human evolution

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human evolution,

theory of the origins of the human species, Homo sapiens. Modern understanding of human origins is derived largely from the findings of paleontologypaleontology
[Gr.,= study of early beings], science of the life of past geologic periods based on fossil remains. Knowledge of the existence of fossils dates back at least to the ancient Greeks, who appear to have regarded them as the remains of various mythological creatures.
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, anthropologyanthropology,
classification and analysis of humans and their society, descriptively, culturally, historically, and physically. Its unique contribution to studying the bonds of human social relations has been the distinctive concept of culture.
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, and geneticsgenetics,
scientific study of the mechanism of heredity. While Gregor Mendel first presented his findings on the statistical laws governing the transmission of certain traits from generation to generation in 1856, it was not until the discovery and detailed study of the
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, and involves the process of natural selection (see DarwinismDarwinism,
concept of evolution developed in the mid-19th cent. by Charles Robert Darwin. Darwin's meticulously documented observations led him to question the then current belief in special creation of each species.
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). Although gaps in the fossil record due to differential preservation prevent the complete specification of the line of human descent, H. sapiens share clear anatomical, genetic, and historic relationships to other primatesprimate,
member of the mammalian order Primates, which includes humans, apes, monkeys, and prosimians, or lower primates. The group can be traced to the late Cretaceous period, where members were forest dwellers.
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. Of all primates, humans bear particularly close affinity to other members of a group known as hominoids, or apesape,
any primate of the superfamily Hominoidea, which includes humans; this article, however, focuses on the nonhuman apes. The small apes, the gibbon and the siamang, and the orangutan, one of the great apes, are found in SE Asia.
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, which includes orangutansorangutan
, an ape of the genus Pongo, found in swampy coastal forests of Borneo and Sumatra. Highly specialized for arboreal life, orangutans usually travel by grasping branches with hands and feet and moving from tree to tree. Adult males are about 4 1-2 ft (1.
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, gibbonsgibbon,
small ape, family Hylobatidae, found in the forests of SE Asia. The gibbons are known as the small, or lesser, apes; they are the most highly adapted of the apes to arboreal life. They are highly endangered because of habitat destruction.
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, gorillasgorilla,
an ape, Gorilla gorilla, native to the lowland and mountain forests of western and central equatorial Africa. It is the largest of the apes, the males reaching a height of 5 to 6 ft (150–190 cm) with a 9-ft (144–cm) arm spread.
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, chimpanzeeschimpanzee,
an ape, genus Pan, of the equatorial forests of central and W Africa. The common chimpanzee, Pan troglodytes, lives N of the Congo River. Full-grown animals of this species are up to 5 ft (1.
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, and humans. Humans and their immediate ancestors, known as hominins, are notable among hominoids for their bipedal locomotion, slow rate of maturation, large brain size, and, at least among the more recent hominins, the development of a relatively sophisticated capacity for language, tool use, and social activity.

The Evolutionary Tree

Humans are mammals of the Primate order. The earliest primates evolved about 65 million years ago in the geological period known as the Paleocene epoch. They were small-brained, arboreal fruit eaters, similar to modern tree shrewstree shrew,
small, arboreal mammal of the family Tupaiidae, found in S Asia. The 17 known species of tree shrews are classified as the order Tupaioidea or Scandentia. Tree shrews superficially resemble squirrels, and are commonly brown, gray, or olive in color.
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. Primates of the Eocene epoch (55 to 38 million years ago) were similar and ancestral to contemporary tarsierstarsier
, small, nocturnal, forest-dwelling prosimian primate, genus Tarsius. There are at least three species found in the Philippines, in Sumatra and Borneo, and in Sulawesi. Tarsiers are about 6 in. (15 cm) long with a 10 in. (25 cm) hairless tail, and weigh about 4.
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, lemurslemur
, name for prosimians, or lower primates, of two related families, found only on Madagascar and adjacent islands. Lemurs have monkeylike bodies and limbs, and most have bushy tails about as long as the body. They have pointed muzzles and large eyes.
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, and tree shrews, and are classified as lower primates or prosimians. During the late Eocene, the higher primates, or anthropoids, developed from prosimian ancestors and, aided by continental driftcontinental drift,
geological theory that the relative positions of the continents on the earth's surface have changed considerably through geologic time. Though first proposed by American geologist Frank Bursley Taylor in a lecture in 1908, the first detailed theory of
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, diverged into New World (or platyrrhine) and Old World (or catarrhine) monkeysmonkey,
any of a large and varied group of mammals of the primate order. The term monkey includes all primates that do not belong to the categories human, ape, or prosimian; however, monkeys do have certain common features.
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. The branching of Old World monkeys and hominoids apparently occurred in the late Oligocene (38 to 25 million years ago) or early Miocene (25 to 8 million years ago), a time period poorly represented in the fossil record. The lesser apes (gibbons and siamangs) and other hominoid lines diverged about 20 million years ago, while the Asian great apes (the orangutan being the only surviving form) diverged from the African hominoids about 15 to 10 million years ago. Genetic evidence suggests that the ancestral lines of gorillas diverged about 8 million years ago and that chimpanzees and hominins diverged about 5 million years ago.

