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Simplified scheme of human evolution

Human evolution, or anthropogenesis, is the origin and evolution of Homo sapiens as a distinct species from other hominids, great apes and placental mammals. The study of human evolution encompasses many scientific disciplines, including physical anthropology, primatology, archaeology, linguistics and genetics.[1]

The term "human" in the context of human evolution refers to the genus Homo, but studies of human evolution usually include other hominids, such as the Australopithecines. The genus Homo had diverged from the Australopithecines by about 2.3 to 2.4 million years ago in Africa.[2][3] Scientists have estimated that humans branched off from their common ancestor with chimpanzees - the only other living hominins - about 5–7 million years ago. Several species of Homo evolved and are now extinct. These include Homo erectus, which inhabited Asia, and Homo neanderthalensis, which inhabited Europe. Archaic Homo sapiens evolved between 400,000 and 250,000 years ago.

The dominant view among scientists concerning the origin of anatomically modern humans is the "Out of Africa" or recent African origin hypothesis,[4][5][6][7] which argues that H. sapiens arose in Africa and migrated out of the continent around 50-100,000 years ago, replacing populations of H. erectus in Asia and H. neanderthalensis in Europe. Scientists supporting the alternative multiregional hypothesis argue that H. sapiens evolved as geographically separate but interbreeding populations stemming from a worldwide migration of H. erectus out of Africa nearly 2.5 million years ago.

Contents

History of ideas about human evolution

The word homo, the name of the biological genus to which humans belong, is Latin for "human". It was chosen originally by Carolus Linnaeus in his classification system. The word "human" is from the Latin humanus, the adjectival form of homo. The Latin "homo" derives from the Indo-European root, dhghem, or "earth".[8]

Carolus Linnaeus and other scientists of his time also considered the great apes to be the closest relatives of human beings due to morphological and anatomical similarities. The possibility of linking humans with earlier apes by descent only became clear after 1859 with the publication of Charles Darwin's On the Origin of Species. This argued for the idea of the evolution of new species from earlier ones. Darwin's book did not address the question of human evolution, saying only that "Light will be thrown on the origin of man and his history".

The first debates about the nature of human evolution arose between Thomas Huxley and Richard Owen. Huxley argued for human evolution from apes by illustrating many of the similarities and differences between humans and apes and did so particularly in his 1863 book Evidence as to Man's Place in Nature. However, many of Darwin's early supporters (such as Alfred Russel Wallace and Charles Lyell) did not agree that the origin of the mental capacities and the moral sensibilities of humans could be explained by natural selection. Darwin applied the theory of evolution and sexual selection to humans when he published The Descent of Man in 1871.[9]

A major problem was the lack of fossil intermediaries. It was only in the 1920s that such fossils were discovered in Africa. In 1925, Raymond Dart described Australopithecus africanus. The type specimen was the Taung Child, an Australopithecine infant discovered in a cave. The child's remains were a remarkably well-preserved tiny skull and an endocranial cast of the individual's brain. Although the brain was small (410 cm³), its shape was rounded, unlike that of chimpanzees and gorillas, and more like a modern human brain. Also, the specimen showed short canine teeth, and the position of the foramen magnum was evidence of bipedal locomotion. All of these traits convinced Dart that the Taung baby was a bipedal human ancestor, a transitional form between apes and humans.

The classification of humans and their relatives has changed considerably over time. The gracile Australopithecines are now thought to be ancestors of the genus Homo, the group to which modern humans belong. Both Australopithecines and Homo sapiens are part of the tribe Hominini. Recent data suggests Australopithecines were a diverse group and that A. africanus may not be a direct ancestor of modern humans. Reclassification of Australopithecines that originally were split into either gracile or robust varieties has put the latter into a family of its own, Paranthropus. Taxonomists place humans, Australopithecines and related species in the same family as other great apes, in the Hominidae.

Hominin species distributed through time edit

Note: 1e+06 years = 1 × 106 years = 1 million years ago = 1 Ma

Before Homo

Evolution of apes

Plesiadapis

The evolutionary history of the primates can be traced back 65 million years, as one of the oldest of all surviving placental mammal groups. The oldest known primate-like mammal species, the Plesiadapis, come from North America, but they were widespread in Eurasia and Africa during the tropical conditions of the Paleocene and Eocene.

Notharctus

With the beginning of modern climates, marked by the formation of the first Antarctic ice in the early Oligocene around 30 million years ago. A primate from this time was Notharctus. Fossil evidence found in Germany in the 1980s was determined to be about 16.5 million years old, some 1.5 million years older than similar species from East Africa and challenging the original theory regarding human ancestry originating on the African continent.

David Begun[10] says that these primates flourished in Eurasia and that the lineage leading to the African apes and humans— including Dryopithecus—migrated south from Europe or Western Asia into Africa. The surviving tropical population, which is seen most completely in the upper Eocene and lowermost Oligocene fossil beds of the Fayum depression southwest of Cairo, gave rise to all living primates—lemurs of Madagascar, lorises of Southeast Asia, galagos or "bush babies" of Africa, and the anthropoids; platyrrhines or New World monkeys, and catarrhines or Old World monkeys and the great apes and humans.

The earliest known catarrhine is Kamoyapithecus from uppermost Oligocene at Eragaleit in the northern Kenya Rift Valley, dated to 24 million years ago. Its ancestry is generally thought to be species related to Aegyptopithecus, Propliopithecus, and Parapithecus from the Fayum, at around 35 million years ago. There are no fossils from the intervening 11 million years.

