Resegmentation of humanity
Humanity had been segmented by the bottleneck that had occurred in Group A of our ancestry, and this segmentation had been cured by the change in climate that had allowed archaic Homo sapiens not only to enter Africa in large numbers but to rapidly spread throughout Africa. Subsequently to that, the climate of Africa, as well as the Arabian Peninsula continued to fluctuate wildly, causing intermittent resegmentation of Homo sapiens within Africa.
This resegmentation was primarily a function of periodically dry condition that broke the human population into segments, and the largest of these segments in southern Africa, south of the Limpopo River. This large segment was completely cut off about every hundred years, and then partially reconnected before dry conditions returned in the vicinity of the Zambezi River.
While the southern segment expanded and contracted with changes of climate, it remained intact throughout the 300,000 year period of Homo sapiens dominnce in Africa. The same can't be said for that part of humanity that was more or less trapped north of the Zambezi River: At times when the southern part of Africa was cut off from the north, the southern part was repeatedly chopped into segments that were divided from each other by uninhabitable territory. This repeated division of the northern part of Africa propelled some parts to the northern population to develop an ability to more quickly adapt to changes so as to avoid becoming extinct during rapid changes of the environment.
Movement and turmoil
In the course of the last 130,000 years climatic oscillations have been accelerating in Africa and the bulk of the southern population remained more or less trapped below the Kalahari Desert, aside from a few that moved north into rock shelters. But at the same time, between 80,000 and 50,000 years ago, some of those in eastern Africa began to move across Africa from east to west, south to north, and again from from west to east, ignoring climatic and ecological limitations that had previously stopped their movement, such as semi-desert areas and forested areas. As they moved from one more habitable area to another they put indigenous populations to flight. Some of the aggressive populations continued to churn through Africa, but others moved out of Africa altogether, into the Levant or along the coast of the Arabian Peninsula, eventually to find their way as far as Australia.
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 longi/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 longi/denisova can be detected in the genomes of many modern people, particularly in Oceania, offshore from Southeast Asia, where some Homo longi/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 Homo sapiens occupied a string of far-north camps across Europe, from modern-day Poland to present-day England.
As they passed through Homo neanderthalensis territory they certainly must have noticed that interbreeding with the indigenous population 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 longi/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 longi/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 longi/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 longi/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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