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Out of Africa Migration

All 7 billion people on the planet today, everyone in every nook and cranny of all the continents and islands has their origin in East Africa. This single origin of modern humans is the predominant position held within the scientific community.

In the popular media, the theory is called the [Recent] Out-of-Africa model. Academically it is referred to as the recent single-origin hypothesis  (RSOH), Replacement Hypothesis, and Recent African Origin (RAO) model.

The concept was speculative from the time Charles Darwin suggested that humans have a single origin (monogenesis) in his book Descent of Man in 1871:

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

Charles Darwin was way ahead in his ideas because in 1871 there were hardly any hominid fossils for study available. The study of present-day mitochondrial DNA and evidence based on physical anthropology of archaic fossil specimens corroborates his idea.

Mitochondrial DNA (mtDNA) is the DNA located in organelles called mitochondria. Mitochondria are structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, that is, adenosine triphosphate (ATP). Most other DNA present in eukaryotic organisms is found in the cell nucleus. Mitochondrial DNA can be regarded as the smallest chromosome, and was the first significant part of the human genome to be sequenced. In most species, including humans, mtDNA is inherited solely from the mother. The DNA sequence of mtDNA allows biologists to explain the evolutionary relationships among species. It also permits an examination of the relatedness of populations, and so has become important in anthropology and field biology.

Genetic and fossil evidence shows that archaic Homo sapiens evolved to anatomically modern humans solely in Africa, between 200,000 and 150,000 years ago. Members of one branch of Homo sapiens left Africa by between 125,000 and 60,000 years ago. Over time these humans replaced earlier human populations such as Neanderthals and Homo erectus. The date of the earliest successful “out of Africa” migration (earliest migrants with living descendents) has generally been placed at 60,000 years ago as suggested by genetics, although attempts at 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.

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. All people alive today have inherited the same Mitochondria from one woman who lived in Africa about 160,000 years ago. She has been named Mitochondrial Eve. All men today have inherited their Y chromosomes from a man who lived 140,000 years ago, probably in Africa. He has been named Y-chromosomal Adam. It is now believed that more men participated in the out of Africa exodus of early humans than women based on comparing non-sex-specific chromosomes with sex-specific ones.

Mitochondrial DNA: 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 of the Congo. 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.

Y-chromosomal DNA: The mutations defining macro-haplogroup CT (all Y haplogroups except A and B) predate the “Out of Africa” migration. Macro-group DE are confined to Africa. The mutations that distinguish Haplogroup C from all other descendants of CR have occurred some 60,000 years ago, shortly after the first Out of Africa migration. Haplogroup F originated some 45,000 years ago, either in North Africa (in which case it would point to a second wave of out-of-Africa migration) or in South Asia. More than 90% of males not native to Africa are descended in direct male line from the first bearer of haplogroup F.

About 70,000 years ago, a part of the bearers of mitochondrial haplogroup L3 migrated from East Africa into the Near East.

Before the first successful migrations, homo sapiens almost went extinct! It has been estimated that from a totala population of 2,000 to 5,000 individuals in Africa, only a small group (haplogroup L3 bearers), possibly as few as 150 to 1,000 people, crossed the Red Sea. This happened about 70,000 years ago. 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.

Today at the Bab-el-Mandeb straits the Red Sea is about 12 miles (20 kilometres) wide, but 50,000 years ago sea levels were 70 meters lower (owing to glaciation) and the water was much narrower. Though the straits were never completely closed, there may have been islands in between which could be reached using simple rafts. Shell middens 125,000 years old have been found in Eritrea, indicating the diet of early humans included seafood obtained by beachcombing.

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.

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. 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. 60% 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. 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, 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.

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.

 

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