Reconstructed tailless Proconsul skeleton

In the early Miocene, after 22 million years ago, the many kinds of arboreally-adapted primitive catarrhines from East Africa suggest a long history of prior diversification. Fossils at 20 million years ago include fragments attributed to Victoriapithecus, the earliest Old World Monkey. Among the genera thought to be in the ape lineage leading up to 13 million years ago are Proconsul, Rangwapithecus, Dendropithecus, Limnopithecus, Nacholapithecus, Equatorius, Nyanzapithecus, Afropithecus, Heliopithecus, and Kenyapithecus, all from East Africa. The presence of other generalized non-cercopithecids of middle Miocene age from sites far distant—Otavipithecus from cave deposits in Namibia, and Pierolapithecus and Dryopithecus from France, Spain and Austria—is evidence of a wide diversity of forms across Africa and the Mediterranean basin during the relatively warm and equable climatic regimes of the early and middle Miocene. The youngest of the Miocene hominoids, Oreopithecus, is from 9 million year old coal beds in Italy.

Molecular evidence indicates that the lineage of gibbons (family Hylobatidae) became distinct from Great Apes between 18 and 12 million years ago, and that of orangutans (subfamily Ponginae) became distinct from the other Great Apes at about 12 million years; there are no fossils that clearly document the ancestry of gibbons, which may have originated in a so-far-unknown South East Asian hominoid population, but fossil proto-orangutans may be represented by Ramapithecus from India and Griphopithecus from Turkey, dated to around 10 million years ago.

Divergence of the human lineage from other Great Apes

Species close to the last common ancestor of gorillas, chimpanzees and humans may be represented by Nakalipithecus fossils found in Kenya and Ouranopithecus found in Greece. Molecular evidence suggests that between 8 and 4 million years ago, first the gorillas, and then the chimpanzees (genus Pan) split off from the line leading to the humans; human DNA is approximately 98.4% identical to that of chimpanzees when comparing single nucleotide polymorphisms (see Human evolutionary genetics). The fossil record of gorillas and chimpanzees is quite limited. Both poor preservation (rain forest soils tend to be acidic and dissolve bone) and sampling bias probably contribute to this problem.

Other hominines likely adapted to the drier environments outside the equatorial belt, along with antelopes, hyenas, dogs, pigs, elephants, and horses. The equatorial belt contracted after about 8 million years ago. Fossils of these hominans - the species in the human lineage following divergence from the chimpanzees - are relatively well known. The earliest are Sahelanthropus tchadensis (7 Ma) and Orrorin tugenensis (6 Ma), followed by:

Genus Homo

Homo sapiens is the only non-extinct species of its genus, Homo. There were other Homo species, all of which are now extinct. While some of these other species might have been ancestors of H. sapiens, many were likely our "cousins", having speciated away from our ancestral line.[11] There is not yet a consensus as to which of these groups should count as separate species and which as subspecies. In some cases this is due to the paucity of fossils, in other cases it is due to the slight differences used to classify species in the Homo genus. The Sahara pump theory (describing an occasionally passable "wet" Sahara Desert) provides an explanation of the early variation in the genus Homo.

Based on archaeological and paleontological evidence, it has been possible to infer the ancient dietary practices of various Homo species and to study the role of diet in physical and behavioral evolution within Homo.[12][13][14][15][16]

Habilis

H. habilis lived from about 2.4 to 1.4 Ma. H. habilis, the first species of the genus Homo, evolved in South and East Africa in the late Pliocene or early Pleistocene, 2.5–2 Ma, when it diverged from the Australopithecines. H. habilis had smaller molars and larger brains than the Australopithecines, and made tools from stone and perhaps animal bones. One of the first known hominids, it was nicknamed 'handy man' by its discoverer, Louis Leakey due to its association with stone tools. Some scientists have proposed moving this species out of Homo and into Australopithecus due to the morphology of its skeleton being more adapted to living on trees rather than to moving on two legs like H. sapiens.[17]

Rudolfensis and Georgicus

These are proposed species names for fossils from about 1.9–1.6 Ma, the relation of which with H. habilis is not yet clear.

  • H. rudolfensis refers to a single, incomplete skull from Kenya. Scientists have suggested that this was another H. habilis, but this has not been confirmed.[18]
  • H. georgicus, from Georgia, may be an intermediate form between H. habilis and H. erectus,[19] or a sub-species of H. erectus.[20]

Ergaster and Erectus

One current view of the temporal and geographical distribution of hominid populations.[21] Other interpretations differ mainly in the taxonomy and geographical distribution of hominid species.

The first fossils of Homo erectus were discovered by Dutch physician Eugene Dubois in 1891 on the Indonesian island of Java. He originally gave the material the name Pithecanthropus erectus based on its morphology that he considered to be intermediate between that of humans and apes.[22] H. erectus lived from about 1.8 Ma to about 70,000 years ago (which would indicate that they were probably wiped out by the Toba catastrophe; however, Homo erectus soloensis and Homo floresiensis survived it). Often the early phase, from 1.8 to 1.25 Ma, is considered to be a separate species, H. ergaster, or it is seen as a subspecies of H. erectus, Homo erectus ergaster.

In the early Pleistocene, 1.5–1 Ma, in Africa, Asia, and Europe, some populations of Homo habilis are thought to have evolved larger brains and made more elaborate stone tools; these differences and others are sufficient for anthropologists to classify them as a new species, H. erectus. In addition H. erectus was the first human ancestor to walk truly upright.[23] This was made possible by the evolution of locking knees and a different location of the foramen magnum (the hole in the skull where the spine enters). They may have used fire to cook their meat.

