Recent African origin of modern humans



In paleoanthropology, the recent single-origin hypothesis (RSOH, or Out-of-Africa model, or Replacement Hypothesis), also  Recent African Origin (RAO) is one of two accounts of the origin of anatomically modern humans, Homo sapiens. According to the RSOH, anatomically modern humans evolved in Africa between 200,000 and 100,000 years ago, with members of one branch leaving Africa around 60,000 years ago.

These emigrants spread to the rest of the world, replacing other Homo species already there, such as Neanderthals and Homo erectus. The hypothesis is derived from research in several disciplines, chiefly genetics, archaeology and linguistics.

Currently available genetic and archaeological evidence is generally interpreted as supportive of a recent single origin of modern humans in East Africa.

Alternative scenarios claim multiregional origin of modern humans, notably including claims of interbreeding of Cro-Magnon and Neanderthals. These push back the original "out of Africa" migration to 2 million years ago.

History of the theory
Charles Darwin was one of the first to suggest that all humans had a common ancestor who lived in Africa. In the Descent of Man he writes: 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.

The prediction was highly insightful because at the time, in 1871, there were hardly any human fossils of ancient hominids available. Almost fifty years later Darwin was vindicated, as anthropologists began finding numerous fossils of ancient hominids all over Africa (list of hominina fossils).

In 19th century scientific racism, "monogeism" was opposed by "polygenism", the idea that the various human races had evolved independently out of archaic hominids. Such views were largely obsolete by the mid 20th century, although there were isolated proponents in the later 20th century such as Carleton Coon who hypothesized as late as 1962 that Homo sapiens arose five separate times from Homo erectus in five separate places.

With the advent of archaeogenetics in the 1990s, it became possible to date the "out of Africa" migration with some confidence. The question whether there may have been admixture (hybridization) of Homo erectus and Homo sapiens remains under debate.

Early Homo sapiens
Homo sapiens 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 cranial expansion and the elaboration of stone tool technologies developed, providing evidence for a transition from H. erectus to H. sapiens. In the RAO scenario, migration within and out of Africa eventually replaced the earlier dispersed H. erectus. Homo sapiens idaltu, from Ethiopia, lived from about 160,000 years ago. It is the oldest known anatomically modern human.

Fossils of modern humans were found in a cave in Israel at Qafzeh and have been dated to 100,000 years ago. However these humans seem to have either gone extinct or retreated back to Africa 80,000 - 70,000 years ago, possibly replaced by south bound Neanderthals escaping the colder regions of ice age Europe. All other fossils of fully modern humans outside of Africa have been dated to more recent times. The next oldest fossil of modern humans outside of Africa are those of Mungo Man found in Australia and have been dated to about 42,000 years ago.

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. By this time the ritual burying of the dead is noted. Stone tools show regular patterns that are reproduced or duplicated with more precision. Tools made of bone and antler appear for the first time. These new changes are suggestive of more advanced behaviour and scientists attribute these changes to the development of language.

Genetic reconstruction
Two pieces of the human genome are particularly useful in deciphering human history. One is the Mitochondrial DNA and the other is the Y chromosome. These are the only two parts of the genome that are not shuffled about by the evolutionary mechanisms designed to generate diversity with each generation. Hence the Mitochondrial DNA and the Y chromosome are passed down generation to generation intact. All 6.5 billion people alive today have inherited the same Mitochondria from one woman who lived in Africa about 150,000 years ago; she has been named Mitochondrial Eve. All men today have inherited their Y chromosomes from a man who lived 60,000 years ago, probably in Africa. He has been named Y-chromosomal Adam.

Mitochondrial DNA


The first lineage to branch off from Eve is L1. This haplogroup is found in high proportions among the San and the Mbuti people. These groups branched off early in human history and have remained relatively isolated genetically since. Haplogroups L2 and L3 are descendents of L1 and are largely confined to Africa. The macro haplogroups M and N, which are the lineages of the rest of the world outside Africa, descended from L3.

Y-chromosomal DNA
The mutations defining macro-haplogroup CR (all Y haplogroups except A and B) predate the "Out of Africa" migration, its descendent macro-group DE being 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.

Exodus from Africa


Some 70 millennia ago, a part of the bearers of mitochondrial haplogroup L3 migrated from East Africa into the Near East.

Some scientists believe that only a few people left Africa in a single migration that went on to populate the rest of the world. It has been estimated that from a population of 2,000 - 5,000 in Africa, only a small group of possibly 150 people crossed the Red Sea. This is because, of all the lineages present in Africa, only the daughters of one lineage, L3, are found outside Africa. Had there been several migrations one would expect more than one African lineage outside Africa. L3's daughters, the M and N lineages, are found in very low frequencies in 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 propose that there were two migrations out of Africa, one across the Red Sea travelling along the coastal regions to India, 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. Today at the Bab-el-Mandeb straits the Red Sea is about 12 miles (20 kilometres) wide but 50,000 years ago it was much narrower and sea levels were 70 meters lower. 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 was sea food obtained by beachcombing.

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

The group that crossed the Red Sea travelled along the coastal route around the coast of Arabia and Iran 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 frizzy hair typically found in Africa, some scientists believe they are all part of the same wave of migrants who departed across the Red Sea. Others suggest that their physical resemblance to Africans is more likely to be an example of convergent evolution.

From Saudi 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 has confounded scientists attempting to trace migratory routes.

Competing hypotheses
The multi-regional (hybrid-origin) hypothesis proposes admixture of archaic Homo sapiens subspecies resulting in hybrids that gave rise to the world's races. This means that proponents reject the assumption of a species barrier between Homo erectus and Homo sapiens and date the first "out of Africa" migration of Homo sapiens subspecies to 2 million years ago. The recent migration out of Africa 60,000 years ago would then have brought previously isolated subspecies into renewed contact, resulting in a hybrid Homo sapiens sapiens, who was superior to both its ancestor subspecies due to what is commonly termed hybrid vigour. Proponents argue that very strong genetic similarities among all humans do not prove recent common ancestry, but rather reflect the interconnectedness of human populations around the world, resulting in relatively constant gene flow (Thorne and Wolpoff 1992). Erik Trinkaus is a proponent of hybridization of Cro-Magnon H. sapiens with Homo neanderthalensis around 30,000 years ago.

These theories are based largely on archaeological and fossil evidence. They are not widely recognized, opponents citing the lack of DNA evidence.

While the hybrid-origin scenario at present cannot be ruled out with certainty, more extreme proposals, known as Polygenism were a topic of academic debate in later 19th century, but are obsolete today as historical examples of scientific racism.