Wednesday, November 21, 2007

Migration of Early Humans From Africa Aided By Wet Weather

Finding: Migrations out of Africa 200,000 to 150,000 was dependent on the the wet climate in the presently hyper-arid Saharan-Arabian desert.

Conventional thinking
This migration was dependent on the occurrence of wetter climate in the region. There is good evidence that the southern and central Saharan-Arabian desert experienced increased monsoon precipitation during this period (200,000 to 150,000), but there is no unequivocal evidence for a corresponding rainfall increase in the northern part of the migration corridor, including the Sinai-Negev land bridge between Africa and Asia.

Passage through this "bottleneck" region would have been dependent on the development of suitable climate conditions.

Uranium series dating method - Speleothems
Scientists a reconstruction of paleoclimate in the Negev Desert based on absolute uranium series dating of carbonate cave deposits (speleothems). Speleothems only form when rainwater enters the groundwater system and vegetation grows above a cave.

Today the climate in the Negev Desert is very arid and speleothems do not form, but their presence in a number of caves clearly indicates that conditions were wetter in the past. Scientists have dated 33 speleothem samples from five caves in the central and southern Negev Desert.

Increased Rainfall in the central and Southern Negev Desert
The ages of these speleothems show that the last main period of increased rainfall occurred between 140,000 and 110,000 years ago. The climate during this time consisted of episodic wet events that enabled the deserts of the northeastern Sahara, Sinai, and the Negev to become more hospitable for the movement of early modern humans.

Wet periods in the North and South parts of the Saharan-Arabian desert
The simultaneous occurrence of wet periods in the northern and southern parts of Saharan-Arabian desert may have led to the disappearance of the desert barrier between central Africa and the Levant.

The humid period in the Negev Desert between 140,000 and 110,000 years ago was preceded and followed by essentially unbroken arid conditions; thus creating a climatic "window" for early modern human migration to the Levant.

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