Although some scientists equate humans with all members of the genus Homo, in common usage, it generally refers to Homo sapiens, the only extant member. H. sapiens emerged around 300,000 years ago in Africa, evolving from Homo heidelbergensis and migrating out of Africa, gradually replacing local populations of archaic humans. For most of history, all humans were nomadichunter-gatherers. The Neolithic Revolution, which began in Southwest Asia around 13,000 years ago, saw the emergence of agriculture and permanent human settlement. As populations became larger and denser, forms of governance developed within and between communities and a number of civilizations have risen and fallen. Humans have continued to expand, with a global population of over 7.9 billion in December 2021.
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), two humans are over 99% genetically similar. Humans are sexually dimorphic: Generally, men have greater body strength and women have a higher body fat percentage. At puberty, humans develop secondary sex characteristics. Women are capable of pregnancy, and undergo menopause and become infertile at around the age of 50.
Humans are omnivorous, capable of consuming a wide variety of plant and animal material, and have used fire and other forms of heat to prepare and cook food since the time of H. erectus. They can survive for up to eight weeks without food, and three or four days without water. Humans are generally diurnal, sleeping on average seven to nine hours per day. Childbirth is dangerous, with a high risk of complications and death. Often, both the mother and the father provide care for their children, who are helpless at birth.
Humans have a large and highly developed prefrontal cortex, the region of the brain associated with higher cognition. They are 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 possible through reason and the transmission of knowledge to future 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.
Etymology and definition
All modern humans are classified into the speciesHomo sapiens, coined by Carl Linnaeus in his 18th-century work Systema Naturae. The generic name "Homo" is a learned 18th-century derivation from Latin hom?, which refers to humans of either sex. The word human can refer to all refer members of the Homo genus, although in common usage it generally just refers to Homo sapiens, the only extant species. The name "Homosapiens" means 'wise man' or 'knowledgeable man'. There is disagreement if extinct members of the genus, namely Neanderthals, should be included as subspecies of H. sapiens or not.
Human is a Middle Englishloanword from Old Frenchhumain, ultimately from Latinh?m?nus, the adjectival form of hom? ('man' -- in the sense of humankind). The native English term man can refer to the species generally (a synonym for humanity) as well as to human males. It may also refer to individuals of either sex, though this latter form is less common in contemporary English.
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).
Humans are apes (superfamilyHominoidea). The gibbons (family Hylobatidae) and orangutans (genus Pongo) were the first living groups to split from this lineage, then gorillas, and finally, chimpanzees (genus Pan). The splitting date between human and chimpanzee lineages is placed 8-4 million years ago, during the late Miocene epoch, with a more constrained interval 8-7 million proposed by some geneticists. 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.
The genus Homo evolved from Australopithecus. Though fossils from the transition are scarce, the earliest members of Homo share several key traits with Australopithecus. The earliest record of Homo is the 2.8 million-year-old specimen LD 350-1 from Ethiopia, and the earliest named species are Homo habilis and Homo rudolfensis which evolved by 2.3 million years ago.H. erectus (the African variant is sometimes called H. ergaster) evolved 2 million years ago and was the first archaic human species to leave Africa and disperse across Eurasia.H. erectus also was the first to evolve a characteristically human body plan. Homo sapiens emerged in Africa around 300,000 years ago from a species commonly designated as either H. heidelbergensis or H. rhodesiensis, the descendants of H. erectus that remained in Africa. H. sapiens migrated out of the continent, gradually replacing local populations of archaic humans.
Human evolution was not a simple linear or branched progression but involved interbreeding between related species. Genomic research has shown that hybridization between substantially diverged lineages was common in human evolution.DNA evidence suggests that several genes of Neanderthal origin are present among all non-African populations, and Neanderthals and other hominins, such as Denisovans, may have contributed up to 6% of their genome to present-day humans.
Humans are one of the most adaptable species, despite having a narrow tolerance to many of the earth's extreme environments. Through invention, humans have been able to extend their tolerance to a wide variety of temperatures, humidity, and altitudes. 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. Most other species are confined to a few geographical areas by their limited adaptability. 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 and the vast swathes of ocean. Most humans (61%) live in Asia; the remainder live in the Americas (14%), Africa (14%), Europe (11%), and Oceania (0.5%).