A famous example of Homo erectus is Peking Man; others were found in Asia (notably in Indonesia), Africa, and Europe. Many paleoanthropologists now use the term H. ergaster for the non-Asian forms of this group, and reserve H. erectus only for those fossils that are found in Asia and meet certain skeletal and dental requirements which differ slightly from H. ergaster.

Cepranensis and Antecessor

These are proposed as species that may be intermediate between H. erectus and H. heidelbergensis.

Heidelbergensis

H. heidelbergensis (Heidelberg Man) lived from about 800,000 to about 300,000 years ago. Also proposed as Homo sapiens heidelbergensis or Homo sapiens paleohungaricus.[27]

Rhodesiensis, and the Gawis cranium

  • H. rhodesiensis, estimated to be 300,000–125,000 years old. Most current experts believe Rhodesian Man to be within the group of Homo heidelbergensis though other designations such as Archaic Homo sapiens and Homo sapiens rhodesiensis have also been proposed.
  • In February 2006 a fossil, the Gawis cranium, was found which might possibly be a species intermediate between H. erectus and H. sapiens or one of many evolutionary dead ends. The skull from Gawis, Ethiopia, is believed to be 500,000–250,000 years old. Only summary details are known, and no peer reviewed studies have been released by the finding team. Gawis man's facial features suggest its being either an intermediate species or an example of a "Bodo man" female.[28]

Neanderthalensis

H. neanderthalensis lived from 400,000[29] years ago. Also proposed as Homo sapiens neanderthalensis: there is ongoing debate over whether the Neanderthal Man was a separate species, Homo neanderthalensis, or a subspecies of H. sapiens.[30] While the debate remains unsettled, evidence from sequencing mitochondrial DNA indicates that no significant gene flow occurred between H. neanderthalensis and H. sapiens, and, therefore, the two were separate species that shared a common ancestor about 660,000 years ago.[31][32] In 1997, Mark Stoneking stated: "These results [based on mitochondrial DNA extracted from Neanderthal bone] indicate that Neanderthals did not contribute mitochondrial DNA to modern humans… Neanderthals are not our ancestors." Subsequent investigation of a second source of Neanderthal DNA supported these findings.[33] However, supporters of the multiregional hypothesis point to recent studies indicating non-African nuclear DNA heritage dating to one Ma,[34] although the reliability of these studies has been questioned.[35] Competition from Homo sapiens probably contributed to Neanderthal extinction.[36][37]

Sapiens

H. sapiens (sapiens is Latin for wise or intelligent) has lived from about 250,000 years ago to the present. Between 400,000 years ago and the second interglacial period in the Middle Pleistocene, around 250,000 years ago, the trend in skull expansion and the elaboration of stone tool technologies developed, providing evidence for a transition from H. erectus to H. sapiens. The direct evidence suggests there was a migration of H. erectus out of Africa, then a further speciation of H. sapiens from H. erectus in Africa. A subsequent migration within and out of Africa eventually replaced the earlier dispersed H. erectus. This migration and origin theory is usually referred to as the recent single origin or Out of Africa theory. Current evidence does not preclude some multiregional evolution or some admixture of the migrant H. sapiens with existing Homo populations. This is a hotly debated area of paleoanthropology.

Current research has established that human beings are genetically highly homogenous; that is, the DNA of individuals is more alike than usual for most species, which may have resulted from their relatively recent evolution or the possibility of a population bottleneck resulting from cataclysmic natural events such as the Toba catastrophe.[38][39][40] Distinctive genetic characteristics have arisen, however, primarily as the result of small groups of people moving into new environmental circumstances. These adapted traits are a very small component of the Homo sapiens genome, but include various characteristics such as skin color and nose form, in addition to internal characteristics such as the ability to breathe more efficiently in high altitudes.

H. sapiens idaltu, from Ethiopia, is a possible extinct sub-species who lived from about 160,000 years ago.

Floresiensis

H. floresiensis, which lived from approximately 100,000 to 12,000 before present, has been nicknamed hobbit for its small size, possibly a result of insular dwarfism.[41] H. floresiensis is intriguing both for its size and its age, being a concrete example of a recent species of the genus Homo that exhibits derived traits not shared with modern humans. In other words, H. floresiensis share a common ancestor with modern humans, but split from the modern human lineage and followed a distinct evolutionary path. The main find was a skeleton believed to be a woman of about 30 years of age. Found in 2003 it has been dated to approximately 18,000 years old. The living woman was estimated to be one meter in height, with a brain volume of just 380 cm3 (considered small for a chimpanzee and less than a third of the H. sapiens average of 1400 cm3).

However, there is an ongoing debate over whether H. floresiensis is indeed a separate species.[42] Some scientists presently believe that H. floresiensis was a modern H. sapiens suffering from pathological dwarfism.[43] This hypothesis is supported in part, because some modern humans who live on Flores, the island where the skeleton was found, are pygmies. This coupled with pathological dwarfism could indeed create a hobbit-like human. The other major attack on H. floresiensis is that it was found with tools only associated with H. sapiens.[43]

