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Human evolution
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Human evolution, or anthropogenesis, is the part of biological evolution concerning the emergence of Homo sapiens as a distinct species from other hominans, great apes and placental mammals. It is the subject of a broad scientific inquiry that seeks to understand and describe how this change occurred. The study of human evolution encompasses many scientific disciplines, most notably physical anthropology, primatology, linguistics and genetics.
The term "human", in the context of human evolution, refers to the genus Homo, but studies of human evolution usually include other hominins, such as the australopithecines.
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Human evolution, or anthropogenesis, is the part of biological evolution concerning the emergence of Homo sapiens as a distinct species from other hominans, great apes and placental mammals. It is the subject of a broad scientific inquiry that seeks to understand and describe how this change occurred. The study of human evolution encompasses many scientific disciplines, most notably physical anthropology, primatology, linguistics and genetics.
The term "human", in the context of human evolution, refers to the genus Homo, but studies of human evolution usually include other hominins, such as the australopithecines. The Homo genus diverged from the australopithecines about 2 million years ago in Africa. Several typological species of Homo, now extinct, evolved. 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 is the recent African origin of modern humans (RAO) that H. sapiens evolved in Africa and spread across the globe, replacing populations of H. erectus and H. neanderthalensis. Scientists supporting the alternative hypothesis on the multiregional origin of modern humans (MTO) view modern humans as having evolved as a single, widespread population from existing Homo species, particularly H. erectus. The fossil evidence is insufficient to resolve this vigorous debate, while studies of human population genetics have largely supported a recent African Origin explanation.
History of paleoanthropology
Paleoanthropology is the study of ancient humans based on fossil evidence, tools, and other signs of human habitation. The modern field of paleoanthropology began with the discovery of a Neanderthal skeleton in 1856, although there had been finds elsewhere since 1830.
By 1859, the morphological similarity of humans to certain great apes had been discussed and argued for some time, but the idea of the biological evolution of species in general was not legitimized until Charles Darwin published On the Origin of Species in November of that year. Darwin's first book on evolution did not address the specific question of human evolution: "Light will be thrown on the origin of man and his history", was all Darwin wrote on the subject. Nevertheless, the implications of evolutionary theory were clear to contemporary readers.
Debates between Thomas Huxley and Richard Owen focused on human evolution. 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. By the time Darwin published his own book on the subject, The Descent of Man, it was already a well-known interpretation of his theory, and the interpretation which made the theory highly controversial. Even many of Darwin's original supporters (such as Alfred Russel Wallace and Charles Lyell) did not like the idea that human beings could have evolved their impressive mental capacities and moral sensibilities through natural selection.
Since the time of Carolus Linnaeus, scientists have considered the great apes to be the closest relatives of human beings because they look very similar. In the 19th century, they speculated that the closest living relatives of humans are chimpanzees. Based on the morphological similarities with chimpanzees and on their natural range, in addition to other paleontological evidence, evolutionary biologists surmised that humans share a common ancestor with African great apes and that fossils of these shared ancestors would be found in Africa. Taxonomists today place human beings in the same family as the other great apes, the Hominidae.
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. This cave, in Taung, South Africa, was being mined for raw materials used to make concrete. 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.
Another 20 years would pass before Dart's claims were taken seriously, following the discovery of more fossils that resembled his find. The prevailing view of the time was that a large brain evolved before bipedality. It was thought that intelligence on par with 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. 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 dead-end cousin. 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.
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.
Before Homo
The evolutionary history of the primates can be traced back for some 85 million years, as one of the oldest of all surviving placental mammal groups. The oldest known primates come from North America, but they were widespread in Eurasia and Africa as well, during the tropical conditions of the Paleocene and Eocene.
With the beginning of modern climates, marked by the formation of the first Antarctic ice in the early Oligocene around 30 million years ago, primates went extinct everywhere but Africa and southern Asia. One such primate from this time was Notharctus. Fossil evidence found in Germany 20 years ago was determined to be about 16.5 million years old, some 1.5 million years older than similar species from East Africa. It suggests that the primate lineage of the great apes first appeared in Eurasia and not Africa.
