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Language, involving syntax and grammar, is one notably unique characteristics of humans. Other animals species that sometimes are said to have a "language"-such as the "language of bees"-merely have systems of giving and receiving signals; they lack a system of communication with syntax and grammar, which is required to properly be a language (Mayr 2001, p. 253). Thus, chimpanzees, despite decades of attempts to teach them language, cannot talk about the future or the past; they seem to lack the ability to adopt syntax (Mayr 2001).

Other animals have intelligence and think, including highly developed intelligence in various mammals and birds (corvids, parrots, and so on) (Mayr 2001). But human intelligence is greater by orders of magnitude. Humans have self-awareness, can reason abstractly, are capable of introspection, and appreciate beauty and aesthetics. They desire to understand the world, including both past, present, and future, and even study other animals and themselves. They have developed complex systems of governance and law, established sciences, and express feelings through art, music, and literature. They have developed complex technologies.

Human beings, unlike any other animals, transfer a great deal of cultural information, utilizing language in the process. Many animals, such as most invertebrates, do not even have any relationship with their parents, which die before their are hatched, and thus the parents do not transmit information to their offspring. Humans, on the other hand, form monogamous pair bonds and have extensive parental care, raising their young in families where both parents educate the youngsters.

However, even in species with highly developed parental care, such as in certain mammals and birds, the amount of information that is handed down from generation to generation (nongenetic information transfer) is quite limited (Mayr 2001, 253). For humans, there is a great deal of information that is transferred. Unlike chimpanzee young, which become independent of their mothers within the first year of life, human young require many years to reach maturity, during which the parents transmit language, culture, and skills that make up the greater part of human personhood. Information is even transferred by the use of symbols, and in written languages in books.

Humans beings also practice altruism, not only for the benefit of an individual's own offspring, or the close relatives, or members of the same social group, but even towards outsiders and competitors or enemies. In chimpanzees, there is a practice of maiming or killing of former alpha males after they have been supplanted by a new leader. Human males, on the other hand, typically protect the children of other families of their tribe, and former male leaders are respected as honored elders. Respect for elderly males, codified in human morality as filial piety, is another means by which humans propagate and transmit culture.

Many religious hold that the most essential characteristic that makes humans unique is an immaterial essence: A soul, spirit, atman, qi, or so forth. It is this inner aspect that is considered to separate humans from animals. For example, there is a concept that humans have not only a physical body with physical senses, but also an immaterial or spiritual body with spiritual senses. This spiritual body is considered to mirror the appearance of the physical body, but also exists after the death of the material form. An example of such is found in the Bible: "It is sown a physical body, but it is raised a spiritual body. If there is a physical body, there is also a spiritual body" (1 Corinthians 15:44).

Thus, although there are close anatomical similarities between humans and other primates, particularly chimpanzees, the gap between humans and apes in terms of culture, mental capacity, and various spiritual, emotional, and technological aspects is so large as to dwarf differences between apes and other animals. In this sense, philosophers have recognized humans as distinct from animals generally.

The name Homo sapiens is Latin for "wise human" or "knowing human," emphasizing the importance of intelligence in separating humans and other animals. Mayr (2001) states that "it has long been appreciated that it is our brain that makes us human. Any other part of our anatomy can be matched or surpassed by a corresponding structure in some other animal." However, many theologians and philosophers would emphasize the inner aspects of humans as the most distinctive factor, or emphasize the essence of humans in the ability to love.

Biology

Genetics and physiology

See also: Human body

An old diagram of a male human skeleton.

Humans are an 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 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.

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 average height of an adult human is about 5 to 6 feet (1.5 to 1.8 meters) tall, although this varies significantly from place to place (de Beer 2004). Humans are capable of fully bipedal locomotion, thus leaving their arms available for manipulating objects using their hands, aided especially by opposable thumbs.

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 (Wade 2003).

Skin color, hair color, and "races"

An Inuit woman, circa 1907.

The hue of human hair and skin is determined by the presence of pigments called melanins. Human skin hues can range from very dark brown to very pale pink, while human hair ranges from blond to brown to red to, most commonly, black (Rogers et al. 2004).

The differences in skin color between various people is due to one type of cell, the melanocyte. The number of melanocytes in human skin is believed to be the same for all people. However, the amount of pigment, or melanin, within the melanocytes is different. People with black skin have the most pigment and people with white skin have the least amount of pigment (Astner and Anderson 2004).

