The new influx 300,000 years ago
By the time of the second major influx of archaic homo sapiens into Africa -- about 300,000 years ago -- some archaic Homo sapiens from Group A of our ancestry had already recombined with African Homo heidelbergensis while moving into west-central Africa. But by 300,000 years ago, when the new influx of Homo sapiens arrived from the Middle East, some of the earlier migrants from Group A had moved back into eastern Africa, where the climate had dried considerably and the African Homo heidelbergensis had abandoned the region. Consequently, when the second wave arrived they found archaic Homo sapiens already in possession of eastern Africa.
By 300,000 years ago the archaic in Africa had already developed effective methods of hunting the smaller game that they had found in Africa, both in the grassy strips that had opened up in the forests of west-central Africa and in the dry conditions of eastern Africa.
Recombination of Group A with Group B
Rapid changes in climate continued to segmentize human populations in Africa as parts of Africa became excessively wet and humid or excessively dry. But Homo sapiens were better able to hunt relatively small and fast-moving animals, so they were able to survive in dry climates that blocked Homo heidelbergensis.
As archaic Homo sapiens took over Africa they also interbred extensively withe the more numerous indigenous humans, but the two species were largely incompatible, most likely because of differences in their behavior and linguistic capability, and most of the Homo heidelbergensis DNA that had been picked up by the Homo sapiens population quickly became non-functional.
New segmentation, within Africa
The southern part of Africa, south of the Limpopo River, was the last part of Africa that was taken over because even found it difficult to survive in the persistently dry desert regions regions just north of there, bur eventually archaic took over southern Africa as well.
Meanwhile, the archaic Homo sapiens population continued to to be broken into segments by the vacillating climate as various other parts became excessively dry or excessively wet, but the division between southern Africa and the rest of the continent was the most persistent division.
The emergence of modern Homo sapiens
in general, the idea of coevolution has been discarded as an explanation for the emergence of modern in various parts of the world. But as population densities have increased everywhere in response to to the regular cooking of foods, coevolution is certainly a convenient explanation for some of the changes that have occurred in the human genome. The most obvious of these changes are changes in human physiognomy: the emergence of rounder heads, reduction in tooth sizes and further enlargement and sharpening of the chin. The rounder heads accompanied episodes of domestication as population densities increased, and changes in the jaw and tee continued earlier changes that resulted from the regular coking of food.
At the same time effective group sizes skyrocketed as human population increased rapidly but expansion of successful subgroups at the expense of their neighbors came to depend more on circumstances and luck and les on factors that had often differentiated the smaller groups. These differences included genetic differences related to brain size, which was linked to random proliferations of neurons, but once populations and group sizes became very large, these factors became les relevant to the success of particular groups. The result was that genetic drift took charge of brain size evolution, and brain-size evolution drifted into a broader spectrum with a larger standard deviation. Indeed, average brain sizes began to decline about 50,000 years ago, most likely because of the high energy demands of larger brains, in competition with other short-term and long-term energy demands.
Departures from Africa
Between 120,000 and 75,000 years ago as population density increased and small societies combined into larger more versatile societies the bow-and-arrow killing machine and other new technologies were invented (but not fully perfected. The very large society at the southern tip of Africa invented the bow-and-arrow killing machine, among other things, and began to colonize rock shelters to the north, and also develop fishing technologies which enabled them to move along the shoreline in boats while they lived on resources of the sea.
Starting about 80,000 years ago they moved slowly along the coast to east and west. But especially to east, because they eventually reached Southeast Asia, where they populated Australia, New Zealand, and the nearby islands of Oceania such as New Guinea (which at that time was connected to Australia by a land bridge.
These boat people also moved north along the coast of East Asia, and were probably the first people to reach the Americas.
At the same time as these boat people were moving north along the coast of Africa small societies in in eastern Africa completed a merger into larger, more versatile societies, and they also picked up innovations as a result of contact with people from the south. These more versatile societies began to expand rapidly, and they found an outlet by driving across Africa from east to west.
After this dynamic combination of societies drove across Africa from east to west displacing or absorbing smaller societies along the way they moved north through grasslands that broke the forests of west-central Africa before driving back across the continent from west to east. In eastern Africa some of them then moved out of Africa entirely, and ended up in the Levant, in Anatolia or on the Iranian Plateau.
First Anatolians and first Iranians
It has often been assumed that the Out-Of-Africa movement began about 50,000 years ago, but the first fully successful out-of-Africa movement occurred more than 72,000 years ago, just before the explosion of the Toba Super-volcano.
The early pioneers moved north into the Levant and then moved east and further north, into mountainous regions that are now in Syria, Iraq and Turkey. Some of them moved still further east, to the Persian Plateau.
Those who moved into the mountains found the hunting good there, because the rugged terrain provided many opportunities for trapping sheep and goats. Many descendants of these sheep- and goat-hunters are still there today, even though they were reduced to cooking and eating grass seeds with the onset of the Younger Dryas, 12,900 years ago -- about 55,000 years after they had arrived in the region.
