human evolution, theory of the origins of the human species, Homo sapiens. Modern understanding of human origins is derived largely from the findings of paleontology paleontology (pā'lēəntŏl`əjē) [Gr.
..... Click the link for more information. , anthropology anthropology, 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 .
..... Click the link for more information. , and genetics genome, or characteristic set of genes, that contains the total genetic information for an individual organism. In many familiar organisms two genes for each trait are present in each individual, and these paired genes, both governing the same trait, are called

alleles.
..... Click the link for more information. , and involves the process of natural selection (see Darwinism Darwinism, 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.
..... Click the link for more information. ). 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 primates primate, 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.
..... Click the link for more information. . Of all primates, humans bear particularly close affinity to other members of a group known as hominoids, or apes ape, any primate of the subfamily Hominoidea, with the possible exception of humans. The small apes, the gibbon and the siamang, and the orangutan , one of the great apes, are found in SE Asia.
..... Click the link for more information. , which includes orangutans orangutan (ōrăng`
..... Click the link for more information. , gibbons gibbon, small ape , genus Hyloblates, found in the forests of SE Asia. The gibbons, including the siamang, are known as the small, or lesser, apes; they are the most highly adapted of the apes to arboreal life.
..... Click the link for more information. , gorillas gorilla, 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.
..... Click the link for more information. , chimpanzees chimpanzee, 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.
..... Click the link for more information. , and humans. Humans and their immediate ancestors, known as hominids, are notable among hominoids for their bipedal locomotion, slow rate of maturation, large brain size, and, at least among the more recent hominids, 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 shrews tree 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.
..... Click the link for more information. . Primates of the Eocene epoch (55 to 38 million years ago) were similar and ancestral to contemporary tarsiers tarsier (tär`sēər), small, nocturnal, forest-dwelling prosimian primate , genus Tarsius.
..... Click the link for more information. , lemurs lemur (lē`mər), name for prosimians, or lower primates , of two related families, found only on Madagascar and adjacent islands.
..... Click the link for more information. , 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 drift continental 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
..... Click the link for more information. , diverged into New World (or platyrrhine) and Old World (or catarrhine) monkeys monkey, 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.
..... Click the link for more information. . 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 hominids diverged about 5 million years ago.

Hominid Evolution

The earliest known hominids are members of the genus Australopithecus Australopithecus (ôstrā'lōpĭth`əkəs, –pəthē`kəs)
..... Click the link for more information. , the earliest of which date to more than 4 million years ago. Unlike other primates, but like all hominids, 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 hominid 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 hominid to exhibit the marked expansion of the brain (with an average cranial capacity of about 750 cc) that would become a hallmark of subsequent hominid 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 erectus Homo erectus (hō`mō ērĕk`təs), extinct hominid living between 1.6 million and 250,000 years ago.
..... Click the link for more information. . 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.

Between 500,000 and 250,000 years ago, H. erectus evolved into H. sapiens. Transitional forms between H. erectus and H. sapiens are referred to as archaic H. sapiens. With the exception of H. sapiens neandertalensis (see Neanderthal man Neanderthal man (nēăn`dərthôl', –tôl') or Neandertal man
..... Click the link for more information. ), no additional subspecies are recognized. Indeed, some scientists consider Neanderthal a separate species. Archaic H. sapiens changed gradually, becoming somewhat larger, more gracile and larger-brained through time. Cranial capacity, for example, increased from about 1150 cc in early transitional forms to the current world average of just over 1350 cc. By 150,000 years ago in Africa and Asia and 28,000 years ago in Europe (see Cro-Magnon man Cro-Magnon man (krō-măg`nən, –măn`yən), an early Homo sapiens
..... Click the link for more information. ), the transition to H. sapiens was complete, and fully modern humans became the single surviving hominid species.

The Evolution of Culture

Among hominids, 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 culture culture, 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.
..... Click the link for more information. ). 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).

human evolution

Evolution of modern human beings from extinct nonhuman and humanlike forms. Genetic evidence points to an evolutionary divergence between the lineages of humans and the great apes on the African continent 8–5 million years ago (mya). The earliest fossils considered to be remains of hominins (members of the human lineage) date to at least 4 mya in Africa; they are classified as genus Australopithecus. The next major evolutionary stage, Homo habilis, inhabited sub-Saharan Africa about 2–1.5 mya. Homo habilis appears to have been supplanted by a taller and more humanlike species, Homo erectus, which lived from c. 1,700,000 to 200,000 years ago, gradually migrating into Asia and parts of Europe. Between c. 600,000 and 200,000 years ago, Homo heidelbergensis, sometimes called archaic Homo sapiens, lived in Africa, Europe, and perhaps parts of Asia. Having features resembling those of both H. erectus and modern humans, H. heidelbergensis may have been an ancestor of modern humans and also of the Neanderthals (H. neanderthalensis), who inhabited Europe and western Asia from c. 200,000 to 28,000 years ago. Fully modern humans (H. sapiens) seem to have emerged in Africa only c. 150,000 years ago, perhaps having descended directly from H. erectus or from an intermediate species such as H. heidelbergensis.