Many researchers believe that skin darkening was an adaptation that evolved as a protection against ultraviolet solar radiation, as melanin is an effective sun-block (Jablonski and Chaplin 2000). 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 (Harding et al. 2000; Robins 1991).

Historically, efforts have been made to designate various human populations as distinct "races" based on skin color, along with such other observable physical traits as hair type, facial features, and body proportions. However, today many scientists from diverse fields, such as genetics, physical anthropology, sociology, and biology, believe that the concept of distinct human races is unscientific and that there are no distinct races as previously claimed (O'Campo 2005; Keita et al. 2004). The concept of "race" is a valid taxonomic concept in other species. However, in humans only a small proportion of the genetic variability of humans occurs between so-called races, there is much greater variability among members of a race than between members of different races, and racial traits overlap without discrete boundaries-making genetic differences among groups biologically meaningless (O'Campo 2005; Schwartz and Vissing 2002; Smedley and Smedley 2005; Lewontin 1972). In addition, so-called races are freely interbreeding. On the other hand, other geneticists argue that categories of self-identified race/ethnicity or biogeographic ancestry are both valid and useful (Risch et al. 2002; Bamshad 2005), and that arguments against delineating races could also be made regarding making distinctions based on age or sex (Risch et al. 2002).

Rather than delineating races, there is a current tendency to identify ethnic groups, with members defined by shared geographical origin or cultural history, such as common language and religion (O'Campo 2005), and there is a tendency to recognize a graded serious of differences (a cline) along geographical or environmental ranges.

The recognition of different races, along with preferences toward particular groups, or exploitation or domination of other groups, is sometimes identified with the term racism. From a biological point of view, in which species are recognized as actually or potentially interbreeding natural populations, one might define someone as a "racist" on the basis of whether the person is willing to marry, and to have their children marry, someone of any other "race." From a biblical point of view, all people are descended from one common pair of ancestors (O'Campo 2005).

From the point of view of some religions, the essential part of humans is the soul, which counters a fixation on physiology and observable physical characteristics alone (O'Campo 2005).

Life cycle

Human embryo at 5 weeks.

The human life cycle is similar to that of other placental mammals. New humans develop viviparously (live birth) from fertilization of an egg by a sperm (conception). An egg is usually fertilized inside the female by sperm from the male through sexual intercourse, though the recent technology of in vitro fertilization is occasionally used.

The fertilized egg, called a zygote, divides inside the female's uterus to become an embryo that is implanted on the uterine wall. The fetal stage of prenatal development (fetus) begins about seven or eight weeks after fertilization, when the major structures and organ systems have formed, until birth. After about nine months of gestation, the fully-grown fetus is expelled from the female's body and breathes independently as a "neonate" or 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 personhood to human fetuses while they remain in the uterus.

Two young human girls.

Compared with that of other species, human childbirth can be dangerous. Painful labors lasting twenty-four hours or more are not uncommon, and may result in injury, or even death, to the child and/or mother. 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 (LaVelle 1995; Correia et al. 2005). 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 relatively hazardous ordeals in developing regions of the world, with maternal death rates approximately 100 times more common than in developed countries (Rush 2000).

In developed countries, infants are typically 3-4 kilograms (6-9 pounds) in weight and 50-60 centimeters (20-24 inches) in height at birth. However, low birth weight is common in developing countries, and contributes to the high levels of infant mortality in these regions (Khor 2003).

Helpless at birth, humans continue to grow for some years, typically reaching sexual maturity at 12 to 15 years of age. Human girls continue to grow physically until around the age of 18, and human boys 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, are not fixed, and particularly the later stages.

There are striking differences in life expectancy around the world, ranging from as high as over 80 years to less than 40 years.

The number of centenarians (humans of age 100 years or older) in the world was estimated at nearly half a million 2015 (Stepler 2016). 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.

The philosophical questions of when human personhood begins and whether it persists after death are the subject of considerable debate. The prospect of death causes unease or fear for most humans. Burial ceremonies are characteristic of human societies, often accompanied by beliefs in an afterlife or immortality.

Diet

Early Homo sapiens employed a "hunter-gatherer" method as their primary means of food collection, involving combining stationary plant and fungal food sources (such as fruits, grains, tubers, and mushrooms) with wild game, which must be hunted and killed in order to be consumed. It is believed that humans have used fire to prepare and cook food prior to eating since possibly the time of Homo erectus.

