Humans
Humans (Homo sapiens) are the most common and widespread species of primate in the great ape family Hominidae, and also the most common species of primate overall. Humans are broadly characterized by their bipedalism and high intelligence. Humans' large brain and resulting cognitive skills have allowed them to thrive in a variety of environments and develop complex societies and civilizations. Humans are highly social and tend to live in complex social structures composed of many cooperating and competing groups, from families and kinship networks to political states. As such, social interactions between humans have established a wide variety of values, social norms, languages, and rituals, each of which bolsters human society. The desire to understand and influence phenomena has motivated humanity's development of science, technology, philosophy, mythology, religion, and other conceptual frameworks.
Humans
Genes and the environment influence human biological variation in visible characteristics, physiology, disease susceptibility, mental abilities, body size, and life span. Though humans vary in many traits (such as genetic predispositions and physical features), any two humans are at least 99% genetically similar. Humans are sexually dimorphic: generally, males have greater body strength and females have a higher body fat percentage. At puberty, humans develop secondary sexual characteristics. Females are capable of pregnancy, usually between puberty, at around 12 years old, and menopause, around the age of 50.
Humans have a large, highly developed, and complex prefrontal cortex, the region of the brain associated with higher cognition. Humans are highly intelligent, capable of episodic memory, have flexible facial expressions, self-awareness, and a theory of mind. The human mind is capable of introspection, private thought, imagination, volition, and forming views on existence. This has allowed great technological advancements and complex tool development to be possible through complex reasoning and the transmission of knowledge to subsequent generations. Language, art, and trade are defining characteristics of humans. Long-distance trade routes might have led to cultural explosions and resource distribution that gave humans an advantage over other similar species.
All modern humans are classified into the species Homo sapiens, coined by Carl Linnaeus in his 1735 work Systema Naturae.[2] The generic name "Homo" is a learned 18th-century derivation from Latin homō, which refers to humans of either sex.[3][4] The word human can refer to all members of the Homo genus,[5] although in common usage it generally just refers to Homo sapiens, the only extant species.[6] The name "Homo sapiens" means 'wise man' or 'knowledgeable man'.[7] There is disagreement if certain extinct members of the genus, namely Neanderthals, should be included as a separate species of humans or as a subspecies of H. sapiens.[5]
Despite the fact that the word animal is colloquially used as an antonym for human,[10] and contrary to a common biological misconception, humans are animals.[11] The word person is often used interchangeably with human, but philosophical debate exists as to whether personhood applies to all humans or all sentient beings, and further if one can lose personhood (such as by going into a persistent vegetative state).[12]
Human evolution was not a simple linear or branched progression but involved interbreeding between related species.[36][37][38] Genomic research has shown that hybridization between substantially diverged lineages was common in human evolution.[39] DNA evidence suggests that several genes of Neanderthal origin are present among all non sub-Saharan African populations, and Neanderthals and other hominins, such as Denisovans, may have contributed up to 6% of their genome to present-day non sub-Saharan African humans.[36][40][41]
Human evolution is characterized by a number of morphological, developmental, physiological, and behavioral changes that have taken place since the split between the last common ancestor of humans and chimpanzees. The most significant of these adaptations are obligate bipedalism, increased brain size and decreased sexual dimorphism (neoteny). The relationship between all these changes is the subject of ongoing debate.[42]
Until about 12,000 years ago, all humans lived as hunter-gatherers.[43][44] The Neolithic Revolution (the invention of agriculture) first took place in Southwest Asia and spread through large parts of the Old World over the following millennia.[45] It also occurred independently in Mesoamerica (about 6,000 years ago),[46] China,[47][48] Papua New Guinea,[49] and the Sahel and West Savanna regions of Africa.[50][51][52] Access to food surplus led to the formation of permanent human settlements, the domestication of animals and the use of metal tools for the first time in history. Agriculture and sedentary lifestyle led to the emergence of early civilizations.[53][54][55]
