Out of Africa I

 

From Wikipedia, the free encyclopedia
 

In paleoanthropology, Out of Africa I is the first hominin expansion into Eurasia, taking place between 1.8 and 0.8 million years ago. It is thought that Homo erectus developed a flexible adaptation to the open grounds, descending from the older Homo habilis lineage, which was strictly adapted to the dense woodlands. Such an adaptation would have allowed Homo erectus to leave Africa and expand its range into Eurasia. According to the recent African origin of modern humans hypothesis (Out of Africa II), the first hominin expansion out of Africa is followed by the dispersal into Eurasia and replacement of these previous hominins by anatomically modern humans, starting about 100,000 years ago. Without further specification, Out of Africa is usually held to mean Out of Africa II, the expansion of modern humans into Eurasia.

Movements out of Africa by early hominins seem to have occurred in at least three waves. Primitive chopper producers were first out by c. 1.8 Ma, followed by early Acheulean industries c. 1.4 Ma, and various cleaver-producing Acheulean groups around 0.8 Ma.^[1]

Until the early 1980s, hominins were assumed to have been restricted to the African continent for the whole of the Early Pleistocene (until about 0.8 Ma), and so much archaeological effort has disproportionately focused on Africa. Compounded with hominins probably being rare out of East Africa in the Early Pleistocene,^[2] we are left with a sequence of events broken in space and time.

Sites

Hominin sites are oldest in East Africa. The earliest evidence for retouched tools are from Kada Gona, Ethiopia, dating back to 2.6 – 2.5 Ma in the very early Pleistocene. They might be the product of Australopithecus garhi or Paranthropus aethiopicus, the two known hominins contemporary with the tools.^[3]

Homo habilis is the first member of the Homo line and could have descended from the Australopithecus as early as 2.3 Ma; it is first attested in Lake Turkana, Kenya. Homo erectus seems to appear a little later, its earliest remains dating to c. 1.9 – 1.6 Ma, from Koobi Fora, Kenya.^[4] The two species would have lived face to face in East Africa for nearly half a million years.^[5]

Dmanisi in Georgia, the earliest hominin site out of Africa, dates back to 1.81 Ma^[6]

Well before Homo habilis disappeared (c. 1.4 Ma), Homo erectus had made it into Eurasia. The earliest well-dated Eurasian site is that of Dmanisi in Georgia, and is securely dated to 1.81 Ma.^[6]
There, some evidence of caring for the old was found. The skull of an old Homo erectus had lost all but one teeth years before his death, and is perhaps unlikely to have survived on his own (but such caring is not yet certain – a partially paralysed chimpanzee at the Gombe reserve survived for years without help^[7]).

Early Pleistocene sites in North Africa, the geographical intermediate of East Africa and Georgia, are in poor stratigraphic context. The earliest of the dated is Ain Hanech in northern Algeria (c. 1.8^[8] – 1.2 Ma^[9]), an Oldowan grade layer. It attests that early hominins have crossed the northern African tracts, which are usually hot and dry. Hominins were part of the East African biome, and so a flux in climate could have momentarily expanded their environment, giving them the chance to move north.
There is very little time between Homo erectus’ apparent arrival in South Caucasus, and its probable arrival in East and Southeast Asia. There is evidence of hominins in Yuanmou, China, dating to 1.7 Ma and in Sangiran, on Java, Indonesia, dating to 1.66 Ma.^[10] It appears hominins took longer to move into Europe, with the earliest site in Barranco León in southeastern Spain dated to 1.4 Ma^[11] and a more controversial Pirro Nord in southern Italy, allegedly from 1.3 – 1.7 Ma.^[12]

In any case, by 1 Ma, hominins had settled in most of the Old World. In Western Europe, it is hard to say, however, if settlement was continuous or if successive waves repopulated the territory in glacial interludes. Early Acheulean tools were present at Ubeidiya by 1.4 Ma^[13] and it seems likely that successive waves out of Africa after then would have brought Acheulean technology to Western Europe, but handaxes, which are typical of the Acheulean industry, are absent in early Western European sites.