Hominin Evolution

The earliest known hominins are members of the genus AustralopithecusAustralopithecus
, an extinct hominin genus found in Africa between about 4 and 1 million years ago. At least seven species of australopithecines are now generally recognized, including Australopithecus afarensis, A. africanus, A. bahrelghazali, A.
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, the earliest of which date to more than 4 million years ago. Unlike other primates, but like all hominins, australopithecines were bipedal. Their crania, however, were small and apelike, with an average cranial capacity of about 450 cc in the gracile species and 600 cc in the robust forms. Australopithecines that have been considered ancestral in the lineage leading to the human genus Homo include A. afarensis (an important skeleton of which is popularly known as Lucy) and A. africanus. The exact position of these and other early species on the hominin family tree continues to be disputed.

The first member of the genus Homo, a small gracile species known as H. habilis, was present in east Africa at least 2 million years ago. H. habilis was the first hominin to exhibit the marked expansion of the brain (with an average cranial capacity of about 750 cc) that would become a hallmark of subsequent hominin evolutionary history. By about 1.6 million years ago, H. habilis had evolved into a larger, more robust, and larger-brained species known as Homo erectusHomo erectus
, extinct hominin living between 1.6 million and 250,000 years ago. Homo erectus is thought to have evolved in Africa from H. habilis, the first member of the genus Homo. African forms of H.
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 (African members of the species are sometimes called H. ergaster). Cranial capacities ranged from about 900 cc in early specimens to 1050 cc in later ones. H. erectus persisted for well over a million years and migrated off the African continent into Asia, Indonesia, and Europe. H. heidelbergensis is believed to arisen from H. erectus as far back as 1.3 million years ago.

Between 400,000 and 350,000 years ago, H. heidelbergensis is believed to have given rise to H. neandertalensis, or Neanderthal manNeanderthal man
or Neandertal man
, a species of Homo, the genus to which contemporary humans belong, known as H. neandertalensis after Neanderthal (now Neandertal), Germany, the valley where the first specimen was found.
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, in Europe, and to an branch in Africa that eventually became H. sapiens. By about 150,000 years ago in Africa and Asia and 40,000 years ago in Europe (see Cro-Magnon manCro-Magnon man
, an early Homo sapiens (the species to which modern humans belong) that lived about 40,000 years ago. Skeletal remains and associated artifacts of the of the Aurignacian culture were first found in 1868 in Les Eyzies, Dordogne, France.
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), the transition to H. sapiens was complete, and fully modern humans became the single surviving hominin species. (The possible exception is the humans represented by the remains found on FloresFlores
, island, 6,627 sq mi (17,164 sq km), E Indonesia, one of the Lesser Sunda Islands. Flores is heavily wooded, rugged, and mountainous, rising to 7,872 ft (2,399 m); there are active volcanoes.
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, Indonesia, a dwarf hominin species that survived until about 50,000 years ago.)

The Evolution of Culture

Among hominins, a parallel evolutionary process involving increased intelligence and cultural complexity is apparent in the material record. Evidence of greater behavioral flexibility and adaptability presumably reflects the decreased influence of genetically encoded behaviors and the increased importance of learning and social interaction in transmitting and maintaining behavioral adaptations (see cultureculture,
in anthropology, the integrated system of socially acquired values, beliefs, and rules of conduct which delimit the range of accepted behaviors in any given society. Cultural differences distinguish societies from one another.
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). Because the organization of neural circuitry is more significant than overall cranial capacity in establishing mental capabilities, direct inferences from the fossil record are likely to be misleading. Contemporary humans, for example, exhibit considerable variability in cranial capacity (1150 cc to 1600 cc), none of which is related to intelligence.

Tool use was once thought to be the hallmark of members of the genus Homo, beginning with H. habilis, but is now known to be common among chimpanzees. The earliest stone tools of the lower Paleolithic, known as Oldowan tools and dating to about 2 to 2.5 million years ago, were once thought to have been manufactured by H. habilis. Recent finds suggest that Oldowan tools may also have been made by robust australopithecines. The simultaneous emergence of H. erectus and the more complex Achuelian tool tradition may indicate shifting adaptations as much as increased intelligence.

While it is clear that H. erectus was much more versatile than any of its predecessors, adapting its technologies and behaviors to diverse environmental conditions, the extent and limitations of its intellectual endowment remain a subject of heated debate. This is also the case for both archaic H. sapiens and Neanderthals, the latter associated with the more sophisticated technologies of the middle Paleolithic. However impressive the achievements of H. erectus and early H. sapiens, most material remains predating 40,000 years ago reflect utilitarian concerns. Nonetheless, there is now scattered African archaeological evidence from before that time (in one case as early as 90,000 years ago) of the production by H. sapiens of beads and other decorative work, perhaps indicating a gradual development of the aesthetic concerns and other symbolic thinking characteristic of later human societies. Whether the emergence of modern H. sapiens corresponds to the explosion of technological innovations and artistic activities associated with Cro-Magnon culture or was a more prolonged process of development is a subject of archaeological debate.

Bibliography

See R. Lewin, Human Evolution (2d ed. 1989) and, with R. Leakey, Origins Reconsidered (1992); I. Tattersall, The Fossil Trail: How We Know What We Think We Know about Human Evolution (1995); A. Walker and P. Shipman, The Wisdom of the Bones: In Search of Human Origins (1996); C. Stringer and R. McKie, African Exodus: The Origins of Modern Humanity (1997); L. R. Berger and B. Hilton-Barber, In the Footsteps of Eve: The Mystery of Human Origins (2000); I. Tattersall and J. H. Schwartz, Extinct Humans (2000); H. Gee, The Accidental Species: Misunderstandings of Human Evolution (2013).

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