Comparative table of Homo species

Comparative table of Homo species
Species Lived when (Ma) Lived where Adult height Adult mass Brain volume (cm³) Fossil record Discovery / publication of name
H. rudolfensis 1.9 Kenya 1 skull 1972/1986
H. ergaster 1.9 – 1.4 Eastern and Southern Africa 1.9 m (6.2 ft) 700–850 Many 1975
H. habilis 1.7? – 1.4 Africa 1.0–1.5 m (3.3–4.9 ft) 33–55 kg (73–120 lb) 660 Many 1960/1964
H. georgicus 1.8 Georgia 600 4 individuals 1999/2002
H. erectus 1.5 – 0.2 Africa, Eurasia (Java, China, India, Caucasus) 1.8 m (5.9 ft) 60 kg (130 lb) 850 (early) – 1,100 (late) Many 1891/1892
H. antecessor 1.2 – 0.8 Spain 1.75 m (5.7 ft) 90 kg (200 lb) 1,000 2 sites 1997
H. cepranensis 0.9 – 0.8? Italy 1,000 1 skull cap 1994/2003
H. heidelbergensis 0.6 – 0.35 Europe, Africa, China 1.8 m (5.9 ft) 60 kg (130 lb) 1,100–1,400 Many 1908
H. neanderthalensis 0.35 – 0.03 Europe, Western Asia 1.6 m (5.2 ft) 55–70 kg (120–150 lb) (heavily built) 1,200–1,900 Many (1829)/1864
H. rhodesiensis 0.3 – 0.12 Zambia 1,300 Very few 1921
H. sapiens sapiens 0.2 – present Worldwide 1.4–1.9 m (4.6–6.2 ft) 50–100 kg (110–220 lb) 1,000–1,850 Still living —/1758
H. sapiens idaltu 0.16 – 0.15 Ethiopia 1,450 3 craniums 1997/2003
H. floresiensis 0.10? – 0.012 Indonesia 1.0 m (3.3 ft) 25 kg (55 lb) 400 7 individuals 2003/2004

Use of tools

Using tools has been interpreted as a sign of intelligence, and it has been theorized that tool use may have stimulated certain aspects of human evolution—most notably the continued expansion of the human brain. Paleontology has yet to explain the expansion of this organ over millions of years despite being extremely demanding in terms of energy consumption. The brain of a modern human consumes about 20 watts (400 kilocalories per day), which is one fifth of the energy consumption of a human body. Increased tool use would allow hunting for energy-rich meat products, and would enable processing more energy-rich plant products. Researchers have suggested that early hominids were thus under evolutionary pressure to increase their capacity to create and use tools.[44]

Precisely when early humans started to use tools is difficult to determine, because the more primitive these tools are (for example, sharp-edged stones) the more difficult it is to decide whether they are natural objects or human artifacts. There is some evidence that the australopithecines (4 Ma) may have used broken bones as tools, but this is debated.

It should be noted that many species make and use tools, but it is the human species that dominates the areas of making and using more complex tools. A good question is, what species made and used the first tools? The oldest known tools are the "Oldowan stone tools" from Ethiopia. It was discovered that these tools are from 2.5 to 2.6 million years old, which predates the earliest known "Homo" species. There is no known evidence that any "Homo" specimens appeared by 2.5 Ma. A Homo fossil was found near some Oldowan tools, and its age was noted at 2.3 million years old, suggesting that maybe the Homo species did indeed create and use these tools. It is surely possible, but not solid evidence. Bernard Wood noted that "Paranthropus" coexisted with the early Homo species in the area of the "Oldowan Industrial Complex" over roughly the same span of time. Although there is no direct evidence that points to Paranthropus as the tool makers, their anatomy lends to indirect evidence of their capabilities in this area. Most paleoanthropologists agree that the early "Homo" species were indeed responsible for most of the Oldowan tools found. They argue that when most of the Oldowan tools were found in association with human fossils, Homo was always present, but Paranthropus was not.[45]

In 1994, Randall Susman used the anatomy of opposable thumbs as the basis for his argument that both the Homo and Paranthropus species were toolmakers. He compared bones and muscles of human and chimpanzee thumbs, finding that humans have 3 muscles that chimps lack. Humans also have thicker metacarpals with broader heads, making the human hand more successful at precision grasping than the chimpanzee hand. Susman defended that modern anatomy of the human thumb is an evolutionary response to the requirements associated with making and handling tools and that both species were indeed toolmakers.[45]

Stone tools

Stone tools are first attested around 2.6 Ma, when H. habilis in Eastern Africa used so-called pebble tools, choppers made out of round pebbles that had been split by simple strikes.[46] This marks the beginning of the Paleolithic, or Old Stone Age; its end is taken to be the end of the last Ice Age, around 10,000 years ago. The Paleolithic is subdivided into the Lower Paleolithic (Early Stone Age, ending around 350,000–300,000 years ago), the Middle Paleolithic (Middle Stone Age, until 50,000–30,000 years ago), and the Upper Paleolithic.

The period from 700,000–300,000 years ago is also known as the Acheulean, when H. ergaster (or erectus) made large stone hand-axes out of flint and quartzite, at first quite rough (Early Acheulian), later "retouched" by additional, more subtle strikes at the sides of the flakes. After 350,000 BP (Before Present) the more refined so-called Levallois technique was developed. It consisted of a series of consecutive strikes, by which scrapers, slicers ("racloirs"), needles, and flattened needles were made.[46] Finally, after about 50,000 BP, ever more refined and specialized flint tools were made by the Neanderthals and the immigrant Cro-Magnons (knives, blades, skimmers). In this period they also started to make tools out of bone.

Modern humans and the "Great Leap Forward" debate

Until about 50,000–40,000 years ago the use of stone tools seems to have progressed stepwise. Each phase (H. habilis, H. ergaster, H. neanderthalensis) started at a higher level than the previous one, but once that phase started further development was slow. These Homo species were culturally conservative, but after 50,000 BP modern human culture started to change at a much greater speed. Jared Diamond, author of The Third Chimpanzee, and some anthropologists characterize this as a "Great Leap Forward."

Modern humans started burying their dead, making clothing out of hides, developing sophisticated hunting techniques (such as using trapping pits or driving animals off cliffs), and engaging in cave painting.[47] As human culture advanced, different populations of humans introduced novelty to existing technologies: artifacts such as fish hooks, buttons and bone needles show signs of variation among different populations of humans, something that had not been seen in human cultures prior to 50,000 BP. Typically, H. neanderthalensis populations do not vary in their technologies.