Routes out of Africa

Sinai Peninsula

The Sinai Peninsula should be the African exit route par excellence, being since the Pliocene the only land bridge between the two continents of the Old World. As detailed below, unless we argue for boats on behalf of Homo erectus, it is surely the only way out. However, it was hard to access until the Middle Pleistocene. The Nile followed a different and pitiful course.

There are two Eurasian entryways that take advantage of the Sinai. First, the Levantine corridor, which moves north along the Eastern Mediterranean. Second, down the eastern bank of the Red Sea. Archaeological efforts in Arabia is limited, and attention is usually given to the Levantine corridor.

Due to the presence of Eurasian sites securely dated to the Early Pleistocene, to reject the Sinai is effectively to affirm that early hominins crossed straits, and this hypothesis has problems of its own.

Bab-el-Mandeb

The Bab-el-Mandeb is a 30 km strait parting East Africa from the Arabian Peninsula, with a small island, Perim, 3 km off the Arabian bank. The strait has a major appeal in the study of Eurasian expansion in that it brings East Africa in direct proximity with Eurasia. It doesn’t require hopping from water body to the next across the North African desert.

The land connection with Arabia has disappeared in the Pliocene,^[14] and though it may have reformed momentarily,^[15] the evaporation of the Red Sea and associated increase in salinity would have left traces in the fossil record after just 200 years and evaporite deposits after 600 years. Neither have been detected.^[16] A strong current flows from the Red Sea into the Indian Ocean and crossing would have been difficult without a land connection.

Oldowan grade tools are reported from Perim Island,^[17] implying that the strait could have been crossed in the Early Pleistocene, though these finds have yet to be confirmed.^[18]

Strait of Gibraltar

The Strait of Gibraltar is the Atlantic entryway to the Mediterranean, where Spanish and Moroccan banks are only 14 km apart. A decrease in sea levels in the Pleistocene due to glaciation would not have brought this down to less than 10 km. Water treadmills at Gibraltar. Deep currents push westwards, and surface water flows strongly back into the Mediterranean. The current would likely lose a swimmer or an unsteered raft.

Entrance into Eurasia across the strait of Gibraltar could explain the hominin remains at Barranco León in southeastern Spain dated to 1.4 Ma^[11] and Sima del Elefante in northern Spain dated to 1.2 Ma.^[19][20] But the site of Pirro Nord in southern Italy, allegedly from 1.3 – 1.7 Ma,^[12] suggests a possible arrival from the East. Resolution is insufficient to settle the matter.^[21]

Strait of Sicily

The modern Strait of Sicily separates Tunisia and Sicily by 145 km, but is shallow and would have been much narrower in glacial maxima. We have a poor understanding of plate tectonics of this area for the greater part of the Pleistocene. But while plate tectonics could have made the strait narrower than predicted by the lowering of sea levels alone, contrast of Pleistocene fauna strongly argues against an actual land bridge. Since the strait is only 400 km away from the North African hominin site of Ain Hanech in Algeria, dating to 1.8 Ma^[8] or 1.2 Ma,^[9] it remains a plausible route for Early Pleistocene expansion into Eurasia. But there is close to no evidence for an actual passing. Alimen based most of his argument^[22] in favour of such migration on Bianchini’s discovery^[23] of Sicilian Oldowan grade tools. But radiometric dates have not been produced, and the artefacts might as well be from the Middle Pleistocene.^[24]

Crossing straits

Presence of hominin remains in Indonesian islands is good evidence for seafaring by Homo erectus late in the Early Pleistocene. Bednarik suggests that navigation had appeared by 1 Ma, possibly to exploit offshore fishing grounds.^[25] He has reproduced a primitive dirigible raft to demonstrate the feasibility of faring across the Lombok Strait on such a device, which he believes to have been done before 850 ka. The strait has maintained a width of at least 20 km for the whole of the Pleistocene.
Such an achievement by Homo erectus in the Early Pleistocene offers some strength to the suggested water routes out of Africa, as the Gibraltar, Sicilian, and Bab-el-Mandeb exit routes are harder to consider if boats are deemed beyond the capacities of Homo erectus.