The discoveries suggest that the early ancestors of the hominids (the family of great apes and humans) migrated to Eurasia from Africa about 17 million years ago, just before these two continents were cut off from each other by an expansion of the Mediterranean Sea. Begun says that these primates flourished in Eurasia and that their lineage leading to the African apes and humans—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 mya (millions of years before present). Its ancestry is generally thought to be close to such genera as Aegyptopithecus, Propliopithecus, and Parapithecus from the Fayum, at around 35 mya. There are no fossils from the intervening 11 million years. No near ancestor to South American platyrrhines, whose fossil record begins at around 30 mya, can be identified among the North African fossil species, and possibly lies in other forms that lived in West Africa that were caught up in the still-mysterious transatlantic sweepstakes that sent primates, rodents, boa constrictors, and cichlid fishes from Africa to South America sometime in the Oligocene.
In the early Miocene, after 22 mya, many kinds of arboreally adapted primitive catarrhines from East Africa suggest a long history of prior diversification. Because the fossils at 20 mya include fragments attributed to Victoriapithecus, the earliest cercopithecoid; the other forms are (by default) grouped as hominoids, without clear evidence as to which are closest to living apes and humans. Among the presently recognized genera in this group, which ranges up to 13 mya, we find 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 mya coal beds in Italy.
Molecular evidence indicates that the lineage of gibbons (family Hylobatidae) became distinct between 18 and 12 Mya, and that of orangutans (subfamily Ponginae) at about 12 Ma; we have no fossils that clearly document the ancestry of gibbons, which may have originated in a so far unknown South East Asian hominid population, but fossil proto-orangutans may be represented by Ramapithecus from India and Griphopithecus from Turkey, dated to around 10 Mya.
It has been suggested that species close to last common ancestors 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 mya, first the gorillas, and then the chimpanzee (genus Pan) split off from the line leading to the humans; human DNA (when comparing single nucleotide polymorphisms) is approximately 98.4 percent identical to the DNA of chimpanzees (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, however, likely adapted (along with antelopes, hyenas, dogs, pigs, elephants, and horses) to the somewhat drier environments outside the equatorial belt (which contracted after about 8 million years ago; reference needed) and their fossils are relatively well known. The earliest are Sahelanthropus tchadensis (7 mya) and Orrorin tugenensis (6 mya), followed by:
- Ardipithecus (5.5–4.4 mya), with species Ar. kadabba and Ar. ramidus;
- Australopithecus (4–2 mya), with species Au. anamensis, Au. afarensis, Au. africanus, Au. bahrelghazali, and Au. garhi;
- Kenyanthropus (3-2.7 mya), with species Kenyanthropus platyops
- Paranthropus (3–1.2 mya), with species P. aethiopicus, P. boisei, and P. robustus;
- Homo (2 mya–present), with species Homo habilis, Homo rudolfensis, Homo ergaster, Homo georgicus, Homo antecessor, Homo cepranensis, Homo erectus, Homo heidelbergensis, Homo rhodesiensis, Homo neanderthalensis, Homo sapiens idaltu, Archaic Homo sapiens, Homo floresiensis
Genus Homo
The word homo is Latin for "human", chosen originally by Carolus Linnaeus in his classification system. It is often translated "man". The word "human" is from the Latin humanus, the adjectival form of homo. (Latin for "man" in the gender-specific sense is vir, as in "virile".) The Latin "homo" derives from the Indo-European root, "dhghem," earth.
In modern taxonomy, Homo sapiens is the only extant species of its genus, Homo. Likewise, the ongoing study of the origins of Homo sapiens often demonstrates that 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. There is not yet a consensus as to which of these groups should count as separate species and which as subspecies of another species. 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 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 human (Homo) physical and behavioral evolution.