Among concrete examples of Modern human behavior, anthropologists include specialization of tools, use of jewelery and images (such as cave drawings), organization of living space, rituals (for example, burials with grave gifts), specialized hunting techniques, exploration of less hospitable geographical areas, and barter trade networks. Debate continues as to whether a "revolution" led to modern humans ("the big bang of human consciousness"), or whether the evolution was more gradual.[48]

Models of human evolution

Today, all humans belong to one, undivided by species barrier, population of Homo sapiens sapiens. However, according to the "Out of Africa" model this is not the first species of hominids: the first species of genus Homo, Homo habilis, evolved in East Africa at least 2 Ma, and members of this species populated different parts of Africa in a relatively short time. Homo erectus evolved more than 1.8 Ma, and by 1.5 Ma had spread throughout the Old World.

Anthropologists have been divided as to whether current human population evolved as one interconnected population (as postulated by the Multiregional Evolution hypothesis), or evolved only in East Africa, speciated, and then migrating out of Africa and replaced human populations in Eurasia (called the "Out of Africa" Model or the "Complete Replacement" Model).

Multiregional model

Multiregional evolution, a model to account for the pattern of human evolution, was proposed by Milford H. Wolpoff[49] in 1988.[50] Multiregional evolution holds that human evolution from the beginning of the Pleistocene 2.5 million years BP to the present day has been within a single, continuous human species, evolving worldwide to modern Homo sapiens.

According to the multiregional hypothesis, fossil and genomic data are evidence for worldwide human evolution and contradict the recent speciation postulated by the Recent African origin hypothesis. The fossil evidence was insufficient for Richard Leakey to resolve this debate.[51] Studies of haplogroups in Y-chromosomal DNA and mitochondrial DNA have largely supported a recent African origin.[52] Evidence from autosomal DNA also supports the Recent African origin. However the presence of archaic admixture in modern humans remains a possibility and has been suggested by some studies.[53]

Out of Africa

According to the Out of Africa model, developed by Chris Stringer and Peter Andrews, modern H. sapiens evolved in Africa 200,000 years ago. Homo sapiens began migrating from Africa between 70,000 – 50,000 years ago and eventually replaced existing hominid species in Europe and Asia.[54][55] Out of Africa has gained support from research using mitochondrial DNA (mtDNA). After analysing genealogy trees constructed using 133 types of mtDNA, researchers concluded that all were descended from a woman from Africa, dubbed Mitochondrial Eve. Out of Africa is also supported by the fact that mitochondrial genetic diversity is highest among African populations.[56]

There are differing theories on whether there was a single exodus or several. A multiple dispersal model involves the Southern Dispersal theory,[57] which has gained support in recent years from genetic, linguistic and archaeological evidence. In this theory, there was a coastal dispersal of modern humans from the Horn of Africa around 70,000 years ago. This group helped to populate Southeast Asia and Oceania, explaining the discovery of early human sites in these areas much earlier than those in the Levant. A second wave of humans dispersed across the Sinai peninsula into Asia, resulting in the bulk of human population for Eurasia. This second group possessed a more sophisticated tool technology and was less dependent on coastal food sources than the original group. Much of the evidence for the first group's expansion would have been destroyed by the rising sea levels at the end of the Holocene era.[57] The multiple dispersal model is contradicted by studies indicating that the populations of Eurasia and the populations of Southeast Asia and Oceania are all descended from the same mitochondrial DNA lineages, which support a single migration out of Africa that gave rise to all non-African populations.[58]

The broad study of African genetic diversity headed by Dr.Sarah Tishkoff found the San people to express the greatest genetic diversity among the 113 distinct populations sampled, making them one of 14 "ancestral population clusters". The research also located the origin of modern human migration in south-western Africa, near the coastal border of Namibia and Angola.[59]

Contemporary human evolution

Natural selection is being observed in contemporary human populations, with recent findings demonstrating the population which is at risk of the severe debilitating disease kuru has significant over representation of an immune variant of the prion protein gene G127V versus non immune alleles. Scientists postulate one of the reasons for the rapid selection of this genetic variant is the lethality of the disease in non-immune persons.[60][61] Other reported evolutionary trends in other populations include a lengthening of the reproductive period, reduction in cholesterol levels, blood glucose and blood pressure.[62]

Genetics

Human evolutionary genetics studies how one human genome differs from the other, the evolutionary past that gave rise to it, and its current effects. Differences between genomes have anthropological, medical and forensic implications and applications. Genetic data can provide important insight into human evolution.

