MONSOON rain, or rather the lack of it, precipitated the rise of great civilisations in what is now the Sahara desert.
Since prehistoric times people have been following the shifting monsoon rains around the Sahara, a practice that triggered the herding of livestock and even the development of the great pharaonic dynasties,say researchers who have re-examined archaeological sites across the region.
Rudolph Kuper and Stefan Kr枚pelin of the Institute of Prehistoric Archaeology at the University of Cologne, Germany, studied geological and ecological data for clues to patterns of past rainfall. They also examined radiocarbon dates of dwellings and artefacts from 150 archaeological sites stretching from the far north of the eastern Sahara to the south, which allowed them to identify four main phases of human occupation of the desert (Science, DOI: 10.1126/science.1130989).
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Starting around 8500 BC, and continuing over the next few centuries, the lower boundary of the desert shifted about 800 kilometres north, bringing monsoon rains to barren lands. People living in the south followed the rains north, rapidly occupying the entire eastern Sahara. For about the next 3000 years, the climate was relatively stable. During this time, human settlements became well established, and people began to keep livestock.
Then, around 5300 BC, monsoons failed to reach the Egyptian Sahara. People began to retreat, along with their cattle, into places such as the banks of the Nile where there was still enough rainfall and surface water to meet their needs. 鈥淲e are convinced that the emergence of the pharaonic civilisation in the Nile Valley in about 3500 BC was not coincidental 鈥� but triggered by the onset of full desert conditions in most of Egypt outside the Nile valley and a few oases,鈥� Kropelin says.
Finally, between 3500 and 1500 BC, lack of rain drove people to maintain permanent settlements in the south of the region only. This exodus introduced the Neolithic way of life into sub-Saharan Africa, including pastoralism 鈥� and even today keeping livestock is one of the most important African economies.
How fire shaped the rainforest
In Australia, too, climate shifts played an important role in shaping the development of prehistoric societies.
Chris Turney and Douglas Hobbs of the University of Wollongong, New South Wales, studied 710 radiocarbon dates from archaeological sites across Queensland, where human settlements date back 35,000 years. There was a sudden increase in inland settlements at the same time as the onset of the modern El Ni帽o / Southern Oscillation about 5000 years ago. The number of settlements matching peaks in ENSO activity 3800, 2500 and 1000 years ago, the researchers report in a future issue of the Journal of Archaeological Science.
El Ni帽os are associated with less summer rain, making fire useful for managing the inland landscape, argue Turney and Hobbs, improving hunting success and regenerating plants. Drier conditions, and increased burning, then opened up the Australian rainforest 鈥� which might explain why people started colonising it 5000 years ago.