It might be tempting to consider a one-off event getting a few hominins across a strait – perhaps an Homo erectus family drifting on flood debris to land on a Eurasian bank. But successful population of Eurasia by such a beginning is unlikely. There are biological constraints to the minimum size a population must maintain to avoid extinction. That is to say, if less than 50 hominins at once made it into Eurasia and loss contact with African hominins, the population would likely undergo an extinction vortex, in part due to inbreeding.^[26]

Causes for hominin dispersals

Climate change and hominin flexibility

For a given species in a given environment, available resources will limit the amount of individuals that can survive indefinitely. This is the carrying capacity. Upon reaching this threshold, individuals may find it easier to gather resources in the poorer less exploited peripheral environment than in the preferred habitat. Homo habilis could have developed some baseline behavioural flexibility prior to its expansion into the peripheries (such as encroaching into the predatory guild^[27][28]). This flexibility could then have been positively selected and amplified, leading to Homo erectus’ adaptation to the peripheral open habitats.^[29] A new and more flexible hominin population could have come back to the old niche and replace the ancestral population.^[30] Moreover, some step-wise shrinking of the woodland and the associated reduction of hominin carrying capacity in the woods around 1.8 Ma, 1.2 Ma, and 0.6 Ma would have stressed the carrying capacity’s pressure for adapting to the open grounds.^[31][32]

With Homo erectus’ new environmental flexibility, it is likely that it saw favourable climate fluxes open it the way to the Levantine corridor at least sporadically in the Early Pleistocene.^[2]

Chasing fauna

Lithic analysis implies that Oldowan hominins weren't predators.^[33] However, Homo erectus appears to have followed animal migrations to the north during wetter periods, likely as a source of scavenged food. The sabre-tooth cat Megantereon was an apex predator of the Early and Middle Pleistocene (before MIS 12). It went extinct in Africa c. 1.5 Ma,^[34] but had already moved out through the Sinai, and is among the faunal remains of the Levantine hominin site of Ubeidiya, c. 1.4 Ma.^[13] It couldn’t break bone marrow and its kills were likely an important food source for hominins,^[35] especially in glacial periods.^[36]

In colder Eurasian times, the hominin diet would have to be principally meat-based and Acheulean hunters must have competed with cats.

Coevolved zoonotic diseases

Bar-Yosef and Cohen^[1] suggest that the success of hominins within Eurasia once out of Africa is in part due to the absence of zoonotic diseases outside their original habitat. Zoonotic diseases are those that are transmitted from animals to humans. While a disease specific to hominins must keep its human host alive long enough to transmit itself, zoonotic diseases won't necessarily do so as they can complete their life cycle without humans. Still, these infections are well accustomed to human presence, having evolved alongside them. The higher an African ape's population density the better a disease fares. Fifty-five percent of chimps at the Gombe reserve die of disease, most of them zoonotic.^[37] The majority of these diseases are still restricted to hot and damp African environments.
Once hominins had moved out into dryer and colder habitats of higher latitudes, one major limiting factor in population growth was out of the equation.

Hominin biology

Homo habilis reconstruction

While Homo habilis was certainly bipedal, its long arms are indicative of an arboreal adaptation.^[38] Homo erectus had longer legs and shorter arms, revealing a transition to obligate terrestriality, though it remains unclear how this change in relative leg length might have been an advantage.^[39] Sheer body size, on the other hand, seems to have allowed for better walking energy efficiency and endurance.^[40] A larger Homo erectus would also dehydrate more slowly and could thus cover greater distances before facing thermoregulatory limitations.^[41] The ability for prolonged walking at a normal pace would have been a decisive factor for effective colonisation of Eurasia.^[42]

Brain thermoregulation

Homo erectus reconstruction

Thermoregulation and dehydration are major problems that need to be dealt with to move into the open grasslands. In particular, vascularisation of the brain is crucial in maintaining it in the narrow frame of tolerable temperatures.

Bones of the higher cranium grow in response to expansion of cerebral mass, in such a way that brain tissue and blood vessels mold the inner brain case. Endocranial casts of fossil skulls allow
approximating brain vascularisation.^[43] Dean Falk noticed that a single large vessel, the occipital marginal sinus, was responsible for irrigating most of the brain in early australopiths
(Australopithecus afarensis, Paranthropus robustus and boisei).^[44] The vessel grew smaller with time so as to be progressively replaced by a network of small veins in later hominins, starting with Homo habilis and continuing well into Eurasia. She interprets the change as an adaptation to cool the brain,^[45] which she uses to advance her “radiator theory” for accelerated encephalisation from Homo habilis onwards.^[46] To Falk, bipedalism, which predates large brains, favoured a rewiring of cerebral blood vessels into a gravity-assisted irrigation network, itself allowing the cool down needed for encephalisation.