Notable human evolution researchers

  • Robert Broom, a Scottish physician and palaeontologist whose work on South Africa led to the discovery and description of the Paranthropus genus of hominins, and of "Mrs. Ples"
  • James Burnett, Lord Monboddo, a British judge most famous today as a founder of modern comparative historical linguistics
  • Raymond Dart, an Australian anatomist and palaeoanthropologist, whose work at Taung, in South Africa, led to the discovery of Australopithecus africanus
  • Charles Darwin, a British naturalist who documented considerable evidence that species originate through evolutionary change
  • Richard Dawkins, a British ethologist, evolutionary biologist who has promoted a gene-centered view of evolution
  • J. B. S. Haldane, a British geneticist and evolutionary biologist
  • William D. Hamilton, a British Evolutionary Biologist who expounded a rigorous genetic basis for kin selection, and on the evolution of HIV and other human diseases.
  • Sir Alister Hardy, a British zoologist, who first hypothesised the aquatic ape theory of human evolution
  • Henry McHenry, an American anthropologist who specializes in studies of human evolution, the origins of bipedality, and paleoanthropology
  • Louis Leakey, an African archaeologist and naturalist whose work was important in establishing human evolutionary development in Africa
  • Mary Leakey, a British archaeologist and anthropologist whose discoveries in Africa include the Laetoli footprints
  • Richard Leakey, an African paleontologist and archaeologist, son of Louis and Mary Leakey
  • Svante Pääbo, a Swedish biologist specializing in evolutionary genetics
  • Jeffrey H. Schwartz, an American physical anthropologist and professor of biological anthropology
  • Chris Stringer, anthropologist, leading proponent of the recent single origin hypothesis
  • Alan Templeton, geneticist and statistician, proponent of the multiregional hypothesis
  • Philip V. Tobias, a South African palaeoanthropologist is one of the world's leading authorities on the evolution of humankind
  • Erik Trinkaus, a prominent American paleoanthropologist and expert on Neanderthal biology and human evolution
  • Alfred Russel Wallace, a British naturalist, sometimes called the "father of biogeography", who independently from Charles Darwin proposed the principles of evolution of animal species
  • Milford H. Wolpoff, an American paleoanthropologist who is the leading proponent of the multiregional evolution hypothesis.

Species list

This list is in chronological order across the page by genus.

See also

References

Notes

  1. ^ Heng HH (May 2009). "The genome-centric concept: resynthesis of evolutionary theory". Bioessays 31 (5): 512–25. doi:10.1002/bies.200800182. PMID 19334004. 
  2. ^ Stringer, C.B. (1994). "Evolution of early humans". in Steve Jones, Robert Martin & David Pilbeam (eds.). The Cambridge Encyclopedia of Human Evolution. Cambridge: Cambridge University Press. p. 242. ISBN 0-521-32370-3.  Also ISBN 0-521-46786-1 (paperback)
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Further reading

External links


Simple English

Human evolution is about the origin of the human species. All humans belong to the same species, which has spread from its birthplace in Africa to almost all parts of the world. Its origin in Africa is proved by the fossils which have been found there.

It was known for a long time – several centuries – that man and the apes were related. At heart, their anatomy is similar, despite many superficial differences. This was the reason why Buffon and Linnaeus, in the 18th century, put them together in one family. Charles Darwin's theory of evolution says that such basic structural similarity comes from the common origin of the group. The apes and man are close relatives, and form one branch of the primates: the order of mammals which includes monkeys and their relatives.

File:Huxley - Mans Place in
Shows similarities of apes and man. The human skeleton is on the right. Figures are drawn to scale, but the gibbon, on the left, is drawn at double size.

The great apes live in tropical rainforests; but we think that human evolution started when a group of apes began to live more in the savannah. Savannah is more open, with trees, shrubs and grass. This group started walking on two legs. They began to use their hands to carry things. Life in the open was quite different, and there was a big advantage in having better brains. Their brains grew to be much larger, and they began to make simple tools. All this began at least 5 million years ago. We have fossils of two or three different groups of walking apes, and one was the ancestor of humans.

The scientific name for "human" or "man" is Homo. The modern human species is called Homo sapiens. "Sapiens" means "thought". Homo sapiens means "the thinking man".

The science that is concerned with finding out how human evolution happened is called Physical anthropology or Paleoanthropology. It explains how the human race developed, by looking at ancient humans fossils, tools, and other signs of human life in the past. The modern field of paleoanthropology began in the 19th century with the discovery of "Neanderthal man". The skeleton that gave the name was found in 1856.

Contents

Humans are similar to great apes

By 1859, zoologists knew that humans were, in their anatomy similar to the great apes. There were also differences: humans can speak, for example. But the similarities were more basic than the differences. The idea that species were the result of evolution had been proposed before Darwin, but his book gave much evidence, and many were persuaded by it. The book was On the Origin of Species by means of Natural Selection, published in November 1859. In this book, Darwin wrote about the idea of evolution in general, rather than the evolution of humans. Light will be thrown on the origin of man and his history, was all Darwin wrote on the subject. Nevertheless, the implications of the theory were clear to the readers of the time.[1]

Different people discussed the evolution of humans. Among them were Thomas Huxley and Charles Lyell. Huxley convincingly illustrated many of the similarities and differences between humans and apes in his 1863 book Evidence as to Man's place in nature. When Darwin published his own book on the subject, The Descent of Man, and selection in relation to sex, the idea of human evolution was already well-known. The theory was controversial. Even some of Darwin's supporters (such as Alfred Russel Wallace and Charles Lyell) did not like the idea that human beings have evolved their impressive mental capacities and moral sensibilities through natural selection.

Since the 18th century, scientists thought the great apes to be closely related to human beings. In the 19th century, they speculated that the closest living relatives of humans were either chimpanzees or gorillas. Both live in central Africa in tropical rainforests, mainly of the Congo. It turns out that the chimpanzee species are closest to us.[2] Biologists believed that humans share a common ancestor with other African great apes and that fossils of these ancestors would be found in Africa, which they have been. It is now accepted by virtually all biologists that humans are not only similar to the great apes, but actually are great apes.

The issue was finally settled in modern times by studies on the sequences of proteins and genes in apes and man. These studies showed that man shares about 98/99% of these structures with chimpanzees.[3] This is a much closer relationship than with any other type of animal, and fully supports the ideas put forward in the 19th century by Darwin and Huxley.

