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Evolution and range Main article: Human evolution Further information: Anthropology, Homo (genus), and Timeline of human evolution The genus Homo diverged from other hominins in Africa, after the human clade split from the chimpanzee lineage of the hominids (great ape) branch of the primates. Modern humans, defined as the species Homo sapiens or specifically to the single extant subspecies Homo sapiens sapiens, proceeded to colonize all the continents and larger islands, arriving in Eurasia 125,000–60,000 years ago,[15][16] 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.[17][18] 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 (genus Pan) and gorillas (genus Gorilla).[19] With the sequencing of both the human and chimpanzee genome, current estimates of similarity between human and chimpanzee DNA sequences range between 95% and 99%.[19][20][21] By using the technique called a molecular clock 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 (Hylobatidae) and orangutans (genus Pongo) were the first groups to split from the line leading to the humans, then gorillas (genus Gorilla) followed by the chimpanzees and bonobos (genus Pan). The splitting date between human and chimpanzee lineages is placed around 4–8 million years ago during the late Miocene epoch.[22][23][24] Evidence from the fossil record Skulls of 1. Gorilla 2. Australopithecus 3. Homo erectus 4. Neanderthal (La Chapelle aux Saints) 5. Steinheim Skull (Archaic Homo sapiens) 6. Caucasoid (H. Sapiens) There is little fossil evidence for the divergence of the gorilla, chimpanzee and hominin lineages.[25][26] The earliest fossils that have been proposed as members of the hominin lineage are Sahelanthropus tchadensis dating from 7 million years ago, and Orrorin tugenensis dating from 5.7 million years ago and Ardipithecus kadabba dating to 5.6 million years ago. Each of these has been argued to be a bipedal ancestor of later hominins, but in each case the claims have been contested. It is also possible that either of these species is an ancestor of another branch of African apes, or that they represent a shared ancestor between hominins and other Hominoidea. The question of the relation between these early fossil species and the hominin lineage is still to be resolved. From these early species the australopithecines arose around 4 million years ago diverged into robust (also called Paranthropus) and gracile branches, one of which (possibly A. garhi) went on to become ancestors of the genus Homo. The earliest members of the genus Homo are Homo habilis which evolved around 2.3 million years ago. Homo habilis is the first species for which we have positive evidence of use of stone tools. The brains of these early hominins 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. Homo 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 are also found at Dmanisi, Georgia. These descendants of African H. erectus spread through Eurasia from ca. 500,000 years ago evolving into H. antecessor, H. heidelbergensis and H. neanderthalensis. The earliest fossils of anatomically modern humans are from the Middle Paleolithic, about 200,000 years ago such as the Omo remains of Ethiopia and the fossils of Herto sometimes classified as Homo sapiens idaltu.[27] Later fossils of archaic Homo sapiens from Skhul in Israel and Southern Europe begin around 90,000 years ago.[28] Anatomical adaptations Reconstruction of Homo habilis, the first human ancestor to use stone tools 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. The relationship between all these changes is the subject of ongoing debate.[29] Other significant morphological changes included the evolution of a power and precision grip, a change first occurring in H. erectus.[30] 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[31] or Orrorin, with Ardipithecus, a full bipedal, coming somewhat later. 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. The early bipedals eventually evolved into the australopithecines and later the genus Homo. 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. The human species developed a much larger brain than that of other primates – typically 1,330 cc in modern humans, over twice the size of that of a chimpanzee or gorilla.[32] The pattern of encephalization started with Homo habilis which at approximately 600 cc had a brain slightly larger than chimpanzees, and continued with Homo erectus (800–1100 cc), and reached a maximum in Neanderthals with an average size of 1200-1900cc, larger even than Homo sapiens. 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.[33][34][35][36] 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.[32] Encephalization has been tied to an increasing emphasis on meat in the diet,[37][38] or with the development of cooking,[39] and it has been proposed 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 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. Rise of Homo sapiens Further information: Recent African origin of modern humans, Multiregional origin of modern humans, Anatomically modern humans, Archaic human admixture with modern humans, and Early human migrations The path followed by humans in the course of history By the beginning of the Upper Paleolithic period (50,000 BP), full behavioral modernity, including language, music and other cultural universals had developed.[40][41] As modern humans spread out from Africa they encountered other hominids such as Homo neanderthalensis and the so-called Denisovans. The nature of interaction between early humans and these sister species has been a long standing source of controversy, the question being whether humans replaced these earlier species or whether they were in fact similar enough to interbreed, in which case these earlier populations may have contributed genetic material to modern humans.[42] Recent studies of the human and Neanderthal genomes suggest gene flow between archaic Homo sapiens and Neanderthals and Denisovans.[43][44][45] This dispersal out of Africa is estimated to have begun about 70,000 years BP from northeast Africa. Current evidence suggests that there was only one such dispersal and that it only involved a few hundred individuals. The vast majority of humans stayed in Africa and adapted to a diverse array of environments.[46] Modern humans subsequently spread globally, replacing earlier hominins (either through competition or hybridization). They inhabited Eurasia and Oceania by 40,000 years BP, and the Americas at least 14,500 years BP.[47][48] Transition to civilization 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 c. 10,000 years ago, humans lived as hunter-gatherers. They generally lived in small nomadic groups known as band societies. The advent of agriculture prompted the Neolithic Revolution, when 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 encouraged trade and cooperation, and led to complex society. Because of the significance of this date for human society, it is the epoch of the Holocene calendar or Human Era. About 6,000 years ago, the first proto-states developed in Mesopotamia, Egypt's Nile Valley 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, India, China, Rome, and Greece, developed through conquest into the first expansive empires. Ancient Greece was the seminal civilization that laid the foundations of Western culture, being the birthplace of Western philosophy, democracy, major scientific and mathematical advances, the Olympic Games, Western literature and historiography, as well as Western drama, including both tragedy and comedy.[49] Influential religions, such as Judaism, originating in West Asia, and Hinduism, originating 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 society promoted innovations and sciences, such as printing and seed drilling. In India, major advancements were made in mathematics, philosophy, religion and metallurgy. The Islamic Golden Age saw major advancements in mathematics, astronomy, optics, biology, medicine, art and architecture in Muslim empires. In Europe, the rediscovery of classical learning and inventions such as the printing press led to the Renaissance in the 14th and 15th centuries. Over the next 500 years, exploration and colonialism brought great parts of the world under European control, leading to later struggles for independence. The Scientific Revolution in the 17th century and the Industrial Revolution in the 18th–19th 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. 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,[50] and 3.3 billion by mobile phone subscriptions.[51] Although interconnection 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. Human civilization has led to environmental destruction and pollution significantly contributing to the ongoing mass extinction of other forms of life called the holocene extinction event,[52] which may be further accelerated by global warming in the future.[53] |
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