Humans are omnivorous, capable of consuming both plant and animal products. The view of humans as omnivores is supported by the evidence that both a pure animal and a pure vegetable diet can lead to deficiency diseases in humans. A pure animal diet can, for instance, lead to scurvy, while a pure plant diet can lead to deficiency of a number of nutrients, including Vitamin B12. Some humans have chosen to abstain from eating some or all meat for religious, ethical, ecological, or health reasons. Supplementation, particularly for vitamin B12, is highly recommended for people living on a pure plant diet.

The human diet is prominently reflected in human culture, and has led to the development of food science.

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, but longer periods are known, including fasting for religious purposes.

Lack of food remains a serious global problem, with about 300,000 people starving to death every year. Childhood malnutrition is also common and contributes to the global burden of disease (Murray and Lopez 1997). However global food distribution is not even, and obesity among some human populations has increased to almost epidemic proportions, leading to health complications and increased mortality in some developed, and a few developing countries. Obesity is caused by consuming more calories than are expended, with many attributing excessive weight gain to a combination of overeating and insufficient exercise.

At least ten thousand years ago, humans developed agriculture (see rise of civilization below), which has substantially altered the kind of food people eat. This has 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.

History

Origin of Homo sapiens sapiens (modern humans)

For more details on this topic, see Human evolution.

The scientific study of human evolution concerns the emergence of humans as a distinct species. It encompasses the development of the genus Homo, as well as studying extinct human ancestors, such as the australopithecines, and even chimpanzees (genus Pan), which are usually classified together with genus Homo in the tribe Hominini. "Modern humans" are defined as the Homo sapiens species, of which the only extant subspecies is Homo sapiens sapiens.

There is substantial evidence for a primate origin of humans (Mayr 2001):

  1. Anatomical evidence: Human beings exhibit close anatomical similarities with the African apes, and particularly the chimpanzee. Compared to apes, the few unique physical characteristics of humans are the proportion of arms and legs, opposable thumbs, body hair, skin pigmentation, and size of the central nervous system, such as the forebrain.
  2. Fossil evidence: Numerous fossils have been found sharing human and primate characteristics.
  3. Molecular evidence: Human molecules are very similar to that of chimpanzees. In some, such as hemoglobin, they are virtually identical.

The closest living relatives of Homo sapiens are two distinct species of the genus Pan: the bonobo (Pan paniscus) and the common chimpanzee (Pan troglodytes). Through a study of proteins, comparison of DNA, and use of a molecular clock (a method of calculating evolution based on the speed at which genes mutate), scientists believe thePan/Homo split happened about 5 to 8 million years ago (Mayr 2001, Physorg 2005). (See Pan/Homo split.)

Well-known members of the Homo genus include Homo habilis (about 2.4 to 1.5 mya), Homo erectus (1.8 mya to 70,000 years ago), Homo heidelbergensis (800,000 to 300,000 years ago), and Homo neanderthalensis (250,000 to 30,000 years ago).

H. sapiens have lived from about 250,000 years ago to the present. Between 400,000 years ago and the second interglacial period in the Middle Pleistocene, around 250,000 years ago, the trend in cranial expansion and the elaboration of stone tool technologies developed, providing evidence for a transition from H. erectus to H. sapiens. Based on molecular evidence, the calculation of the time of divergence of all modern human populations from a common ancestor typically yields dates around 200,000 years (Disotell 1999).

Notably, however, about 50,000 to 40,000 years ago, human beings appeared to have taken a Great Leap Forward, when human culture apparently changed at a much greater speed. Humans started to bury their dead carefully, made clothing out of hides, developed sophisticated hunting techniques (such as pitfall traps, or driving animals to fall off cliffs), and made cave paintings. Additionally, human culture began to become more technologically advanced, in that different populations of humans begin to create novelty in existing technologies. Artifacts such as fish hooks, buttons, and bone needles begin to show signs of variation among different population of humans, something what had not been seen in human cultures prior to 50,000 BP. This "Great Leap Forward" seems connected to the arrival of modern humans beings: Homo sapiens sapiens. (See modern man and the great leap forward.)

The Cro-Magnons form the earliest known European examples of Homo sapiens sapiens. The term falls outside the usual naming conventions for early humans and is used in a general sense to describe the oldest modern people in Europe. Cro-Magnons lived from about 40,000 to 10,000 years ago in the Upper Paleolithic period of the Pleistocene epoch. For all intents and purposes these people were anatomically modern, only differing from their modern day descendants in Europe by their slightly more robust physiology and larger brain capacity than that of modern humans. When they arrived in Europe about 40,000 years ago, they brought with them sculpture, engraving, painting, body ornamentation, music, and the painstaking decoration of utilitarian objects.