Humans are one of the most adaptable species, despite having a low or narrow tolerance for many of the earth's extreme environments.[122] Through advanced tools, humans have been able to extend their tolerance to a wide variety of temperatures, humidity, and altitudes.[122] As a result, humans are a cosmopolitan species found in almost all regions of the world, including tropical rainforest, arid desert, extremely cold arctic regions, and heavily polluted cities; in comparison, most other species are confined to a few geographical areas by their limited adaptability.[123] The human population is not, however, uniformly distributed on the Earth's surface, because the population density varies from one region to another, and large stretches of surface are almost completely uninhabited, like Antarctica and vast swathes of the ocean.[122][124] Most humans (61%) live in Asia; the remainder live in the Americas (14%), Africa (14%), Europe (11%), and Oceania (0.5%).[125]
Within the last century, humans have explored challenging environments such as Antarctica, the deep sea, and outer space.[126] Human habitation within these hostile environments is restrictive and expensive, typically limited in duration, and restricted to scientific, military, or industrial expeditions.[126] Humans have briefly visited the Moon and made their presence felt on other celestial bodies through human-made robotic spacecraft.[127][128][129] Since the early 20th century, there has been continuous human presence in Antarctica through research stations and, since 2000, in space through habitation on the International Space Station.[130]
In 2018, 4.2 billion humans (55%) lived in urban areas, up from 751 million in 1950.[140] The most urbanized regions are Northern America (82%), Latin America (81%), Europe (74%) and Oceania (68%), with Africa and Asia having nearly 90% of the world's 3.4 billion rural population.[140] Problems for humans living in cities include various forms of pollution and crime,[141] especially in inner city and suburban slums. Humans have had a dramatic effect on the environment. They are apex predators, being rarely preyed upon by other species.[142] Human population growth, industrialization, land development, overconsumption and combustion of fossil fuels have led to environmental destruction and pollution that significantly contributes to the ongoing mass extinction of other forms of life.[143][144]
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 about 100 trillion (1014) cells. The most commonly defined body systems in humans are the nervous, the cardiovascular, the digestive, the endocrine, the immune, the integumentary, the lymphatic, the musculoskeletal, the reproductive, the respiratory, and the urinary system.[145][146] 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 third molars, with some individuals having them congenitally absent.[147]
Humans share with chimpanzees a vestigial tail, appendix, flexible shoulder joints, grasping fingers and opposable thumbs.[148] Apart from bipedalism and brain size, humans differ from chimpanzees mostly in smelling, hearing and digesting proteins.[149] While humans have a density of hair follicles comparable to other apes, it is predominantly vellus hair, most of which is so short and wispy as to be practically invisible.[150][151] Humans have about 2 million sweat glands spread over their entire bodies, many more than chimpanzees, whose sweat glands are scarce and are mainly located on the palm of the hand and on the soles of the feet.[152]
Like most animals, humans are a diploid and eukaryotic species. Each somatic cell has two sets of 23 chromosomes, each set received from one parent; gametes have only one set of chromosomes, which is a mixture of the two parental sets. Among the 23 pairs of chromosomes, there are 22 pairs of autosomes and one pair of sex chromosomes. Like other mammals, humans have an XY sex-determination system, so that females have the sex chromosomes XX and males have XY.[163] Genes and environment influence human biological variation in visible characteristics, physiology, disease susceptibility and mental abilities. The exact influence of genes and environment on certain traits is not well understood.[164][165]
The human genome was first sequenced in 2001[174] and by 2020 hundreds of thousands of genomes had been sequenced.[175] In 2012 the International HapMap Project had compared the genomes of 1,184 individuals from 11 populations and identified 1.6 million single nucleotide polymorphisms.[176] African populations harbor the highest number of private genetic variants. While many of the common variants found in populations outside of Africa are also found on the African continent, there are still large numbers that are private to these regions, especially Oceania and the Americas.[177] By 2010 estimates, humans have approximately 22,000 genes.[178] By comparing mitochondrial DNA, which is inherited only from the mother, geneticists have concluded that the last female common ancestor whose genetic marker is found in all modern humans, the so-called mitochondrial Eve, must have lived around 90,000 to 200,000 years ago.[179][180][181][182] 041b061a72