"Currently available genetic and archaeological evidence is generally interpreted as supportive of a recent single origin of modern humans in East Africa. However, this is where the near consensus on human settlement history ends, and considerable uncertainty clouds any more detailed aspect of human colonization history".[4]

Distinguishing features

Primates have diversified in habitats such as trees and bushes. They retain many features which are adaptations to this environment.[5] Here are some of those traits:

  • Shoulder joints which allow high degrees of movement in all directions.[5]
  • Five digits on the fore and hind limbs with opposable thumbs and big toes; hands can grasp, and usually big toes as well.[5]
  • Nails on the fingers and toes (in most species).[6]
  • Sensitive tactile pads on the ends of the digits.[5]
  • Sockets of eyes encircled in bone.[7]
  • A trend towards a reduced snout and flattened face, attributed to a reliance on vision at the expense of smell.[7]
  • A complex visual system with binocular (stereoscopic) vision, high visual acuity and color vision.[5]
  • Brain with a well developed cerebellum for good balance.[7]
  • Brain large in comparison to body size, especially in simians (old world monkeys and apes).[5]
  • Enlarged cerebral cortex (brain): learning, problem solving.[5]
  • Reduced number of teeth compared to primitive mammals;.[5]
  • A well-developed cecum: vegetable digestion.[7]
  • Two pectoral mammary glands.[5]
  • Typically one young per pregnancy.[5]
  • A pendulous penis and scrotal testes.[7]
  • Long gestation and developmental period.[5] and
  • A trend towards holding the torso upright leading to bipedalism.[5]

Not all primates exhibit these anatomical traits, nor is every trait unique to primates. In regard to behavior, primates are frequently highly social, live in groups with 'flexible dominance hierarchies'.[8][9]

Other similarities

Closely related animals almost always have closely related parasites. This usually comes about because parasites evolve with their hosts, and when host populations split, their parasites split also.[10] It is also possible for parasites to get from one species to another. Two of the most serious parasitic infections of humans in Africa have originated in apes. Each may have been transferred to humans by a single cross-species event.

There are several species of mosquito, and several species of the malarial parasite Plasmodium. The most serious type, P. falciparum, which kills many millions of people each year, originated in gorillas.[11] It is now virtually certain that chimpanzees are the source of HIV-1, the major cause of AIDS.[12] This information is got by the sequence analysis of the nucleic acid of ape and human parasites.

The relevance of this to evolution is that our physiology is so close to the apes that their parasites were able to transfer to humans with great success. Humans have much less resistance to these parasites, which are ancient in origin, but comparatively new to our species.

Immediate ancestors of the genus Homo

File:Sahelanthropus tchadensis - TM
A cast of the cranium of "Tournai", Sahelanthropus tchadensis, a member of an extinct hominid species who lived about 7 million years ago.
File:A.
A reconstruction of Australopithecus afarensis.

It was not until the 1920s that hominid fossils were discovered in Africa. In 1924, Raymond Dart described Australopithecus africanus.[13] The specimen was called the Taung Child, an australopithecine infant discovered in a cave deposit being mined for concrete at Taung, South Africa. The remains were a remarkably well-preserved tiny skull and a cast of the inside of the individual's skull. Although the brain was small (410 cm³), its shape was rounded, unlike that of chimpanzees and gorillas, and more like a modern human brain. Also, the specimen exhibited short canine teeth, and the position of the foramen magnum was evidence of bipedal locomotion. All of these traits convinced Dart that the Taung baby was a bipedal human ancestor, a transitional form between apes and humans.

It took another 20 years before Dart's claims were taken seriously. This was after other similar skeletons had been found. The most common view of the time was that a large brain evolved before bipedality, the ability to move on two feet. It was thought that intelligence similar to that of modern humans was a prerequisite to bipedalism.

The australopithecines are now thought to be immediate ancestors of the genus Homo, the group to which modern humans belong.[14] Both australopithecines and Homo sapiens are part of the tribe Hominini, but recent data has brought into doubt the position of A. africanus as a direct ancestor of modern humans; it may well have been a cousin.[15] The australopithecines were originally classified as either gracile or robust. The robust variety of Australopithecus has since been reclassified as Paranthropus, although it is still regarded as a subgenus of Australopithecus by some authors.[16]

In the 1930s, when the robust specimens were first described, the Paranthropus genus was used. During the 1960s, the robust variety was moved into Australopithecus. The recent trend has been back to the original classification as a separate genus.

The genus Homo

[[File:|thumbnail|left|A chart showing different types of the genus homo, classified by where they lived, and when they lived]] It was Carolus Linnaeus who chose the name Homo. Today, there is only one species in the genus: Homo sapiens. There were other species, but they became extinct. The figure shows where some of them lived and at what time. Sone of the other species might have been ancestors of H. sapiens. Many were likely our "cousins", they developed away from our ancestral line.[17] Scientists are still discussing the exact line of development. A consensus, which of these groups should count as separate species and which as subspecies of another species has not been reached yet. In some cases this is because there are very few fossils, in other cases it is due to the slight differences used to classify species in the Homo genus.

The evolution of the genus Homo took place mostly in the Pleistocene. The whole genus is characterised by its use of stone tools, initially crude, and becoming ever more sophisticated. So much so that in archaeology and anthropology the Pleistocene is most often referred to as the Palaeolithic, or the Stone Age.[18][19]

Homo habilis

Homo habilils was likely the first species of Homo. It developed from the Australopithecus, about 2.5 million years ago. It lived until about 1.4 million years ago. It had smaller molars (back teeth) and larger brains than the Australopithecines.