The Earth, as seen from space in 2016, showing the extent of human occupation of the planet. The bright lights signify both the most densely inhabited and/or illuminated regions.
Estimates of the population at the time agriculture emerged in around 10,000 BC have ranged between 1 million and 15 million. Around 50-60 million people lived in the combined eastern and western Roman Empire in the 4th century AD.Bubonic plagues, first recorded in the 6th century AD, reduced the population by 50%, with the Black Death killing 75-200 million people in Eurasia and North Africa alone. Human population was believed to have reached one billion in 1800. It has since then increased exponentially, reaching two billion in 1930 and three billion in 1960, four in 1975, five in 1987 and six billion in 1999. It passed seven billion in 2011 and in 2020 there were 7.8 billion humans. 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.
Basic anatomical features of female and male humans. These models have had body hair and male facial hair removed and head hair trimmed. The female model is wearing red nail polish on her toenails and a ring.
Humans share with chimpanzees a vestigial tail, appendix, flexible shoulder joints, grasping fingers and opposable thumbs. Apart from bipedalism and brain size, humans differ from chimpanzees mostly in smelling, hearing and digestingproteins. While humans have a density of hair follicles comparable to other apes, it is predominately vellus hair, most of which is so short and wispy as to be practically invisible. 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.
It is estimated that the worldwide average height for an adult human male is about 171 cm (5 ft 7 in), while the worldwide average height for adult human females is about 159 cm (5 ft 3 in). Shrinkage of stature may begin in middle age in some individuals but tends to be typical in the extremely aged. Throughout history, human populations have universally become taller, probably as a consequence of better nutrition, healthcare, and living conditions. The average mass of an adult human is 59 kg (130 lb) for females and 77 kg (170 lb) for males. Like many other conditions, body weight and body type are influenced by both genetic susceptibility and environment and varies greatly among individuals.
Humans have a far faster and more accurate throw than other animals. Humans are also among the best long-distance runners in the animal kingdom, but slower over short distances. Humans' thinner body hair and more productive sweat glands help avoid heat exhaustion while running for long distances.
A graphical representation of the standard human karyotype, including both the male (XY) and female (XX) sex chromosomes.
The human genome was first sequenced in 2001 and by 2020 hundreds of thousands of genomes had been sequenced. 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. 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. By 2010 estimates, humans have approximately 22,000 genes. 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.
Compared with other species, human childbirth is dangerous, with a much higher risk of complications and death. The size of the fetus's head is more closely matched to the pelvis than other primates. The reason for this is not completely understood,[n 3] but it contributes to a painful labor that can last 24 hours or more. 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 greater than in developed countries.
Both the mother and the father provide care for human offspring, in contrast to other primates, where parental care is mostly done by the mother.Helpless at birth, humans continue to grow for some years, typically reaching sexual maturity at 15 to 17 years of age. The human life span has been split into various stages ranging from three to twelve. Common stages include infancy, childhood, adolescence, adulthood and old age. The lengths of these stages have varied across cultures and time periods but is typified by an unusually rapid growth spurt during adolescence. Human females undergo menopause and become infertile at around the age of 50. 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 in turn their children (the grandmother hypothesis), rather than by continuing to bear children into old age.
The life span of an individual depends on two major factors, genetics and lifestyle choices. For various reasons, including biological/genetic causes, women live on average about four years longer than men. As of 2018[update], the global average life expectancy at birth of a girl is estimated to be 74.9 years compared to 70.4 for a boy. There are significant geographical variations in human life expectancy, mostly correlated with economic development--for example, life expectancy at birth in Hong Kong is 87.6 years for girls and 81.8 for boys, while in the Central African Republic, it is 55.0 years for girls and 50.6 for boys. 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. While one in five Europeans is 60 years of age or older, only one in twenty Africans is 60 years of age or older. 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.
Humans are omnivorous, capable of consuming a wide variety of plant and animal material. Human groups have adopted a range of diets from purely vegan to primarily carnivorous. In some cases, dietary restrictions in humans can lead to deficiency diseases; however, stable human groups have adapted to many dietary patterns through both genetic specialization and cultural conventions to use nutritionally balanced food sources. The human diet is prominently reflected in human culture and has led to the development of food science.
