Human

## What is a human?

The **Human** (*Homo sapiens*) is a highly intelligent primate that has become the dominant species on the Earth. They are the only extant members of the subtribe Hominina and—together with chimpanzees, gorillas, and orangutans—are part of the family Hominidae (the great apes, or *hominids*). Humans are terrestrial animals, characterized by their erect posture and bipedal locomotion; high manual dexterity and heavy tool

### Status

Least Concern

use compared to other animals; open-ended and complex language use compared to other animal communications; larger, more complex brains than other primates; and highly advanced and organized societies.[3][4]

Several early hominins used fire and occupied much of Eurasia. *Homo sapiens* (sometimes also known as "modern humans") are thought to have diverged in Africa from an earlier hominin around 300,000 years ago, with the earliest fossil evidence of *Homo sapiens* also appearing around 300,000 years ago in Africa.[5] Humans began to exhibit evidence of behavioral modernity at least by about 100,000–70,000 years ago.[6][7][8][9][10] In several waves of migration, *H. sapiens* ventured out of Africa and populated most of the world.[11][12] The spread of the large and increasing population of humans has profoundly affected the biosphere and millions of species worldwide. Advantages that explain this evolutionary success include a larger, well-developed brain, which enables advanced abstract reasoning, language, problem solving, sociality, and culture through social learning. Humans use tools more frequently and effectively than any other animal: they are the only extant species to build fires, cook food, clothe themselves, and create and use numerous other technologies and arts.

Humans uniquely use systems of symbolic communication as language and art to express themselves and exchange ideas, and also organize themselves into purposeful groups. Humans create complex social structures composed of many cooperating and competing groups, from families and kinship networks to political states. Social interactions between humans have established an extremely wide variety of values,[13] social norms, and rituals, which together undergird human society. Curiosity and the human desire to understand and influence the environment and to explain and manipulate phenomena (or events) have motivated humanity's development of science, philosophy, mythology, religion, and other fields of knowledge.

Though most of human existence has been sustained by hunting and gathering in band societies,[14] many human societies transitioned to sedentary agriculture approximately 10,000 years ago,[15] domesticating plants and animals, thus enabling the growth of civilization. These human societies subsequently expanded, establishing various forms of government, religion, and culture around the world, and unifying people within regions to form states and empires. The rapid advancement of scientific and medical understanding in the 19th and 20th centuries permitted the development of fuel-driven technologies and increased lifespans, causing the human population to rise exponentially. The global human population was estimated to be near 7.8 billion in 2019.

## History

### **Evolution and range**

The genus *Homo* evolved and diverged from other hominins in Africa several million years ago, after the human clade split from the chimpanzee lineage of the hominids (great apes) branch of the primates.[22] Modern humans, specifically the subspecies *Homo sapiens sapiens*, proceeded to colonize all the continents and larger islands, arriving in Eurasia 125,000–60,000 years ago,[23][24] Australia around 40,000 years ago, the Americas around 15,000 years ago, and remote islands such as Hawaii, Easter Island, Madagascar, and New Zealand between the years 300 and 1280.[25][26]

#### **Evidence from molecular biology**

Family tree showing the extant hominoids: humans (genus *Homo*), chimpanzees and bonobos (genus *Pan*), gorillas (genus *Gorilla*), orangutans (genus *Pongo*), and gibbons (four genera of the family Hylobatidae: *Hylobates*, *Hoolock*, *Nomascus*, and *Symphalangus*). All except gibbons are hominids.

The closest living relatives of humans are chimpanzees and bonobos (genus *Pan*)[27][28], as well as gorillas (genus *Gorilla*).[29] With the sequencing of human and chimpanzee genomes, current estimates of similarity between human and chimpanzee DNA sequences are between 95% and 99%.[29][30][31] By the molecular clock technique, which estimates the time required for the number of divergent mutations to accumulate between two lineages, the approximate date for the split between lineages can be calculated. The gibbons (family Hylobatidae) and orangutans (genus *Pongo*) were the first groups to split from the lineage leading to humans, then gorillas, and finally, chimpanzees. The splitting date between human and chimpanzee lineages is placed 4–8 million years ago, during the late Miocene epoch.[32][33] During this split, chromosome 2 was formed from the joining of two other chromosomes, leaving humans with only 23 pairs of chromosomes, compared to 24 for the other apes.[34]