Towards Homo erectus

There are two proposed species that lived from 1.9 to 1.6 million years ago. Their relation has not been clarified. One of them is called Homo rudolfensis. It is known from a single incomplete skull from Kenya. Scientists have suggested that this was just another habilis, but this has not been confirmed.[20] The other is currently called Homo georgicus. It is from Georgia and may be an intermediate form between H. habilis and H. erectus,[21] or a sub-species of H. erectus.[22]

Homo ergaster and Homo erectus

Homo erectus was first discovered on the island of Java in Indonesia, in 1891. The discoverer, Eugene Dubois originally called it Pithecanthropus erectus based on its morphology that he considered to be intermediate between that of humans and apes.[23] Homo erectus lived lived from about 1.8 million to 70,000 years ago. The earlier specimens (from 1.8 to 1.2 million years ago) are sometimes seen as a different species, or a subspecies. called Homo ergaster, or Homo erectus ergaster'.

In the Early Pleistocene, 1.5–1 mya, in Africa, Asia, and Europe, presumably, some populations of Homo habilis evolved larger brains and made more elaborate stone tools; these differences and others are sufficient for anthropologists to classify them as a new species, H. erectus. In addition H. erectus was the first human ancestor to walk truly upright.[24] This was made possible by the evolution of locking knees and a different location of the foramen magnum (the hole in the skull where the spine enters). They may have used fire to cook their meat.

A famous example of Homo erectus is Peking Man; others were found in Asia (notably in Indonesia), Africa, and Europe. Many paleoanthropologists are now using the term Homo ergaster for the non-Asian forms of this group. They reserve H. erectus only for those fossils found in the Asian region that meet certain requirements (as to skeleton and skull) which differ slightly from ergaster.

Neanderthal Man

Homo neaderthalensis (usually called Neanderthal man) lived from about 250,000 to about 30,000 years ago. Also, less usual, as Homo sapiens neanderthalensis: there is still some discussion if it was a separate species Homo neanderthalensis, or a subspecies of H. sapiens.[25] While the debate remains unsettled, evidence from mitochondrial DNA and Y-chromosomal DNA sequencing indicates that little or no gene flow occurred between H. neanderthalensis and H. sapiens, and, therefore, the two were separate species.[26] In 1997, Dr. Mark Stoneking, then an associate professor of anthropology at Pennsylvania State University, stated:

"These results [based on mitochondrial DNA extracted from Neanderthal bone] indicate that Neanderthals did not contribute mitochondrial DNA to modern humans… Neanderthals are not our ancestors."

More investigation of a second source of Neanderthal DNA supported these findings.[27]

A third species

A genetic analysis of a piece of finger bone found in Siberia has produced a surprise result. It dates to about 40,000 years ago, at a time when Neanderthals and modern man were living in the area. German researchers found its mitochondrial DNA did not match either our species or Neanderthals. The possibility is that the bone may belong to a previously unknown species. The degree of difference in the DNA suggests this species split off from our family tree about a million years ago, well before the split between our species and Neanderthals.[28]

Homo floresiensis

Homo floresiensis, which lived about 100,000–12,000 years ago has been nicknamed hobbit for its small size. Its size may be a result of island dwarfism, the tendency for large mammals to evolve smaller forms on islands.[29] H. floresiensis is intriguing both for its size and its age. It is a concrete example of a recent species of the genus Homo that shows derived traits not shared with modern humans. In other words, H. floresiensis share a common ancestor with modern humans, but split from the modern human lineage and followed a different evolutionary path. The main find was a skeleton believed to be a woman of about 30 years of age. Found in 2003 it has been dated to approximately 18,000 years old. The living woman was estimated to be one meter in height, with a brain volume of just 380 cm3 (considered small for a chimpanzee and less than a third of the H. sapiens average of 1400 cm3).

There is an ongoing debate over whether H. floresiensis is indeed a separate species.[30] Some scientists believe that H. floresiensis was a modern H. sapiens suffering from pathological dwarfism.[31] Modern humans who live on Flores, the island where the skeleton was found, are pygmies. This fact is consistent with either theory. One line of attack on H. floresiensis is that it was found with tools only associated with H. sapiens.[31]

Human arrival on Flores

Stone artefacts have now been found on Flores which can be dated to a million years ago. These artefacts are proxies; which means there were no skeletons of humans, but only a species of Homo could have made the artefacts. The artefacts are flakes and other implements, 48 in all, some of which show signs of being worked to produce a cutting edge. This means that humans were present on Flores by that date, but it does not tell us which species that was.[32]

Homo sapiens

Homo sapiens has lived from about 250,000 years ago to the present. Between 400,000 years ago and the second warm period in the Middle Pleistocene, around 250,000 years ago, its skull grew and more sophisticated technologies based on stone tools developed. One possibility is that a transition between H. erectus to H. sapiens occurred. The evidence of Java Man suggests there was an initial migration of H. erectus out of Africa. Then, much later, a further development of H. sapiens from H. erectus in Africa. Then a subsequent migration within and out of Africa eventually replaced the earlier H. erectus. Studies of human Y-chromosome DNA and mitochondrial DNA have largely supported a recent African origin.[33] Evidence from autosomal DNA also supports the recent African origin. The details of this great saga are not established yet.

Current research has established that human beings are genetically rather homogenous. The DNA of individuals is more alike than usual for most species. This may have resulted from their relatively recent evolution or the Toba catastrophe. Distinctive genetic characteristics have arisen, however, primarily as the result of small groups of people moving into new environmental circumstances. These adapted traits are a very small component of the Homo sapiens genome and include such outward "racial" characteristics as skin color and nose form in addition to internal characteristics such as the ability to breathe more efficiently in high altitudes.

H. sapiens idaltu, from Ethiopia, about 160,000 years ago, is a proposed subspecies. It is the oldest known anatomically modern human.

Species list

This list is in chronological order by genus.

References

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