Until the development of agriculture approximately 10,000 years ago, Homo sapiens employed a hunter-gatherer method as their sole means of food collection. This involved combining stationary food sources (such as fruits, grains, tubers, and mushrooms, insect larvae and aquatic mollusks) with wild game, which must be hunted and captured in order to be consumed. It has been proposed that humans have used fire to prepare and cook food since the time of Homo erectus. Around ten thousand years ago, humans developed agriculture, which substantially altered their diet. This change in diet may also have altered human biology; with the spread of dairy farming providing a new and rich source of food, leading to the evolution of the ability to digest lactose in some adults. The types of food consumed, and how they are prepared, have varied widely by time, location, and culture.
In general, humans can survive for up to eight weeks without food, depending on stored body fat. Survival without water is usually limited to three or four days, with a maximum of one week. In 2020 it is estimated 9 million humans die every year from causes directly or indirectly related to starvation. Childhood malnutrition is also common and contributes to the global burden of disease. However, global food distribution is not even, and obesity among some human populations has increased rapidly, leading to health complications and increased mortality in some developed and a few developing countries. Worldwide, over one billion people are obese, while in the United States 35% of people are obese, leading to this being described as an "obesity epidemic." Obesity is caused by consuming more calories than are expended, so excessive weight gain is usually caused by an energy-dense diet.
Changes in the number and order of genes (A-D) create genetic diversity within and between population
There is evidence that populations have adapted genetically to various external factors. The genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication and are more dependent on cow milk.Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that have for a long time inhabited specific climates tend to have developed specific phenotypes that are beneficial for those environments--short stature and stocky build in cold regions, tall and lanky in hot regions, and with high lung capacities at high altitudes. Some populations have evolved highly unique adaptations to very specific environmental conditions, such as those advantageous to ocean-dwelling lifestyles and freediving in the Bajau.
Human hair ranges in color from red to blond to brown to black, which is the most frequent. Hair color depends on the amount of melanin, with concentrations fading with increased age, leading to grey or even white hair. Skin color can range from darkest brown to lightest peach, or even nearly white or colorless in cases of albinism. It tends to vary clinally and generally correlates with the level of ultraviolet radiation in a particular geographic area, with darker skin mostly around the equator. Skin darkening may have evolved as protection against ultraviolet solar radiation. Light skin pigmentation protects against depletion of vitamin D, which requires sunlight to make. Human skin also has a capacity to darken (tan) in response to exposure to ultraviolet radiation.
There is relatively little variation between human geographical populations, and most of the variation that occurs is at the individual level. Much of human variation is continuous, often with no clear points of demarcation. Genetic data shows that no matter how population groups are defined, two people from the same population group are almost as different from each other as two people from any two different population groups. Dark-skinned populations that are found in Africa, Australia, and South Asia are not closely related to each other.
Genetic research has demonstrated that human populations native to the African continent are the most genetically diverse and genetic diversity decreases with migratory distance from Africa, possibly the result of bottlenecks during human migration. These non-African populations acquired new genetic inputs from local admixture with archaic populations and have much greater variation from Neanderthals and Denisovans than is found in Africa, though Neanderthal admixture into African populations may be underestimated. Furthermore, recent studies have found that populations in sub-Saharan Africa, and particularly West Africa, have ancestral genetic variation which predates modern humans and has been lost in most non-African populations. Some of this ancestry is thought to originate from admixture with an unknown archaic hominin that diverged before the split of Neanderthals and modern humans.
Humans are a gonochoric species, meaning they are divided into male and female sexes. The greatest degree of genetic variation exists between males and females. While the nucleotide genetic variation of individuals of the same sex across global populations is no greater than 0.1%-0.5%, the genetic difference between males and females is between 1% and 2%. Males on average are 15% heavier and 15 cm (6 in) taller than females. On average, men have about 40-50% more upper body strength and 20-30% more lower body strength than women at the same weight, due to higher amounts of muscle and larger muscle fibers. Women generally have a higher body fat percentage than men. Women have lighter skin than men of the same population; this has been explained by a higher need for vitamin D in females during pregnancy and lactation. As there are chromosomal differences between females and males, some X and Y chromosome-related conditions and disorders only affect either men or women. After allowing for body weight and volume, the male voice is usually an octave deeper than the female voice. Women have a longer life span in almost every population around the world.