Current research establishes that human beings are highly genetically homogeneous, meaning that the DNA of individual Homo sapiens is more alike than usual for most species. Geneticists Lynn Jorde and Henry Harpending of the University of Utah, noting that the variation in human DNA is minute compared to that of other species, propose that during the Late Pleistocene, the human population was reduced to a small number of breeding pairs-no more than 10,000 and possibly as few as 1,000-resulting in a very small residual gene pool. Various reasons for this hypothetical bottleneck have been postulated, one of those is the Toba catastrophe theory.

There are two major scientific challenges in deducing the pattern of human evolution. For one, the fossil record remains fragmentary. Mayr (2001) notes that no fossils of hominids have been found for the period between 6 and 13 million years ago (mya), the time when branching between the chimpanzee and human lineages is expected to have taken place. Furthermore, as Mayr notes, "most hominid fossils are extremely incomplete. They may consist of part of a mandible, or the upper part of a skull without face and teeth, or only part of the extremities." Coupled with this is a recurrent problem that interpretation of fossil evidence is heavily influenced by personal beliefs and prejudices. Fossil evidence often allows a variety of interpretations, since the individual specimens may be reconstructed in a variety of ways (Wells 2000).

There are two dominant, and one might say polarizing, general views on the issue of human origins, the Out of Africa position and the multiregional position.

The Out of Africa, or Out of Africa II, or replacement model holds that after there was a migration of Homo erectus (or H. ergaster) out of Africa and into Europe and Asia, these populations did not subsequently contribute significant amounts of genetic material (or, some say, contributed absolutely nothing) to later populations along the lineage to Homo sapiens (Kreger 2005). Later, approximately 200,000 years ago, there was a second migration of hominids out of Africa, and this was modern H. sapiens that replaced the populations that then occupied Europe and Asia (Kreger 2005). This view maintains a specific speciation event that led to H. sapiens in Africa, and this is the modern human.

The multiregional or continuity camp hold that since the origin of H. erectus, there have been populations of hominids living in the Old World and that these all contributed to successive generations in their regions (Kreger 2005). According to this view, hominids in China and Indonesia are the most direct ancestors of modern East Asians, those in Africa are the most direct ancestors of modern Africans, and the European populations either gave rise to modern Europeans or contributed significant genetic material to them, while their origins were in Africa or West Asia (Kreger 2005). There is genetic flow to allow for the maintenance of one species, but not enough to prevent racial differentiation.

There are various combinations of these ideas.

Overall, human evolution theory comprises two principal theories: Those related to the pattern of evolution and those related to the process of evolution. The theory of descent with modification addresses the pattern of evolution, and as applied to humans the theory is strongly supported by the fossil record, which provides evidence of skeletons that through time become more and more like the modern human skeleton. In contrast, the theory of natural selection, which relates to the process of evolution is intrinsically more speculative as it relates to presumed causes.

Substantial evidence has been marshaled for the fact that humans have descended from common ancestors by a process of branching (descent with modification) and for a primate origin of humans. However, proposals for the specific ancestral-descendant relationships and for the process leading to humans tend to be speculative. And, while the theory of natural selection typically is central to scientific explanations for the process, evidence for natural selection being the directive or creative force is limited to extrapolation from the microevolutionary level (changes within the level of species). Historically, a major source of controversy has been the process by which humans have developed, whether by physical forces with an exclusively random component (natural selection) or by the creative force of a Creator God. (Abrahamic religions believe that modern humans derive from an original couple Adam and Eve into whose material bodies God breathed spiritual life (added a spirit or soul) to complete the creation of a being uniquely different from animals.)

Rise of civilization

The rise of agriculture led to the foundation of stable human settlements.

Up until only around 10,000 years ago, all humans lived as hunter-gatherers (with some communities persisting until this day). They generally lived in small, nomadic groups. The advent of agriculture prompted the Neolithic Revolution. Developed independently by geographically distant populations, evidence suggests that agriculture first appeared in Southwest Asia, in the Fertile Crescent. Around 9500 B.C.E., farmers first began to select and cultivate food plants with specific characteristics. Though there is evidence of earlier use of wild cereals, it was not until after 9500 B.C.E. that the eight so-called Neolithic founder crops of agriculture appeared: first emmer wheat and einkorn wheat, then hulled barley, peas, lentils, bitter vetch, chick peas, and flax. By 7000 B.C.E., sowing and harvesting reached Mesopotamia. By 6000 B.C.E., farming was entrenched on the banks of the Nile River. About this time, agriculture was developed independently in the Far East, with rice, rather than wheat, the primary crop.