#### **Evidence from the fossil record**

There is little fossil evidence for the divergence of the gorilla, chimpanzee, and hominin lineages.[35][36] The earliest fossils that have been proposed as members of the hominin lineage are *Sahelanthropus tchadensis*, dating from 7 million years ago; *Orrorin tugenensis*, dating from 5.7 million years ago; and *Ardipithecus kadabba*, dating to 5.6 million years ago. Each of these species has been claimed to be a bipedal ancestor of later hominins, but all such claims are contested. It is also possible that any one of the three is an ancestor of another branch of apes, or is an ancestor shared between hominins and other apes. The relation between these early fossil species and the hominin lineage remains unresolved. From these early species, the australopithecines arose around 4 million years ago, diverging into robust (*Paranthropus*) and gracile (*Australopithecus*) branches,[37] possibly one of which—such as *A. garhi*, dating to 2.5 million years ago—is a direct ancestor of the genus *Homo*.[38]

The earliest members of *Homo* are *Homo habilis*, which evolved around 2.8 million years ago.[39] H. *habilis* has been considered the first species for which there is clear evidence of the use of stone tools. However, in 2015, stone tools have been discovered in northwestern Kenya that have been dated to 3.3 million years old, perhaps predating *H. habilis*.[40] Nonetheless, the brains of *H. habilis* were about the same size as that of a chimpanzee, and their main adaptation was bipedalism as an adaptation to terrestrial living. During the next million years a process of encephalization began, and with the arrival of *Homo erectus* in the fossil record, cranial capacity had doubled. *H. erectus* were the first of the hominina to leave Africa, and these species spread through Africa, Asia, and Europe between 1.3 to 1.8 million years ago. One population of *H. erectus*, also sometimes classified as a separate species *Homo ergaster*, stayed in Africa and evolved into *Homo sapiens*. It is believed that these species were the first to use fire and complex tools. The earliest transitional fossils between *H. ergaster/erectus* and archaic humans are from Africa, such as *Homo rhodesiensis*, but seemingly transitional forms have also been found in Dmanisi, Georgia. These descendants of African *H. erectus* spread through Eurasia c. 500,000 years ago, evolving into *H. antecessor*, *H. heidelbergensis* and *H. neanderthalensis*. Fossils of anatomically modern humans that date from the Middle Paleolithic (about 200,000 years ago) include the Omo-Kibish I remains of Ethiopia[41][42][43] and the fossils of Herto Bouri, Ethiopia. Earlier remains now classified as early *Homo sapiens*, such as the Jebel Irhoud remains from Morocco and the Florisbad Skull from South Africa, have been dated to about 300,000 and 259,000 years old respectively.[44][5][45][46][47][48] Fossil records of archaic *Homo sapiens* from Skhul in Israel and Southern Europe begin around 90,000 years ago.[49]

#### **Anatomical adaptations**

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 1. bipedalism, 2. increased brain size, 3. lengthened ontogeny (gestation and infancy), 4. decreased sexual dimorphism (neoteny). The relationship between all these changes is the subject of ongoing debate.[50] Other significant morphological changes included the evolution of a power and precision grip, a change first occurring in *H. erectus*.[51]

Bipedalism is the basic adaption of the hominin line, and it is considered the main cause behind a suite of skeletal changes shared by all bipedal hominins. The earliest bipedal hominin is considered to be either *Sahelanthropus*[52] or *Orrorin*, with *Ardipithecus*, a full bipedal,[53] coming somewhat later.[*citation needed*] The knuckle walkers, the gorilla and chimpanzee, diverged around the same time, and either *Sahelanthropus* or *Orrorin* may be humans' last shared ancestor with those animals.[*citation needed*] The early bipedals eventually evolved into the australopithecines and later the genus *Homo*.[*citation needed*] There are several theories of the adaptational value of bipedalism. It is possible that bipedalism was favored because it freed up the hands for reaching and carrying food, because it saved energy during locomotion, because it enabled long-distance running and hunting, or as a strategy for avoiding hyperthermia by reducing the surface exposed to direct sun.[*citation needed*]