Drawing of the human brain, showing several important structures
Humans have a larger and more developed prefrontal cortex than other primates, the region of the brain associated with higher cognition. This has led humans to proclaim themselves to be more intelligent than any other known species. Objectively defining intelligence is difficult, with other animals adapting senses and excelling in areas that humans are unable to.
There are some traits that, although not strictly unique, do set humans apart from other animals. Humans may be the only animals who have episodic memory and who can engage in "mental time travel". Even compared with other social animals, humans have an unusually high degree of flexibility in their facial expressions. Humans are the only animals known to cry emotional tears. Humans are one of the few animals able to self-recognize in mirror tests and there is also debate over to what extent humans are the only animals with a theory of mind.
Sleep and dreaming
Humans are generally diurnal. The average sleep requirement is between seven and nine hours per day for an adult and nine to ten hours per day for a child; elderly people usually sleep for six to seven hours. Having less sleep than this is common among humans, even though sleep deprivation can have negative health effects. A sustained restriction of adult sleep to four hours per day has been shown to correlate with changes in physiology and mental state, including reduced memory, fatigue, aggression, and bodily discomfort.
During sleep humans dream, where they experience sensory images and sounds. Dreaming is stimulated by the pons and mostly occurs during the REM phase of sleep. The length of a dream can vary, from a few seconds up to 30 minutes. Humans have three to five dreams per night, and some may have up to seven; however most dreams are immediately or quickly forgotten. They are more likely to remember the dream if awakened during the REM phase. The events in dreams are generally outside the control of the dreamer, with the exception of lucid dreaming, where the dreamer is self-aware. Dreams can at times make a creative thought occur or give a sense of inspiration.
Consciousness and thought
Human consciousness, at its simplest, is "sentience or awareness of internal or external existence". Despite centuries of analyses, definitions, explanations and debates by philosophers and scientists, consciousness remains puzzling and controversial, being "at once the most familiar and most mysterious aspect of our lives". The only widely agreed notion about the topic is the intuition that it exists. Opinions differ about what exactly needs to be studied and explained as consciousness. Some philosophers divide consciousness into phenomenal consciousness, which is sensory experience itself, and access consciousness, which can be used for reasoning or directly controlling actions. It is sometimes synonymous with 'the mind', and at other times, an aspect of it. Historically it is associated with introspection, private thought, imagination and volition. It now often includes some kind of experience, cognition, feeling or perception. It may be 'awareness', or 'awareness of awareness', or self-awareness. There might be different levels or orders of consciousness, or different kinds of consciousness, or just one kind with different features.
The process of acquiring knowledge and understanding through thought, experience, and the senses is known as cognition. The human brain perceives the external world through the senses, and each individual human is influenced greatly by his or her experiences, leading to subjective views of existence and the passage of time. The nature of thought is central to psychology and related fields. Cognitive psychology studies cognition, the mental processes underlying behavior. Largely focusing on the development of the human mind through the life span, developmental psychology seeks to understand how people come to perceive, understand, and act within the world and how these processes change as they age. This may focus on intellectual, cognitive, neural, social, or moral development. Psychologists have developed intelligence tests and the concept of intelligence quotient in order to assess the relative intelligence of human beings and study its distribution among population.
Human motivation is not yet wholly understood. From a psychological perspective, Maslow's hierarchy of needs is a well-established theory that can be defined as the process of satisfying certain needs in ascending order of complexity. From a more general, philosophical perspective, human motivation can be defined as a commitment to, or withdrawal from, various goals requiring the application of human ability. Furthermore, incentive and preference are both factors, as are any perceived links between incentives and preferences. Volition may also be involved, in which case willpower is also a factor. Ideally, both motivation and volition ensure the selection, striving for, and realization of goals in an optimal manner, a function beginning in childhood and continuing throughout a lifetime in a process known as socialization.
