[56] 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.[57] [58] Humans tend to be physically weaker than other similarly sized primates, with young, conditioned male humans having been shown to be unable to match the strength of female orangutans which are at least three times stronger.[59]

The construction of the human pelvis differs from other primates, as do the toes. As a result, humans are slower for short distances than most other animals, but are among the best long-distance runners in the animal kingdom.[60] Humans' thinner body hair and more productive sweat glands also helps avoid heat exhaustion while running for long distances. For this reason persistence hunting was most likely a very successful strategy for early humans – in this method, prey is chased until it is literally exhausted. This may have also helped the early human Cro-Magnon population out-compete the Neanderthal population for food. The otherwise physically stronger Neanderthal would have much greater difficulty hunting in this way, and much more likely hunted larger game in close quarters. A trade-off for these advantages of the modern human pelvis is that childbirth is more difficult and dangerous.

The construction of modern human shoulders enable throwing weapons, which also were much more difficult or even impossible for Neanderthal competitors to use effectively.[61] 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 specimens. Humans are gradually losing their wisdom teeth, with some individuals having them congenitally absent.[63]

Physiology
Main article: Human physiologyHuman physiology is the science of the mechanical, physical, and biochemical functions of humans in good health, their organs, and the cells of which they are composed. The principal level of focus of physiology is at the level of organs and systems. Most aspects of human physiology are closely homologous to corresponding aspects of animal physiology, and animal experimentation has provided much of the foundation of physiological knowledge. Anatomy and physiology are closely related fields of study: anatomy, the study of form, and physiology, the study of function, are intrinsically tied and are studied in tandem as part of a medical curriculum.

Genetics
Main article: Human geneticsHumans 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. 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 carries many genes not on the Y chromosome, which means thatrecessive diseases associated with X-linked genes, such as haemophilia, affect men more often than women.

Life cycle
A 10 mm human embryo at 5 weeksThe human life cycle is similar to that of other placental mammals. The zygote divides inside the female's uterus to become an embryo, which over a period of thirty-eight 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 twenty-four hours or more are not uncommon and sometimes leads to the death of the mother, or the child.[64] This is because of both the relatively large fetal head circumference (for housing the brain) and the mother's relatively narrow pelvis (a trait required for successful bipedalism, by way of natural selection).[65] [66] 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 more common than in developed countries.[67]

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.<sup class="reference" id="cite_ref-67" style="line-height: 1em; font-weight: normal; font-style: normal; ">[68] However, low birth weight is common in developing countries, and contributes to the high levels of infant mortality in these regions.<sup class="reference" id="cite_ref-68" style="line-height: 1em; font-weight: normal; font-style: normal; ">[69] 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.<sup class="reference" id="cite_ref-69" style="line-height: 1em; font-weight: normal; font-style: normal; ">[70] 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.<sup class="reference" id="cite_ref-70" style="line-height: 1em; font-weight: normal; font-style: normal; ">[71] <sup class="reference" id="cite_ref-71" style="line-height: 1em; font-weight: normal; font-style: normal; ">[72]

There are significant differences in life expectancy around the world. 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. Life expectancy at birth in Hong Kong is 84.8 years for a female and 78.9 for a male, while in Swaziland, primarily because of AIDS, it is 31.3 years for both sexes.<sup class="reference" id="cite_ref-72" style="line-height: 1em; font-weight: normal; font-style: normal; ">[73] While one in five Europeans is 60 years of age or older, only one in twenty Africans is 60 years of age or older.<sup class="reference" id="cite_ref-73" style="line-height: 1em; font-weight: normal; font-style: normal; ">[74] 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.<sup class="reference" id="cite_ref-74" style="line-height: 1em; font-weight: normal; font-style: normal; ">[75] At least one person, Jeanne Calment, is known to have reached the age of 122 years; higher ages have been claimed but they are not well substantiated. Worldwide, there are 81 men aged 60 or older for every 100 women of that age group, and among the oldest, there are 53 men for every 100 women.External links
 * Wikipedia article about Humans