The human species developed a much larger brain than that of other primates—typically 1,330 cm3 (81 cu in) in modern humans, over twice the size of that of a chimpanzee or gorilla.[54] The pattern of encephalization started with *Homo habilis* which at approximately 600 cm3 (37 cu in) had a brain slightly larger than chimpanzees, and continued with *Homo erectus* (800–1,100 cm3 (49–67 cu in)), and reached a maximum in Neanderthals with an average size of 1,200–1,900 cm3 (73–116 cu in), larger even than *Homo sapiens* (but less encephalized).[55] The pattern of human postnatal brain growth differs from that of other apes (heterochrony), and allows for extended periods of social learning and language acquisition in juvenile humans. However, the differences between the structure of human brains and those of other apes may be even more significant than differences in size.[56][57][58][59] The increase in volume over time has affected different areas within the brain unequally—the temporal lobes, which contain centers for language processing have increased disproportionately, as has the prefrontal cortex which has been related to complex decision making and moderating social behavior.[54] Encephalization has been tied to an increasing emphasis on meat in the diet,[60][61] or with the development of cooking,[62] and it has been proposed [63] that intelligence increased as a response to an increased necessity for solving social problems as human society became more complex.

The reduced degree of sexual dimorphism is primarily visible in the reduction of the male canine tooth relative to other ape species (except gibbons). Another important physiological change related to sexuality in humans was the evolution of hidden estrus. Humans are the only ape in which the female is intermittently fertile year round, and in which no special signals of fertility are produced by the body (such as genital swelling during estrus). Nonetheless humans retain a degree of sexual dimorphism in the distribution of body hair and subcutaneous fat, and in the overall size, males being around 25% larger than females. These changes taken together have been interpreted as a result of an increased emphasis on pair bonding as a possible solution to the requirement for increased parental investment due to the prolonged infancy of offspring.[*citation needed*]

### **Rise of *Homo sapiens***

World map of early human migrations according to mitochondrial population genetics (numbers are millennia before present, the North Pole is at the center).

Main article: Homo sapiens

Further information: Archaic human admixture with modern humans, Early human migrations, Multiregional origin of modern humans, Prehistoric autopsy, and Recent African origin of modern humans

By the beginning of the Upper Paleolithic period (50,000 BP), and likely significantly earlier by 100–70,000 years ago[8][9][6][7][10][64] or possibly by about 300,000 years ago[65][66][67] behavioral modernity, including language, music and other cultural universals had developed.[68][69] As early *Homo sapiens* dispersed, it encountered varieties of archaic humans both in Africa and in Eurasia, in Eurasia notably *Homo neanderthalensis*. Since 2010, evidence for gene flow between archaic and modern humans during the period of roughly 100,000 to 30,000 years ago has been discovered. This includes modern human admixture in Neanderthals, Neanderthal admixture in all modern humans outside Africa, [70][71] Denisova hominin admixture in Melanesians[72] as well as admixture from unnamed archaic humans to some Sub-Saharan African populations.[73]

The "out of Africa" migration of *Homo sapiens* took place in at least two waves, the first around 130,000 to 100,000 years ago, the second (Southern Dispersal) around 70,000 to 50,000 years ago,[74][75][76] resulting in the colonization of Australia around 65–50,000 years ago,[77][78][79] This recent out of Africa migration derived from East African populations, which had become separated from populations migrating to Southern, Central and Western Africa at least 100,000 years earlier.[80] Modern humans subsequently spread globally, replacing archaic humans (either through competition or hybridization). They inhabited Eurasia and Oceania by 40,000 years ago, and the Americas at least 14,500 years ago.[81][82]

### **Transition to modernity**

Main articles: Neolithic Revolution and Cradle of civilization

Further information: History of the world

The rise of agriculture, and domestication of animals, led to stable human settlements.

Until about 12,000 years ago (the beginning of the Holocene), all humans lived as hunter-gatherers, generally in small nomadic groups known as band societies, often in caves.