Endocranial casts of Homo habilis and Homo erectus differ in the organisation of the frontal lobe, in particular in the prefrontal cortex where higher mental functions of consciousness and abstraction occur.^[47] By themselves, mental capacities have likely played a role in the success of Eurasian colonisation. They would have allowed for greater social complexity,^[48] predation and sharing prey,^[49] and an overall higher quality diet.^[50] If we are to believe Bednarik and his seafaring Indonesian Homo erectus, then the brain must have played a role in crossing channels.

According to Wheeler,^[51] loss of functional body hair would have helped prevent hyperthermia, since hair will obstruct air flow over the skin and restrict cooling by evaporation. He further suggests that body cooling due to hair loss has relieved a thermal constraint on brain size (but in a response to Falk’s radiator hypothesis, Ralph Holloway maintains that there is no evidence for a temperature constraint on brain size^[52]). However, differences in body hair between Homo habilis and Homo erectus are impossible to test, and it will remain unclear whether hair loss was part of the hominin adaptation or preadaptation to Eurasia.

Recent African origin of modern humans

From Wikipedia, the free encyclopedia

In paleoanthropology, the recent African origin of modern humans, or the "Out of Africa" theory, is the most widely accepted model of the geographic origin and early migration of anatomically modern humans. This model has incorporated the 2010 discovery of genetic evidence for some archaic human admixture with modern Homo sapiens.^[1] The theory is called the "(Recent) Out-of-Africa" model in the popular press, and academically the "recent single-origin hypothesis" (RSOH), "Replacement Hypothesis", and "Recent African Origin" (RAO) model. The concept was speculative until the 1980s, when it was corroborated by a study of present-day mitochondrial DNA, combined with evidence based on physical anthropology of archaic specimens.

Genetic studies and fossil evidence show that archaic Homo sapiens evolved to anatomically modern humans solely in Africa, between 200,000 and 60,000 years ago,^[2] that members of one branch of Homo sapiens left Africa by between 125,000 and 60,000 years ago, and that over time these humans replaced earlier human populations such as Neanderthals and Homo erectus.^[3] The date of the earliest successful "out of Africa" migration (earliest migrants with living descendants) has generally been placed at 60,000 years ago as suggested by genetics, although migration out of the continent may have taken place as early as 125,000 years ago according to Arabian archaeology finds of tools in the region.^[4] A 2013 paper reported that a previously unknown lineage had been found, which pushed the estimated date for the most recent common ancestor (Y-MRCA) back to 338,000 years ago.^[5]

The recent single origin of modern humans in East Africa is the predominant position held within the scientific community.^{[6][7][8][9][10]} There are differing theories on whether there was a single exodus or several. A multiple dispersal model involves the Southern Dispersal theory,^[11] which has gained support in recent years from genetic, linguistic and archaeological evidence. A growing number of researchers also suspect that "long-neglected North Africa" was the original home of the modern humans who first trekked out of the continent.^[12][13][14]

The major competing hypothesis is the multiregional origin of modern humans, which envisions a wave of Homo sapiens migrating earlier from Africa and interbreeding with local Homo erectus populations in multiple regions of the globe. Most multiregionalists still view Africa as a major wellspring of human genetic diversity, but allow a much greater role for hybridization.^[15][16]

Genetic testing in the last decade has revealed that several now extinct archaic human species may have interbred with modern humans. These species have been claimed to have left their genetic imprint in different regions across the world: Neanderthals in all humans except Sub-Saharan Africans, Denisova hominin in Australasia (for example, Melanesians, Aboriginal Australians and some Negritos) and there could also have been interbreeding between Sub-Saharan Africans and an as-yet-unknown hominin (possibly remnants of the ancient species Homo heidelbergensis). However, the rate of interbreeding was found to be relatively low (1-10%) and other studies have suggested that the presence of Neanderthal or other archaic human genetic markers in modern humans can be attributed to shared ancestral traits originating from a common ancestor 500,000 to 800,000 years ago.^{[17][18][19][20][21]}

History of the theory

With the development of anthropology in the early 19th century, scholars disagreed vigorously about different theories of human development. Those such as Johann Friedrich Blumenbach and James Cowles Prichard held that since the creation, the various human races had developed as different varieties sharing descent from one people (monogenism). Their opponents, such as Louis Agassiz and Josiah C. Nott, argued for polygenism, or the separate development of human races as separate species or had developed as separate species through transmutation of species from apes, with no common ancestor.