Access to food surplus led to the formation of permanent human settlements, the domestication of animals, and the use of metal tools. Agriculture also encouraged trade and cooperation, leading to complex societies. Villages developed into thriving civilizations in regions such as the Middle East's Fertile Crescent.

Around 6,000 years ago, the first proto-states developed in Mesopotamia, Egypt, and the Indus Valley. Military forces were formed for protection and government bureaucracies for administration. States cooperated and competed for resources, in some cases waging wars. Around 2,000-3,000 years ago, some states, such as Persia, China, and Rome, developed through conquest into the first expansive empires. Influential religions, such as Judaism, originating in the Middle East, and Hinduism, a religious tradition that originated in South Asia, also rose to prominence at this time.

The late Middle Ages saw the rise of revolutionary ideas and technologies. In China, an advanced and urbanized economy promoted innovations such as printing and the compass, while the Islamic Golden Age saw major scientific advancements in Muslim empires. In Europe, the rediscovery of classical learning and inventions such as the printing press led to the Renaissance in the fourteenth century. Over the next 500 years, exploration and imperialistic conquest brought much of the Americas, Asia, and Africa under European control, leading to later struggles for independence.

The Scientific Revolution in the seventeenth century and the Industrial Revolution in the eighteenth-nineteenth centuries promoted major innovations in transport, such as the railway and automobile; energy development, such as coal and electricity; and government, such as representative democracy and Communism.

As a result of such changes, modern humans live in a world that has become increasingly globalized and interconnected. Although this has encouraged the growth of science, art, and technology, it has also led to culture clashes, the development and use of weapons of mass destruction, and increased environmental destruction and pollution.

Habitat and population

Humans have structured their environment in extensive ways in order to adapt to problems such as high population density, as shown in this image of the Asian city, Hong Kong.

Early human settlements were dependent on proximity to water and, depending on the lifestyle, other natural resources, such as fertile land for growing crops and grazing livestock, or populations of prey for hunting. However, humans have a great capacity for altering their habitats by various methods, such as through irrigation, urban planning, construction, transport, and manufacturing goods. With the advent of large-scale trade and transport infrastructure, proximity to these resources has become unnecessary, and in many places these factors are no longer a driving force behind the growth and decline of a population. Nonetheless, the manner in which a habitat is altered is often a major determinant in population change.

Technology has allowed humans to colonize all of the continents and adapt to all climates. Within the last few decades, humans have explored Antarctica, the ocean depths, and space, although long-term habitation of these environments is not yet possible.

With a population of over seven billion, humans are among the most numerous of the large mammals. Most humans (61 percent) live in Asia. The vast majority of the remainder live in the Americas (14 percent), Africa (13 percent), and Europe (12 percent), with 0.5 percent in Oceania.

Human habitation within closed ecological systems in hostile environments, such as Antarctica and outer space, is expensive, typically limited in duration, and restricted to scientific, military, or industrial expeditions. Life in space has been very sporadic, with no more than thirteen humans in space at any given time. Between 1969 and 1972, two humans at a time spent brief intervals on the Moon. As of 2007, no other celestial body has been visited by human beings, although there has been a continuous human presence in outer space since the launch of the initial crew to inhabit the International Space Station on October 31, 2000; however, humans have made robots that have visited other celestial bodies.

From 1800 to 2012 C.E., the human population increased from one billion to seven billion. In 2004, around 2.5 billion out of 6.3 billion people (39.7 percent) lived in urban areas, and this percentage is expected to rise throughout the twenty-first century. Problems for humans living in cities include various forms of pollution and crime, especially in inner city and suburban slums. Benefits of urban living include increased literacy, access to the global canon of human knowledge, and decreased susceptibility to rural famines.

Humans have had a dramatic effect on the environment. The extinction of a number of species has been attributed to anthropogenic factors, such as human predation and habitat loss, and other negative impacts include pollution, widespread loss of wetlands and other ecosystems, alteration of rivers, and introduction of invasive species. On the other hand, humans in the past century have made considerable efforts to reduce negative impacts and provide greater protection for the environment and other living organisms, through such means as environmental law, environmental education, and economic incentives.

Psychology

For more details on this topic, see Brain and Mind.

The brain is a centralized mass of nerve tissue enclo

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