The Neolithic Revolution (the invention of agriculture) took place beginning about 10,000 years ago, first in the Fertile Crescent, spreading through large parts of the Old World over the following millennia, and independently in Mesoamerica about 6,000 years ago. 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 (the development of urban development, complex society, social stratification and writing) from about 5,000 years ago (the Bronze Age), first beginning in Mesopotamia.[83]

Few human populations progressed to historicity, with substantial parts of the world remaining in a Neolithic, Mesolithic or Upper Paleolithic stage of development until the advent of globalisation and modernity initiated by European exploration and colonialism.

The Scientific Revolution, Technological Revolution and the Industrial Revolution brought such discoveries as imaging technology, major innovations in transport, such as the airplane and automobile; energy development, such as coal and electricity.[84] This correlates with population growth (especially in America)[85] and higher life expectancy, the World population rapidly increased numerous times in the 19th and 20th centuries as nearly 10% of the 100 billion people who ever lived lived in the past century.[86]

With the advent of the Information Age at the end of the 20th century, modern humans live in a world that has become increasingly globalized and interconnected. As of 2010, almost 2 billion humans are able to communicate with each other via the Internet,[87] and 3.3 billion by mobile phone subscriptions.[88] Although connection between humans has encouraged the growth of science, art, discussion, and technology, it has also led to culture clashes and the development and use of weapons of mass destruction.[*citation needed*] Human population growth and industrialisation has led to environmental destruction and pollution significantly contributing to the ongoing mass extinction of other forms of life called the Holocene extinction event,[89] which may be further accelerated by global warming in the future

## Habitat and Population

Early human settlements were dependent on proximity to water and—depending on the lifestyle—other natural resources used for subsistence, such as populations of animal prey for hunting and arable land for growing crops and grazing livestock. Modern humans, however, have a great capacity for altering their habitats by means of technology, irrigation, urban planning, construction, deforestation and desertification.[91] Human settlements continue to be vulnerable to natural disasters, especially those placed in hazardous locations and with low quality of construction.[92] Deliberate habitat alteration is often done with the goals of increasing comfort or material wealth, increasing the amount of available food, improving aesthetics, or improving ease of access to resources or other human settlements. 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 success of a population. Nonetheless, the manner in which a habitat is altered is often a major determinant in population change.[*citation needed*]

Technology has allowed humans to colonize six of the Earth's seven continents and adapt to virtually all climates. The human population is not, however, uniformly distributed on the Earth's surface, because the population density varies from one region to another and there are large areas almost completely uninhabited, like Antarctica.[93][94] Most humans (61%) live in Asia; the remainder live in the Americas (14%), Africa (14%), Europe (11%), and Oceania (0.5%).[95]

Within the last century, humans have explored challenging environments such as Antarctica, the deep sea, and outer space. Human habitation within these hostile environments 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.[96] Between 1969 and 1972, two humans at a time spent brief intervals on the Moon. As of September 2020, no other celestial body has been visited by humans, although there has been a continuous human presence in space since the initial manning of the International Space Station on 31 October 2000.[97] However, other celestial bodies have been visited by human-made spacecraft.[98][99][100]

Since 1800, the human population has increased from one billion[101] to over seven billion.[102] The combined biomass of the carbon of all the humans on Earth in 2018 was estimated at 60 million tons, about 10 times larger than that of all non-domesticated mammals.[103]

In 2004, some 2.5 billion out of 6.3 billion people (39.7%) lived in urban areas. In February 2008, the U.N. estimated that half the world's population would live in urban areas by the end of the year.[104] Problems for humans living in cities include various forms of pollution and crime,[105] especially in inner city and suburban slums. Both overall population numbers and the proportion residing in cities are expected to increase significantly in the coming decades.[106]

Humans have had a dramatic effect on the environment. They are apex predators, being rarely preyed upon by other species.[107] Currently, through land development, combustion of fossil fuels, and pollution, humans are thought to be the main contributor to global climate change.[108] If this continues at its current rate, it is predicted that climate change will wipe out half of all plant and animal species over the next century