The frontispiece to Huxley's Evidence as to Man's Place in Nature (1863): the image compares the skeletons of apes to humans.

Charles Darwin was one of the first to propose common descent of living organisms, and among the first to suggest that all humans had in common ancestors who lived in Africa.^[22] Darwin first suggested the "Out of Africa" hypothesis after studying the behaviour of African apes, one of which was displayed at the London Zoo. The anatomist Thomas Huxley had also supported the hypothesis and suggested that African apes have a close evolutionary relationship with humans.^[23] These views were however opposed by Ernst Haeckel the German biologist who was a proponent of the Out of Asia theory. Haeckel argued that humans were more closely related to the primates of Southeast Asia and rejected Darwin’s hypothesis of Africa.^[24][25]

In the Descent of Man, Darwin speculated that humans had descended from apes which still had small brains but walked upright, freeing their hands for uses which favoured intelligence. Further, he thought such apes were African:^[26]

In each great region of the world the living mammals are closely related to the extinct species of the same region. It is, therefore, probable that Africa was formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee; and as these two species are now man's nearest allies, it is somewhat more probable that our early progenitors lived on the African continent than elsewhere. But it is useless to speculate on this subject, for an ape nearly as large as a man, namely the Dryopithecus of Lartet, which was closely allied to the anthropomorphous Hylobates, existed in Europe during the Upper Miocene period; and since so remote a period the earth has certainly undergone many great revolutions, and there has been ample time for migration on the largest scale.

—Charles Darwin, Descent of Man^[27]

The prediction was insightful, because in 1871 there were hardly any human fossils of ancient hominids available. Almost fifty years later, Darwin's speculation was supported when anthropologists began finding numerous fossils of ancient small-brained hominids in several areas of Africa (list of hominina fossils).

The debate in anthropology had swung in favour of monogenism by the mid-20th century. Isolated proponents of polygenism held forth in the mid-20th century, such as Carleton Coon, who hypothesized as late as 1962 that Homo sapiens arose five times from Homo erectus in five places.^[28]
The "Recent African origin" of modern humans means "single origin" (monogenism) and has been used in various contexts as an antonym to polygenism.

In the 1980s Allan Wilson together with Rebecca L. Cann and Mark Stoneking worked on the so-called "Mitochondrial Eve" hypothesis. In his efforts to identify informative genetic markers for tracking human evolutionary history, he started to focus on mitochondrial DNA (mtDNA) – genes that sit in the cell, but not in the nucleus, and are passed from mother to child. This DNA material is important because it mutates quickly, thus making it easy to plot changes over relatively short time spans. By comparing differences in the mtDNA Wilson believed it was possible to estimate the time, and the place, modern humans first evolved. With his discovery that human mtDNA is genetically much less diverse than chimpanzee mtDNA, he concluded that modern human populations had diverged recently from a single population while older human species such as Neandertals and Homo erectus had become extinct. He and his team compared mtDNA in people of different ancestral backgrounds and concluded that all modern humans evolved from one 'lucky mother' in Africa about 150,000 years ago.^[29] With the advent of archaeogenetics in the 1990s, scientists were able to date the "out of Africa" migration with some confidence.

