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Origin and Migration of the Indigenous

Peoples of Africa

Stes de Necker

In paleoantropologie, the recent “(Recent) Out-of-Africa” model is the mainstream model describing the origin and early dispersal of anatomically modern humans. 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 hypothesis that humans have a single origin (monogenesis) was published in Charles Darwin's Descent of Man (1871). 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.

According to genetic and fossil evidence, archaic Homo sapiens evolved to anatomically modern humans solely in Africa, between 200,000 and 150,000 years ago.

Members of one branch left Africa by between 125,000 and 60,000 years ago and over time replaced earlier human populations such as Neanderthals and Homo erectus. The date of this original "out of Africa" migration has mostly been dated to between 70,000 and 60,000 years ago, but a 2011 study suggested that an early wave of migration may have taken place as early as 125,000 years ago.

The recent single origin of modern humans in East Africa is the near-consensus position held within the scientific community.

However, recent sequencing of the full Neanderthal Genome suggests Neanderthals and some modern humans share some ancient genetic lineages. The authors of the study suggest that their findings are consistent with Neanderthal admixture of up to 4% in some populations. But the study also suggests that there may be other reasons why humans and Neanderthals share ancient genetic lineages.

The competing hypothesis is the multiregional origin of modern humans. Some push back the original "out of Africa" migration (in this case, by Homo erectus, not by Homo sapiens) to two million years ago.

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.

In the Recent African Origin (RAO) scenario, migration within and out of Africa eventually replaced the earlier dispersed H. erectus.

Exodus from Africa

Some 70,000 years ago, a number of these H. sapiens migrated from East Africa into the Near East. The date of this first wave of "out of Africa" migration has been 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.

Some scientists believe that only a few people left Africa in a single migration that went on to populate the rest of the world based on the fact that only descents of these early H. sapiens are found outside Africa. From that settlement, some others point to the possibility of several waves of expansion. For example, Wells says that the early travelers followed the southern coastline of Asia, crossed about 250 kilometers [155 miles] 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.

It has been estimated that from a population of 2,000 to 5,000 individuals in Africa, only a small group, possibly as few as 150 to 1,000 people, crossed the Red Sea into Saudi Arabia.

Of all the lineages present in Africa only the female descendants of one lineage, mtDNA haplogroup, 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 traveling 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 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.

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 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.

Subsequent Expansion

Subsequent expansion 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 traveled 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. 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 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 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.

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.