Humanity's unprecedented set of intellectual skills were a key factor in the species' eventual technological advancement and concomitant domination of the biosphere. Disregarding extinct hominids, humans are the only animals known to teach generalizable information, innately deploy recursive embedding to generate and communicate complex concepts, engage in the "folk physics" required for competent tool design, or cook food in the wild. Teaching and learning preserves the cultural and ethnographic identity of human societies. Other traits and behaviors that are mostly unique to humans include starting fires,phoneme structuring and vocal learning.
The division of humans into male and female gender roles has been marked culturally by a corresponding division of norms, practices, dress, behavior, rights, duties, privileges, status, and power. Cultural differences by gender have often been believed to have arisen naturally out of a division of reproductive labor; the biological fact that women give birth led to their further cultural responsibility for nurturing and caring for children. Gender roles have varied historically, and challenges to predominant gender norms have recurred in many societies.
While many species communicate, language is unique to humans, a defining feature of humanity, and a cultural universal. Unlike the limited systems of other animals, human language is open--an infinite number of meanings can be produced by combining a limited number of symbols. Human language also has the capacity of displacement, using words to represent things and happenings that are not presently or locally occurring but reside in the shared imagination of interlocutors.
Language differs from other forms of communication in that it is modality independent; the same meanings can be conveyed through different media, audibly in speech, visually by sign language or writing, and through tactile media such as braille. Language is central to the communication between humans, and to the sense of identity that unites nations, cultures and ethnic groups. There are approximately six thousand different languages currently in use, including sign languages, and many thousands more that are extinct.
Art is a defining characteristic of humans and there is evidence for a relationship between creativity and language. The earliest evidence of art was shell engravings made by Homo erectus 300,000 years before modern humans evolved. Art attributed to H. sapiens existed at least 75,000 years ago, with jewellery and drawings found in caves in South Africa. There are various hypotheses as to why humans have adapted to the arts. These include allowing them to better problem solve issues, providing a means to control or influence other humans, encouraging cooperation and contribution within a society or increasing the chance of attracting a potential mate. The use of imagination developed through art, combined with logic may have given early humans an evolutionary advantage.
Evidence of humans engaging in musical activities predates cave art and so far music has been practiced by virtually all known human cultures. There exists a wide variety of music genres and ethnic musics; with humans' musical abilities being related to other abilities, including complex social human behaviours. It has been shown that human brains respond to music by becoming synchronized with the rhythm and beat, a process called entrainment. Dance is also a form of human expression found in all cultures and may have evolved as a way to help early humans communicate. Listening to music and observing dance stimulates the orbitofrontal cortex and other pleasure sensing areas of the brain.
Unlike speaking, reading and writing does not come naturally to humans and must be taught. Still literature has been present before the invention of words and language, with 30,000-year-old paintings on walls inside some caves portraying a series of dramatic scenes. One of the oldest surviving works of literature is the Epic of Gilgamesh, first engraved on ancient Babylonian tablets about 4,000 years ago. Beyond simply passing down knowledge the use and sharing of imaginative fiction through stories might have helped develop humans' capabilities for communication and increased the likelihood of securing a mate. Storytelling may also be used as a way to provide the audience with moral lessons and encourage cooperation.
Humans and humanity itself are often the subject of the arts. While most art focuses on individual humans or a small group, in literature, the genra of science fiction is known for tackling issues related to the humanity as a whole - for example topics such as human evolution or the future of civilization.
Stone tools were used by proto-humans at least 2.5 million years ago. The use and manufacture of tools has been put forward as the ability that defines humans more than anything else and has historically been seen as an important evolutionary step. The technology became much more sophisticated about 1.8 million years ago, with the controlled use of fire beginning around 1 million years ago. The wheel and wheeled vehicles appeared simultaneously in several regions sometime in the fourth millennium BC. The development of more complex tools and technologies allowed land to be cultivated and animals to be domesticated, thus proving essential in the development of agriculture--what is known as the Neolithic Revolution.
Although the exact level of religiosity can be hard to measure, a majority of humans professes some variety of religious or spiritual belief. In 2015 the plurality were Christian followed by Muslims, Hindus and Buddhists. As of 2015, about 16%, or slightly under 1.2 billion humans, were irreligious, including those with no religious beliefs or no identity with any religion.
