hypothesis called "The Saharan pump theory" explains how flora and fauna have migrated through a physical link between Eurasia and Africa. This hypothesis postulates that long periods of heavy rainfall for thousands of years that have alternated with periods of drought in Africa, are associated with a so-called "wet Sahara" phase, during which large lakes and rivers such as mega-lake Chad, existed alternately with an immense desert, the Sahara. The wet Sahara was probably a mosaic landscape of rivers, lakes, swamps, woodlands, forest islands, wooded savannas and grasslands. Earth regions higher than 45° of latitude where under a heavy shelf of ice. Mediterranean sea was much lower than today.
The Middle Paleolithic was a period of African prehistory that began about 280,000 years BCE and ended approximately between 50,000 years and 25,000 years BCE.
Even during periods of drought, humans were most often able to follow the Nile to cross the Sahara, as flora and fauna persisted on its banks. Migrations between Eurasia and Africa were, however, interrupted when, during a desert phase of 1.8 - 0.8 million years, the Nile at times stopped flowing completely and at another period because of a geological uplift (movement of elevation) of the Nile region.
This has resulted in changes in the flora and fauna in the region which have made traveling at great distances very difficult. Evaporation exceeds precipitation, water levels in lakes like Lake Chad, fall very low and rivers become dry wadis. The once widespread flora and fauna must be retreated northward into the Atlas Mountains and south into West and East Africa in the Nile Valley and from there to South-East to the plateaus of Ethiopia and Kenya or north-east to Asia via Sinai.
This separates the populations of the different species in zones with different climates, thus requiring them to adapt either by migration or speciation (evolution towards new species) or by exploiting different resources.
The Saharan pump was invoked to explain three waves of human migrations outside Africa, namely: Homo Erectus to Southeast Asia, perhaps twice, once as far as China and India, once again to Pakistan. Homo heidelbergensis to the Middle East and Western Europe. Homo Sapiens Sapiens towards the Middle East and Western Europe, the so-called "out of Africa"
Between approximately 133 and 122 000 years BCE, the southern parts of the Saharan desert had the beginning of the so-called "Abbassia Pluvial" period, which is a very wet period with monsoon precipitation. This allowed the Eurasian animals to travel to Africa and vice versa.
The Abbassia Pluvial period brought humid and fertile conditions to what is today the Sahara desert, which then benefited from lush vegetation, fed by lakes, swamps and river systems, many of which disappeared later in the drier climate that followed the Abbassia Pluvial period. African wildlife, now associated with the savannas, meadows and woods of the southern Sahara, had penetrated all of North Africa during this period.
The Stone Age cultures, especially the Mousterian and Aterian have grown significantly in Africa during the Abbassia Pluvial period. The transition to more severe climatic conditions that accompanies the end of this humid period may have encouraged the emigration of Homo Sapiens away from Africa.
The coastal road around the western Mediterranean was open during the last glacial period and may have promoted exchanges. Wet periods were limited to only tens or hundreds of years.
During the Pluvial Mousterian, the dried-up regions of North Africa became again very humid, as during the rainy Abbassia period 50,000 years earlier. There were lakes and even small inland seas (mega lake Chad), swamps and hydrographic networks that no longer exist today. Where the Sahara Desert is located today, there was the African wildlife typical of meadows and woods, herbivores such as the gazelle, the giraffe or the ostrich, predators of the lion to the jackal, hippopotamuses and crocodiles, As well as species that have now disappeared, such as the Pleistocene camel.
The Mousterian Pluvial was caused by large-scale climatic changes during the last glacial period. At about 50,000 BCE the Würm glaciation was well advanced in the northern hemisphere. The ice caps in North America and Europe were increasingly shifting the climatic zones favorable to human life to the southern hemisphere. Temperate zones in Europe and North America were transformed into arctic tundra, and rainfall strips typical of temperate zones declined sharply at latitudes in North Africa.
Curiously, the same influences that created the Mousterian Pluvial also seem to have made it disappear. At its maximum development, there are between 30 and 18,000 BCE, the Laurentide ice sheet covers not only an enormous geographical area but reaches an altitude of 1750 meters. This creates a specific meteorological system that affects the jet stream on the North American continent. The jet stream splits into two entities, creating a new climate over the northern hemisphere, which sets harsher conditions in several regions of Central Asia and the Middle East, the end of the Mousterian Pluvial and a Return to a more arid climate in North Africa.
Human settlements then move northwards. The late Paleolithic began in Egypt about 30,000 BCE as evidenced by the skeleton of Nazlet Khater. The excavation of the Nile exposed the first stone tools of that period.
Another example of climate change introduced by the Saharan pump occurred after the last glacial maximum around 22,500 - 17,000 BCE. During the last glacial maximum the Sahara desert was more extensive than it is today, with a considerably weaker extent of tropical forests than today. During this period, lower temperatures reduce the strength of Hadley's atmospheric cell, whereby tropical climbing air brings rain in the tropics, while dry air descending to about 20 degrees north latitude, Flows back to Ecuador and brings desert conditions to this region. This phase is associated with high levels of wind-borne mineral dust found in marine cores from the northern tropical Atlantic.
Around 12,500 BCE, begins a period of much more humid conditions in the Sahara, bringing a savannah climate to the Sahara.
Analysis of Nile sediment deposited in the delta also shows that this period had a higher proportion of sediments from the Blue Nile, suggesting higher precipitation on the highlands of Ethiopia. This was mainly due to a stronger monsoon activity in all tropical regions, affecting India, Arabia and the Sahara.
The African wet period that took place between 12,800 and 3,500 years BCE, was the last occurrence of a "Green Sahara". Climatic conditions in the Sahara during the African wet season were dominated by a strong monsoon with heavy rainfall. With the considerable increase in rainfall, vegetation in North Africa is transformed into vast grasslands. The Sahel region to the south of the Sahara becomes a savanna.
The African wet season was also characterized by a network of vast rivers in the Sahara, large lakes, rivers and deltas. The four largest lakes were Megachad Lake, Megafezzan Lake, Ahnet-Mouydir Lake and Lake Chotts. There were large rivers in the area such as the Senegal River, the Nile River, the Sahabi River and the Kufra River. These river and lake systems have provided corridors that have enabled many animal species, including humans, to extend their geographic range to the north, migrating across the Sahara.
A brutal climatic event occurred around 6,000 BCE. It is characterized by a sudden drop in global temperatures that lasted several millennia. This sudden cooling event may have been caused by the collapse of the Laurentide ice sheet in northeastern North America, probably when the Ojibway and Agassiz glacial lakes suddenly emptied into the North Atlantic Ocean. The melting pulse may have altered the thermohaline circulation of the North Atlantic, reducing the transport of heat from the tropics to the north of the Atlantic.
The sudden movement of the Hadley atmospheric cell towards the south causes sudden cooling followed by slower warming, linked to changes with the El Niño cycle, which leads to a rapid drying up of the Saharan and Arab regions.