In 2000, the mitochondrial DNA (mtDNA) sequence of "Mungo Man 3" (LM3) of ancient Australia was published indicating that Mungo Man was an extinct subspecies that diverged before the most recent common ancestor of contemporary humans. The results, if correct, supports the multiregional origin of modern humans hypothesis.^[30][31] This work was later questioned^[32][33] and explained by W. James Peacock, leader of the team who sequenced Mungo man's aDNA.^[34] In addition, a large-scale genotyping analysis of aboriginal Australians, New Guineans, Southeast Asians and Indians in 2013 showed close genetic relationship between Australian, New Guinean, and the Mamanwa people, with divergence times for these groups estimated at 36,000 y ago. Further, substantial gene flow was detected between the Indian populations and aboriginal Australians, indicating an early "southern route" migration out of Africa, and arrival of other populations in the region by subsequent dispersal. This basically opposes the view that there was an isolated human evolution in Australia.^[35]

The question of whether there was inheritance of other typological (not de facto) Homo subspecies into the Homo sapiens genetic pool is debated.

Early Homo sapiens

 

Anatomical comparison of the skulls of a modern human (left) and Homo neanderthalensis (right).

Anatomically modern humans originated in Africa about 250,000 years ago. The trend in cranial expansion and the acheulean elaboration of stone tool technologies which occurred between 400,000 years ago and the second interglacial period in the Middle Pleistocene (around 250,000 years ago) provide evidence for a transition from H. erectus to H. sapiens.^[36] In the Recent African Origin (RAO) scenario, migration within and out of Africa eventually replaced the earlier dispersed H. erectus.

Homo sapiens idaltu, found at site Middle Awash in Ethiopia, lived about 160,000 years ago.^[37] It is the oldest known anatomically modern human and classified as an extinct subspecies.^[38] Fossils of early Homo sapiens were found in Qafzeh cave in Israel and have been dated to 80,000 to 100,000 years ago. However these humans seem to have either become extinct or retreated back to Africa 70,000 to 80,000 years ago, possibly replaced by south bound Neanderthals escaping the colder regions of ice age Europe.^[39] Hua Liu et al. analyzing autosomal microsatellite markers dates to c. 56,000±5,700 years ago mtDNA evidence. He interprets the paleontological fossil of early modern human from Qafzeh cave as an isolated early offshoot that retracted back to Africa.^[40]

All other fossils of fully modern humans outside Africa have been dated to more recent times. The oldest well dated fossils found outside Africa are from Lake Mungo, Australia, and have been dated to about 42,000 years ago.^[41][42] The Tianyuan cave remains in Liujiang region China have a probable date range between 38,000 and 42,000 years ago. They are most similar in morphology to Minatogawa Man, modern humans dated between 17,000 and 19,000 years ago and found on Okinawa Island, Japan.^[43][44] However, others have dated Liujang Man to 111,000 to 139,000 years before the present.^[45]

Beginning about 100,000 years ago evidence of more sophisticated technology and artwork begins to emerge and by 50,000 years ago fully modern behaviour becomes more prominent. Stone tools show regular patterns that are reproduced or duplicated with more precision while tools made of bone and antler appear for the first time.^[46][47]

Genetic reconstruction

Two pieces of the human genome are quite useful in deciphering human history: mitochondrial DNA and the Y chromosome. These are the only two parts of the genome that are not shuffled about by the evolutionary mechanisms that generate diversity with each generation: instead, these elements are passed down intact. According to the hypothesis, all people alive today have inherited the same mitochondria^[48] from a woman who lived in Africa about 160,000 years ago.^[49][50] She has been named Mitochondrial Eve. All men living today have inherited their Y chromosomes from a man who lived 140,000–500,000 years ago, probably in Africa. He has been named Y-chromosomal Adam. Based on comparisons of non-sex-specific chromosomes with sex-specific ones, it is now believed that more men than women participated in the out-of-Africa exodus of early humans.^[51] [Mendez et al.]

Mitochondrial DNA

 

Map of early diversification of modern humans according to mitochondrial population genetics (see: Haplogroup L).