Homo habilis
H. habilis lived from about 2.4 to 1.4 million years ago (mya). 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 mya, 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 (mode 1). 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.
Homo rudolfensis and Homo georgicus
These are proposed species names for fossils from about 1.9–1.6 mya, 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 just another habilis, but this has not been confirmed.
- H. georgicus, from Georgia, may be an intermediate form between H. habilis and H. erectus, or a sub-species of H. erectus.
Homo ergaster and Homo erectus
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. H. erectus lived from about 1.8 mya to 70,000 years ago. Often the early phase, from 1.8 to 1.25 mya, 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 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. 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, and reserving H. erectus only for those fossils found in the Asian region and meeting certain skeletal and dental requirements which differ slightly from H. ergaster. However, this article does not follow that usage.
Homo cepranensis and Homo antecessor
These are proposed as species that may be intermediate between H. erectus and H. heidelbergensis.
- H. antecessor is known from fossils from Spain and England that are 1.2 mya–500,000 years old.
- H. cepranensis refers to a single skull cap from Italy, estimated to be about 800,000 years old.
Homo 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.
Homo 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.
Homo neanderthalensis
H. neanderthalensis lived from about 250,000 to as recent as 30,000 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 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. 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." Subsequent investigation of a second source of Neanderthal DNA supported these findings. However, supporters of the multiregional hypothesis point to recent studies indicating non-African nuclear DNA heritage dating to one mya, although the reliability of these studies has been questioned.
Homo sapiens
H. sapiens ("sapiens" means 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 (there is little evidence that this speciation occurred elsewhere). Then 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 single-origin theory. However, the current evidence does not preclude multiregional speciation, either. This is a hotly debated area in 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 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, 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, lived from about 160,000 years ago (proposed subspecies). It is the oldest known anatomically modern human.
Homo 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. 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. Some scientists presently believe that H. floresiensis was a modern H. sapiens suffering from pathological dwarfism. 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.
Comparative table of Homo species
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 for hunting and consuming meat, which is more energy-rich than plants. Researchers have suggested that early hominids were thus under evolutionary pressure to increase their capacity to create and use tools.
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 mya) 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 million years ago. 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.
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.
Stone tools
Stone tools are first attested around 2.6 million years ago, when H. habilis in Eastern Africa used so-called pebble tools, choppers made out of round pebbles that had been split by simple strikes. 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. 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 (habilis, ergaster, neanderthal) started at a higher level than the previous one; but once that phase started, further development was slow. In other words, these particular Homo species were culturally conservative. After 50,000 BP, however, human culture apparently started to change at a much greater speed. Jared Diamond, author of The Third Chimpanzee, and other anthropologists characterize this as a "Great Leap Forward." Modern humans started burying their dead carefully, making clothing out of hides, developing sophisticated hunting techniques (such as using trapping pits or driving animals off cliffs), and engaging in cave painting. This speed-up of cultural change seems connected with the arrival of behaviorally modern humans, Homo sapiens. 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, neanderthalensis populations are found with technology similar to other contemporary neanderthalensis populations.
Theoretically, modern human behavior is taken to include four ingredient capabilities: abstract thinking (concepts free from specific examples), planning (taking steps to achieve a further goal), innovation (finding new solutions), and symbolic behaviour (such as images and rituals). Among concrete examples of modern human behaviour, anthropologists include specialization of tools, use of jewelry 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. Nevertheless, debate continues as to whether a "revolution" led to modern humans ("the big bang of human consciousness"), or whether the evolution was more gradual.
