While archaeological and genetic evidence suggest that Homo sapiens originated in Africa around 200,000 years ago, it is more difficult to identify when modern human behaviors emerged. Intriguingly, recent archaeological excavations in southern Africa have shown that technological innovation, linked to the emergence of culture and modern behavior, took place relatively suddenly on more than one occasion. Now, an international team led by researchers from Universitat Autonoma de Barcelona has linked the emergence of tool-making and agriculture to climate change in sub-Saharan Africa.
Over the last million years the global climate has varied between glacial periods and interglacial periods, with changes approximately every 100,000 years. But within these long periods there have been abrupt climate changes, sometimes happening in the space of just a few decades, with variations of up to 10 degrees Celsius in the average temperature in the polar regions caused by changes in the Atlantic ocean circulation. These changes, say the researchers, dramatically affected rainfall in southern Africa.
The researchers have pieced together how rainfall patterns varied over the last 100,000 years by analyzing river delta deposits at the edge of the African continent. Specifically, the ratio of iron (dissolved from the rocks by the water during the rains) to potassium (present in arid soils) is a record of the sediment carried by rivers, and therefore, also of the rainfall throughout that period.
The reconstruction of the rainfall over 100,000 years shows a series of spikes that occurred between 40,000 and 80,000 years ago. These spikes show rainfall levels rising sharply over just a few decades, and falling off again soon afterwards. These changes in climate, contend the researchers, correlate with increases in population and technological innovation as documented from archaeological digs. The end of certain stone tool industries, they add, coincides with the onset of a drier climate.
The new research, appearing in Nature Communications, would appear to confirm one of the principal models of Paleolithic cultural evolution, which correlates technological innovation with the adoption of new refuges and with a resulting increase in population and social networks.
“The innovation and expansion caused by climate change were key factors in the origin of modern human behavior and in the dispersal of Homo sapiens from his ancestral home,” the researchers conclude.
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