The first lineage to branch off from Mitochondrial Eve is L0. This haplogroup is found in high proportions among the San of Southern Africa, the Sandawe of East Africa. It is also found among the Mbuti people.^[52][53]

These groups branched off early in human history and have remained relatively genetically isolated since then. Haplogroups L1, L2 and L3 are descendents of L1-6 and are largely confined to Africa. The macro haplogroups M and N, which are the lineages of the rest of the world outside Africa, descend from L3. L3 is about 84,000 years old, and haplogroup M and N are almost identical in age at about 63,000 years old. ^[54]

Genomic analysis

Although mitochondrial DNA and Y-chromosomal DNA are particularly useful in deciphering human history, data on the genomes of dozens of population groups have also been studied. In June 2009, an analysis of genome-wide SNP data from the International HapMap Project (Phase II) and CEPH Human Genome Diversity Panel samples was published.^[55] Those samples were taken from 1138 unrelated individuals.^[55] Before this analysis, population geneticists expected to find dramatic differences among ethnic groups, with derived alleles shared among such groups but uncommon or nonexistent in other groups.^[56] Instead the study of 53 populations taken from the HapMap and CEPH data revealed that the population groups studied fell into just three genetic groups: Africans, Eurasians (which includes natives of Europe and the Middle East, and Southwest Asians east to present-day Pakistan), and East Asians, which includes natives of Asia, Japan, Southeast Asia, the Americas, and Oceania.^[56] The study determined that most ethnic group differences can be attributed to genetic drift, with modern African populations having greater genetic diversity than the other two genetic groups, and modern Eurasians somewhat more than modern East Asians.^[56] The study suggested that natural selection may shape the human genome much more slowly than previously thought, with factors such as migration within and among continents more heavily influencing the distribution of genetic variations.^[57] A May 2002 study examined three groups, African, European, and Asian. It found greater genetic diversity among Africans than among Eurasians, and that genetic diversity among Eurasians is largely a subset of that among Africans, supporting the 'out of Africa' model.^[58]

Movement out of Africa

Red Sea crossing

By some 70,000 years ago, a part of the bearers of mitochondrial haplogroup L3 migrated from East Africa into the Near East. The date of this first wave of "out of Africa" migration was called into question in 2011, based on the discovery of stone tools in the United Arab Emirates, indicating the presence of modern humans between 100,000 and 125,000 years ago.^[4][59] New research showing slower than previously thought genetic mutations in human DNA published in 2012, indicating a revised dating for the migration of between 90,000 and 130,000 years ago.^[60]

Some scientists believe that only a few people left Africa in a single migration that went on to populate the rest of the world,^[61] based in the fact that only descendents of L3 are found outside Africa. From that settlement, some others point to the possibility of several waves of expansion. For example, geneticist Spencer Wells says that the early travellers followed the southern coastline of Asia, crossed about 250 kilometres (155 mi) of sea, and colonized Australia by around 50,000 years ago. The Aborigines of Australia, Wells says, are the descendants of the first wave of migrations.^[62]
It has been estimated that from a population of 2,000 to 5,000 individuals in Africa,^[63] only a small group, possibly as few as 150 to 1,000 people, crossed the Red Sea.^[64] Of all the lineages present in Africa only the female descendants of one lineage, mtDNA haplogroup L3, are found outside Africa. Had there been several migrations one would expect descendants of more than one lineage to be found outside Africa. L3's female descendants, the M and N haplogroup lineages, are found in very low frequencies in Africa (although haplogroup M1 is very ancient and diversified in North and Northeast Africa) and appear to be recent arrivals. A possible explanation is that these mutations occurred in East Africa shortly before the exodus and by the founder effect became the dominant haplogroups after the exodus from Africa. Alternatively, the mutations may have arisen shortly after the exodus from Africa.

Other scientists have proposed a Multiple Dispersal Model, in which there were two migrations out of Africa, one across the Red Sea travelling along the coastal regions to India (the Coastal Route), which would be represented by Haplogroup M. Another group of migrants with Haplogroup N followed the Nile from East Africa, heading northwards and crossing into Asia through the Sinai.
This group then branched in several directions, some moving into Europe and others heading east into Asia. This hypothesis is supported by relatively late dating of the arrival of modern humans into Europe as well as by both archaeological and DNA evidence. Results from mtDNA collected from aboriginal Malaysians called Orang Asli, and the creation of a phylogentic tree, indicates Hapologroups M and N share characteristics with original African groups dating approximately 85,000 years ago, and sharing characteristics with sub-haplogroups among coastal southeast Asian regions, such as Australasia, the Indian Subcontinent, and throughout continental Asia, which had dispersed and separated from its African origins approximately 65,000 years ago. This southern coastal dispersion would have occurred before the original theory of dispersion through the Levant approximately 45,000 years ago.^[65] This hypothesis attempts to explain why Haplogroup N is predominant in Europe and why Haplogroup M is absent in Europe. Evidence of the coastal migration is hypothesized to have been destroyed by the rise in sea levels during the Holocene epoch.^[11][66] Alternatively, a small European founder population that initially expressed both Haplogroup M and N could have lost Haplogroup M through random genetic drift resulting from a bottleneck (i.e. a founder effect).