Models of human evolution
Today, all humans are classified as belonging to the species Homo sapiens sapiens. However, this is not the first species of hominids: the first species of genus Homo, Homo habilis, evolved in East Africa at least 2 million years ago, and members of this species populated different parts of Africa in a relatively short time. Homo erectus evolved more than 1.8 million years ago, and by 1.5 million years ago had spread throughout the Old World. Virtually all physical anthropologists agree that Homo sapiens evolved out of Homo erectus. Anthropologists have been divided as to whether Homo sapiens evolved as one interconnected species from H. erectus (called the Multiregional Model, or the Regional Continuity Model), or evolved only in East Africa, and then migrated out of Africa and replaced H. erectus populations throughout the Old World (called the Out of Africa Model or the Complete Replacement Model). Anthropologists continue to debate both possibilities, but most anthropologists currently favor the Out of Africa model.
Multiregional model
Advocates of the Multiregional model, primarily Milford Wolpoff and his associates, have argued that the simultaneous evolution of H. sapiens in different parts of Europe and Asia would have been possible if there was a degree of gene flow between archaic populations. Similarities of morphological features between archaic European and Chinese populations and modern H. sapiens from the same regions, Wolpoff argues, support a regional continuity only possible within the Multiregional model. Wolpoff and others further argue that this model is consistent with clinal patterns of phenotypic variation (Wolpoff 1993).
Out of Africa model
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 would eventually replace existing hominid species in Europe and Asia. The Out of Africa Model has gained support by recent research using mitochondrial DNA (mtDNA). After analysing genealogy trees constructed using 133 types of mtDNA, they concluded that all were descended from a woman from Africa, dubbed Mitochondrial Eve.
There are differing theories on whether there was a single exodus, or several (a Multiple Dispersal Model). A Multiple Dispersal Model involves the Southern Dispersal theory, 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. The multiple dispersals models 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. The study further indicates that there was most likely only one single migration out of Africa that gave rise to all Non-African populations.
Comparison of the two models
The two models differ widely. Richard Leakey summarizes the most obvious differences as follows:
The Multiregional model suggests that the fossil record should show the same regional differences in anatomical characteristics that are currently visible today. Racial differences would be deep-rooted, going back as much as two million years. In the Out of Africa model, however, the fossil record would not be expected to show continuity of anatomical characteristics over time; on the contrary, the regional characteristics of older fossils would likely be replaced globally by more modern African ones. Racial differences would be shallow-rooted, having evolved over a relatively short period of time.
Implications for the concept of race
In a 1995 article, Leonard Lieberman and Fatimah Jackson point out that the concepts of cline, population, and ethnicity, as well as humanitarian and political concerns, have led many scientists away from the notion of race; nevertheless, a recent survey showed that physical anthropologists were evenly divided as to whether race is a valid biological concept. They note that among physical anthropologists the vast majority of opposition to the race concept comes from population geneticists, so any new support for a biological concept of race would likely come from another source, namely, the study of human evolution. They therefore ask what, if any, implications the current models of human evolution may have for any biological conception of race.
Lieberman and Jackson identified these implications of the Out of Africa model on the race concept:
Lieberman and Jackson argued that while advocates of both the Multiregional model and the Out of Africa model use the word race and make racial assumptions, none define the term. They conclude, "Each model has implications that both magnify and minimize the differences between races. Yet each model seems to take race and races as a conceptual reality. The net result is that those anthropologists who prefer to view races as a reality are encouraged to do so." However, Lieberman and Jackson conclude that students of human evolution would be better off avoiding the word race, instead describing genetic differences in terms of populations and clinal gradations.
Genetic studies
Groundbreaking work by molecular biologists such as Cann et al. (1987) on mtDNA produced three interesting observations relevant to race and human evolution.