Those who continued on to the Persian Plateau found the hunting good there, also. They interbred with the indigenous Homo neanderthalensis; which caused trouble for both species, because Homo neanderthalensis carried an RhDd antigen on the surface of their red blood cells in the place of the Rh antigen. When a fetus carried one type of antigen and its mother carried the other type, the mother's body manufactured antibodies against her own fetus, which flowed into the unborn child through the umbilical cord.
The populations of both species suffered sharp declines in their populations, but as more and more Homo sapiens arrived on the Persian Plateau that species recovered. Homo neanderthal, who lived in small, isolated groups and could not tolerate a high population density, became extinct on the Persian Plateau.
The same scenario played out later, when Homo sapiens later invaded Europe in large numbers. Moreover, something similar might have occurred when Homo sapiens invaded Africa and interbred with Homo heidelbergensis.
The Toba Super-volcano
About 72,000 years ago the Toba Super-volcano on the Island of Sumatra (in present-day Indonesia) exploded, and South Asia was covered by a layer of volcanic ash five centimeters thick.
Skies darkened, and a volcanic winter intensified the glacial maximum that followed. South Asia was severely affected, and any human population that was moving along the coast there must have ben decimated, but some parts of the Out-of-Africa migration might have already made its way beyond the reach of Toba's wrath. In any event, the migration continued on to to offshore islands, to New Guinea and to Australia.
The volcanic explosion was followed by a glacial maximum, and Africa became very dry. Moreover, as carbon dioxide became tied up in the polar caps Africa a shortage of carbon dioxide in the air caused all kinds of green plants to grow very poorly. Herbivore populations declined drastically and the human population of Africa declined.
Those who had moved north into the Middle East were relatively unscathed by these changes, and indeed people who had moved there became acclimated to cooler temperatures, and were then able to move further north into Eurasia where they were able to compete with the indigenous people there.
Success in Eurasia
About 45,000 years ago modern humans in the Middle East met with some success when they moved further north, particularly those who then moved to the east across central Asia during a warm period and found that an unusually warm climate in that region had created a string of lakes by which they could slip between deserts to the south and Siberian mountains to the north. In that region they came into contact with Homo denisova, and as they moved across what is now China and into southeast Asia they came into contact with more of these archaic humans. Cross-breeding occurred between the two species, and consequently DNA from Homo denisova can be detected in the genomes of many modern people, particularly in Oceania, offshore from Southeast Asia, where some Homo denisova might have been trapped by rising sea levels.
Modern people also met with some success in Europe, in spite of the difficulty of interbreeding with Homo neanderthalensis, who carried the RhD antigen on their red blood cells. Archaeologist have found evidence that one subgroup of Homo sapiens successfully occupied territory further north than that occupied by Homo neanderthalensis, after passing through Homo neanderthalensis territory about 46,000 years ago and picking up DNA from them. These early migrants to Europe occupied a string of far-north camps across Europe, from modern-day Poland to present-day England.
As they had passed through Homo neanderthalensis territory they certainly must have noticed that interbreeding with the indigenous people often led to unfortunate results, and this probably induced them to continue moving north until the climate was too cold for Homo neanderthalensis. Homo Sapiens survived there because they had to, and maybe because their garments were better made.
As Homo sapiens moved into Eurasia in large numbers and interbred with Homo neanderthalensis and Homo denisova they picked up immune-system alleles from them that protected them against novel diseases encountered in Eurasia. These spread rapidly through the Homo sapiens population, so that modern Homo sapiens living in Eurasia now carry these immune-system alleles in their own genomes.
Homo neanderthalensis were driven into extinction by 38,000 years ago, and Homo denisova also became extinct.
Success in Island Southeast Asia
How the Out Of Africa movement first reached Island Southeast Asia, Australia and America is a mystery, but they may have simply sailed along the coast. Along the way to America seafarers may have stopped in the Philippines.
Early modern humans who reached the Philippines may have already possessed seafaring capabilities before they reached the islands more than 30,000 years ago, and they had to have such capabilities in order to get there in the first place. They also had fishing skills that enabled them to catch fish in the open sea. They also maintained a web of social and trading connections between the islands of Wallacea as well as the Philippines.
Archaeologists have also found evidence of early movements along the coast of America, on offshore islands and near the tip of South America. There is also genetic evidence of such an early movement that must have originated in Wallacea or the Philippines -- genetic signals of Australian or New Guinean ancestry, as well as elevated levels of DNA snippets from Homo neanderthalensis and Homo denisova that show up in ancient and modern genomes of some people in Panama and South America are further evidence of such a movement along the coast.
If the early migrants did sail along the coast to get to the Philippines, they must have stopped first in the Middle East or on the Iranian Plateau, where they picked up an admixture of Homo neanderthalensis DNA before sailing along the coast to Island Southeast Asia where they met up with Homo denisova on offshore islands, rather than on the mainland.
Surprisingly, a burnt piece of wood that was found at an archaeological site in southern Chile has been dated at 33,000 years old. This evidence is not conclusive, but it is plausible that people from Wallacea or the Philippines did reach the site at that early date. However, they contributed little of their genome to the later indigenous populations of North and South America, perhaps because they arrived in the New Worle with a limited range of skills that ill-suited them for the many new environments that they found there, and they may have kept on foraging and fishing along the shoreline until after most of North and South America had been populated by later immigrants.
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