Philosophy is a field of study where humans seek to understand fundamental truths about themselves and the world in which they live. Philosophical inquiry has been a major feature in the development of humans intellectual history. It has been described as the "no man's land" between the definitive scientific knowledge and the dogmatic religious teachings. Philosophy relies on reason and evidence unlike religion, but does not require the empirical observations and experiments provided by science. Major fields of philosophy include metaphysics, epistemology, logic, and axiology (which includes ethics and aesthetics).
Humans often live in family-based social structures.
Society is the system of organizations and institutions arising from interaction between humans. Humans are highly social and tend to live in large complex social groups. They can be divided into different groups according to their income, wealth, power, reputation and other factors. The structure of social stratification and the degree of social mobility differs, especially between modern and traditional societies. Human groups range from the size of families to nations. The first forms of human social organization were families living in band societies as hunter-gatherers.
All human societies organize, recognize and classify types of social relationships based on relations between parents, children and other descendants (consanguinity), and relations through marriage (affinity). There is also a third type applied to godparents or adoptive children (fictive). These culturally defined relationships are referred to as kinship. In many societies, it is one of the most important social organizing principles and plays a role in transmitting status and inheritance. All societies have rules of incest taboo, according to which marriage between certain kinds of kin relations are prohibited and some also have rules of preferential marriage with certain kin relations.
Human ethnic groups are a social category that identifies together as a group based on shared attributes that distinguish them from other groups. These can be a common set of traditions, ancestry, language, history, society, culture, nation, religion, or social treatment within their residing area. Ethnicity is separate from the concept of race, which is based on physical characteristics, although both are socially constructed. Assigning ethnicity to certain population is complicated as even within common ethnic designations there can be a diverse range of subgroups and the makeup of these ethnic groups can change over time at both the collective and individual level. Also, there is no generally accepted definition of what constitutes an ethnic group. Ethnic groupings can play a powerful role in the social identity and solidarity of ethnopolitical units. This has been closely tied to the rise of the nation state as the predominant form of political organization in the 19th and 20th centuries.
The early distribution of political power was determined by the availability of fresh water, fertile soil, and temperate climate of different locations. As farming populations gathered in larger and denser communities, interactions between these different groups increased. This led to the development of governance within and between the communities. As communities got bigger the need for some form of governance increased, as all large societies without a government have struggled to function. Humans have evolved the ability to change affiliation with various social groups relatively easily, including previously strong political alliances, if doing so is seen as providing personal advantages. This cognitive flexibility allows individual humans to change their political ideologies, with those with higher flexibility less likely to support authoritarian and nationalistic stances.
Trade, the voluntary exchange of goods and services, is seen as a characteristic that differentiates humans from other animals and has been cited as a practice that gave Homo sapiens a major advantage over other hominids. Evidence suggests early H. sapiens made use of long-distance trade routes to exchange goods and ideas, leading to cultural explosions and providing additional food sources when hunting was sparse, while such trade networks did not exist for the now extinct Neanderthals. Early trade likely involved materials for creating tools like obsidian. The first truly international trade routes were around the spice trade through the Roman and medieval periods.
Humans commit violence on other humans at a rate comparable to other primates, but at a higher rate than most other mammals. It is predicted that 2% of early H. sapiens would be murdered, rising to 12% during the medieval period, before dropping to below 2% in modern times. Unlike most animals, which generally kill infants, adult humans kill other adult humans at a very high rate.[verification needed] There is great variation in violence between human populations with rates of homicide in societies that have legal systems and strong cultural attitudes against violence at about 0.01%.
The willingness of humans to kill other members of their species en masse through organized conflict (i.e., war) has long been the subject of debate. One school of thought is that war evolved as a means to eliminate competitors, and has always been an innate human characteristic. The other suggests that war is a relatively recent phenomenon and appeared due to changing social conditions. While not settled, the current evidence suggests warlike predispositions only became common about 10,000 years ago, and in many places much more recently than that. War has had a high cost on human life; it is estimated that during the 20th century, between 167 million and 188 million people died as a result of war.
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