Firstly, by estimating the rate at which mutations occur in mtDNA Cann et al. were able to estimate the age of the common ancestral mtDNA type: "the common ancestral mtDNA (type a) links mtDNA types that have diverged by an average of nearly 0.57%. Assuming a rate of 2%-4% per million years, this implies that the common ancestor of all surviving mtDNA types existed 140,000-290,000 years ago." This observation is robust, and this common direct female line ancestor (or mitochondrial most recent common ancestor (mtMRCA)) of all extant humans has become known as mitochondrial eve. The observation that the mtMRCA is the direct matrilineal ancestor of all living humans should not be interpreted as meaning that either she was the first anatomically modern human, nor that there were no other female humans living concurrently with her. A more reasonable explanation is that other women who lived at the same time as mtMRCA did indeed reproduce and pass their genes down to living humans, but that their mitochondrial lineages have been lost over time, probably due to random events such as producing only male children. It is impossible to know to what extent these non-extant lineages have been lost or how much they differed from the mtDNA of our mtMRCA. Cann et al.
Secondly, Cann et al. postulate that their work supports an African origin for modern human mtDNA: "We infer from the tree of minimum length... that Africa is a likely source of the human mitochondrial gene pool. This inference comes from the observation that one of the two primary branches leads exclusively to African mtDNAs... while the second primary branch also leads to African mtDNAs... By postulating that the common ancestral mtDNA (type a in Fig. 3) was African, we minimize the number of intercontinental migrations needed to account for the geographic distribution of mtDNA types."
Thirdly, the study shows that mtDNA types (haplogroups) do not cluster by geography, ethnicity or race, implying multiple female lineages were involved in founding modern human populations, with many closely related lineages spread geographically and many populations containing distantly related lineages: "The second implication of the tree (Fig. 3)—that each non-African population has multiple origins—can be illustrated most simply with the New Guineans. Take, as an example, mtDNA type 49, a lineage whose nearest relative is not in New Guinea, but in Asia (type 50). Asian lineage 50 is closer genealogically to this New Guinea lineage than to other Asian mtDNA lineages. Six other lineages lead exclusively to New Guinean mtDNAs, each originating at a different place in the tree (types 12, 13, 26-29, 65, 95 and 127-134 in Fig. 3). This small region of New Guinea (mainly the Eastern Highlands Province) thus seems to have been colonised by at least seven maternal lineages (Tables 2 and 3). In the same way, we calculate the minimum numbers of female lineages that colonised Australia, Asia and Europe (Tables 2 and 3). Each estimate is based on the number of region-specific clusters in the tree (Fig. 3, Tables 2 and 3). These numbers, ranging from 15 to 36 (Tables 2 and 3), will probably rise as more types of human mtDNA are discovered."
The Y chromosome is much larger than mtDNA, and is relatively homogeneous; therefore it has taken much longer to find distinct lineages and to analyse them. Conversely, because the Y chromosome is so large by comparison, it can hold a great deal more genetic information. With regard to the three observations made by Cann et al. concerning mtDNA, Y chromosome studies show similar patterns. The estimate for the age of the ancestral Y chromosome for all extant Y chromosomes is given at about 70,000 years ago and is also placed in Africa, this individual is sometimes referred to as Y chromosome Adam. The difference in dates between Y chromosome Adam and mitochondrial Eve is usually attributed to a higher extinction rate for Y chromosomes due to greater differential reproductive success between individual men, which means that a small number of very successful men may produce a great many children, while a larger number of less successful men will produce far less children. Keita et al. (2004) say, with reference to Y chromosome and mtDNA and concepts of race:
Notable human evolution researchers
- Robert Broom, a Scottish physician and palaeontologist whose work on South Africa led to the discovery and description 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
- Milford H. Wolpoff, an American paleoanthropologist who is the leading proponent of the multiregional evolution hypothesis.
Species list
This list is in chronological order by genus.
Additional notes
See also
Further reading
- Flinn, M. V., Geary, D. C., & Ward, C. V. (2005). Ecological dominance, social competition, and coalitionary arms races: Why humans evolved extraordinary intelligence. Evolution and Human Behavior, 26, 10-46.
- Rozsa L 2008. Medical Hypotheses, 70, 685-690.
—two ancestral ape chromosomes fused to give rise to human chromosome 2.
- 2007-08-08 Homo habilis and Homo erectus are sister species that overlapped in time.
External links
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