Today at the Bab-el-Mandeb straits the Red Sea is about 20 kilometres (12 mi) wide, but 50,000 years ago sea levels were 70 m (230 ft) lower (owing to glaciation) and the water was much narrower. Though the straits were never completely closed, they were narrow enough and there may have been islands in between to have enabled crossing using simple rafts.^[67][68] Shell middens 125,000 years old have been found in Eritrea,^[69] indicating the diet of early humans included seafood obtained by beachcombing.

Subsequent expansion

 

Map of early human migrations^[70]
1. Homo sapiens
2. Neanderthals
3. Early Hominids

From the Near East, these populations spread east to South Asia by 50,000 years ago, and on to Australia by 40,000 years ago, Homo sapiens for the first time colonizing territory never reached by Homo erectus. Europe was reached by Cro-Magnon some 40,000 years ago. East Asia (Korea, Japan) was reached by 30,000 years ago. It is disputed whether subsequent migration to North America took place around 30,000 years ago, or only considerably later, around 14,000 years ago.^[71]

The group that crossed the Red Sea travelled along the coastal route around the coast of Arabia and Persia until reaching India, which appears to be the first major settling point. Haplogroup M is found in high frequencies along the southern coastal regions of Pakistan and India and it has the greatest diversity in India, indicating that it is here where the mutation may have occurred.^[72] Sixty percent of the Indian population belong to Haplogroup M.

The indigenous people of the Andaman Islands also belong to the M lineage. The Andamanese are thought to be offshoots of some of the earliest inhabitants in Asia because of their long isolation from mainland Asia. They are evidence of the coastal route of early settlers that extends from India along the coasts of Thailand and Indonesia all the way to Papua New Guinea. Since M is found in high frequencies in highlanders from New Guinea as well, and both the Andamanese and New Guineans have dark skin and Afro-textured hair, some scientists believe they are all part of the same wave of migrants who departed across the Red Sea ~60,000 years ago in the Great Coastal Migration.

Notably, the findings of Harding et al. (2000, p. 1355) show that, at least with regard to dark skin color, the haplotype background of Papua New Guineans at MC1R (one of a number of genes involved in melanin production) is identical to that of Africans (barring a single silent mutation). Thus, although these groups are distinct from Africans at other loci (due to drift, bottlenecks, etc.), it is evident that selection for the dark skin color trait likely continued (at least at MC1R) following the exodus. This would support the hypothesis that suggests that the original migrants from Africa resembled pre-exodus Africans (at least in skin color), and that the present day remnants of this ancient phenotype can be seen among contemporary Africans, Andamanese and New Guineans.
Others suggest that their physical resemblance to Africans could be the result of convergent evolution.^[73][74]

From Arabia to India the proportion of haplogroup M increases eastwards: in eastern India, M outnumbers N by a ratio of 3:1. However, crossing over into East Asia, Haplogroup N reappears as the dominant lineage. M is predominant in South East Asia but amongst Indigenous Australians N reemerges as the more common lineage. This discontinuous distribution of Haplogroup N from Europe to Australia can be explained by founder effects and population bottlenecks.^[75]

Competing hypotheses

The multiregional hypothesis, initially proposed by Milford Wolpoff, holds that the evolution of humans from H. erectus at the beginning of the Pleistocene 1.8 million years BP has been within a single, continuous worldwide population. Proponents of multiregional origin reject the assumption of an infertility barrier between ancient Eurasian and African populations of Homo. Multiregional proponents point to the fossil record and genetic evidence in chromosomal DNA. One study suggested that at least 5% of the human modern gene pool can be attributed to ancient admixture, which in Europe would be from the Neanderthals.^[76] But the study also suggests that there may be other reasons why humans and Neanderthals share ancient genetic lineages.^[77][78]